Denspressure vs Reality

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

 You see, I class acceleration as one thing. I never use constant acceleration.

Acceleration can be constant. It is a rate of increase. If I drive from a stop to 10 km/h in one second, then that’s 10 km/h/s. If after two seconds I’m going 20 km/h, and after three seconds I’m going 30 km/h, that’s a constant acceleration of 10 km/h/s.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

Acceleration is simply just that. If you're accelerating you are never constant, because your speed is ever changing and this is the only real way you can decelerate.

You can never decelerate from a constant velocity.

Really? So if I’m traveling at 100 km/h for an hour, I can never slow down?

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

As you accelerate vertically you have to add thrust to push the mass or you have to lose the mass of the object (not the fuel) to keep the acceleration. 

Why is rocket fuel not a part of the mass that needs thrusting? Burning fuel reduces the weight of the rocket. By f=ma, acceleration increases exactly as the expending of fuel reduces the mass of the rocket.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

If you can't lose the mass of the object then you have to increase the thrust.

If the thrust is already greater than the weight of the rocket, it will continue to accelerate. Decreasing the mass accelerates it faster.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

However, if you are already at full thrust then all you have left is to lose fuel mass to keep that thrust.

Thrust is a result of the potential chemical energy stored in the fuel and the efficiency of the engines. Losing mass doesn’t affect the amount of thrust (unless you burn all of your fuel).

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

The issue with this is, at best you keep a constant velocity and you can only keep it for each portion of atmosphere you plough through and push against.
A constant velocity cannot ever result in a deceleration because you are never accelerating, so a vertical deceleration that would allow the object an ever slowing forward vertical motion would not be relevant.

If you throttle back on a rocket engine until the thrust is less than the weight of the rocket, it will decelerate.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

It all comes back to the rope climbing and snapping scenario.
If you were to jump on a rope and yank yourself up it and then it snaps, you will still advance a little due to your initial springboard start or accelerated start.

If after your springboard accelerated start you then start the climb and hit a constant velocity, you are required to use more energy with each movement just to keep that constant velocity, because you cannot lose your mass to aid you.
If that rope snaps at any time during any of that climb, you're going to stop dead and then accelerate downwards. You will never gain another inch UP.

That’s because rope climbing involves very short bursts of acceleration followed by short pauses. There’s always a jerkiness to it. Nobody climbs a rope at a steady speed.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

No matter what you're up against in whatever way vertically, you will never keep accelerating unless you lose the mass of the object and the fuel of it at the same time.
You will never do it by losing just fuel. All you can achieve is a constant velocity at best.

Rockets will travel at a constant velocity if their thrust equals their weight. If thrust is greater than their weight, then they will accelerate upward.

Well done JBS...Though I doubt he will understand it.

And it's 22 pages long....War and Peace watch out, Sceptimatic is about....lol

What keeps the ball moving would be its acceleration.

And this is yet to be justified in any way.

An object accelerating up is basically getting loaded up with 'up' velocities. A few downs from resistive forces (gravity/denpressure, air resistance etc) might get into it, but they're outweighed and the net velocity is up, and increasing. When it gets cut off from acceleration the ups on board don't vanish, but there's only a finite number of them and they'd quickly be outweighed by the downs, so the upward motion slowns as more and more ups get cancelled, and then there's nothing, and then there's donwards.
Meanwhile an object moving at a constant speed gets loaded with ups at the same rate the downs cancel them out, so there's no leftover force and as soon as ups stop being added the downs dominate and hence downward velocity.

Except this ignores how it initially got to that point of a constant upwards velocity.

Continuing on from your acceleration example, instead of cutting power, you keep on going, but now at a constant velocity. This means you keep applying "ups" while magic keeps applying "downs", which cancel each other out resulting in the object remaining at constant velocity.
You now cut power (after a period of constant velocity), the object is still loaded with all these ups which need to be cancelled before it starts falling.

Even better, instead of calling these "ups" and "downs", lets give them units?
How about the ups will be +z Ns, and the downs will be -z Ns? (the + and - are to cancel each other instead of up and down, as +z + (-z)=0).
Lets also call them momentum packets.
So if you are initially at rest on the ground this means the ground is giving these +z Ns while "something" is giving -z Ns, and it all cancels out.
Then to accelerate upwards, you are getting more momentum packets applied to you in the +z direction.
This no longer cancels out and thus you establish a surplus of these +z momentum packets corresponding to your upwards motion.

If you no longer have these +z packets applied, so you just have the -z ones applied then you start to slow down, but your surplus keeps you moving.
Eventually you run out and you start falling.

If instead of losing all +z packets, you instead start getting the same amount as the -z packets, then it goes back to cancelling out and you continue moving upwards at a constant velocity.
You still have those initial +z packet surplus which is why you keep going up, with the -z packets cancelling out the additional +z packets.
Now when you stop applying the +z packets you only have the addition of the -z packets.

According to Septi this means you should drop instantly.
But you still have the surplus +z packets that kept you going after acceleration. So these should continue to keep you going, slowly getting cancelled by additional -z packets.

So even after that constant velocity, you should still decelerate, not stop dead.

Quote from: Jonny B Smart on November 25, 2017, 08:18:55 AM

It doesn’t matter which direction you are moving. Moving at a constant velocity requires no force. force = mass x ACCELERATION, remember? No acceleration (change in velocity) means no force.
Moving against a force (gravity or air resistance, for example) requires a balanced force to maintain velocity.

You are massively contradicting yourself.

No he isn't.
F=ma, thus if you have no force acting you stay at constant velocity.
If you have a force acting (like gravity), then you will be accelerated by gravity. Thus in order to maintain velocity you need to apply an additional force to counter gravity, so the net force is 0.

Let's look at decelerating.
In my mind I believe decelerating can only be a thing immediately after the cut off point of acceleration and it's a massive key thought.
If something is building up a mph and the power is immediately cut, that build up will allow deceleration to happen, which means a forward movement to still happen as ever reducing mph.

And you are yet to justify this, and all known observations contradict you.
It doesn't matter if you go at constant velocity or accelerate, you decelerate, not stop dead.

In this case we would see the iron ball sit on the platform all the way up and all the way down to the ground.

On that very same platform going up at a constant velocity, it is under a constant energy load that is keeping that platform moving at a stead mph vertically.
The iron ball on the platform is also moving at the same rate, obviously.
If I was to cut the power in a split second to leave that platform attached to nothing, then that platform and the ball stop dead before accelerating back down.
My argument is that it would not move an inch further vertically after power is cut at a constant rate of mph.

And again, all known observations contradict you.

She's asking questions.
She does not accept my thoughts.
She likes to figure out each and every theory and to do so she asks pertinent questions and queries answers.
She's not your enemy.

The way she humours people like you, asking questions without any form of debate (even when she knows your claims are wrong), and with how she defends you and others like you and attacks the other REers, I would say she is the enemy.

It gives you a false impression of how we should behave which leads you to conclude that we aren't trying to understand.
The simple fact is many of us are trying to understand, but unlike Jane, when we see a problem we point it out.

A constant velocity cannot ever result in a deceleration because you are never accelerating

You are yet to justify why you need an acceleration immediately before to allow it to decelerate.
But another issue you have is how it started moving with a constant vertical velocity without accelerating in the first place?

If that rope snaps at any time during any of that climb, you're going to stop dead and then accelerate downwards. You will never gain another inch UP.

Perhaps not another inch.
As you would accelerate downwards at 9.8 m/s^2, in order to gain an inch (0.0254 m), you need:
vp=0=v0-a*tp
tp=v0/a
dp=v0*tp-0.5*a*tp^2
dp=v0*v0/a-0.5*a*v0^2/a^2
dp=v0^2/a*(1-0.5)=v0^2/2a
v0^2=2*a*dp
v0=sqrt(2*a*dp)

So you need:
v0=sqrt(2*9.8*0.0254)m/s
=0.7 m/s.

Have you tried climbing up a rope that fast at a constant speed?

Obviously space rockets defy this because they kick themselves up their own arses, apparently. lol.

Yes, rockets defy this. That is because they lose mass.
Then there is the issue of reaching that constant velocity.

Objects do not remain at a constant speed due to acceleration.

We're abusing terminology a bit, but that part of what Scepti's saying is entirely correct. Technically he's talking about imparting acceleration, ie a force, and to go at a constant speed you need forces to be balanced.

Momentum is a product of mass and velocity. By “product,” I mean “result of multiplication.”

And that product is going to exist regardless of model. What matters is what it means.

I have to say this is a classic post. Where to start is a difficult one. What it does demonstrate is that Sceptimatic hasnt the first clue about the laws of motion. Jane I hasten to add comes in a constant velocity second place. LoL possibly with a bit of deceleration.
Arguing the toss with Sceptimatic is a bit like chewing on a piece of cardboard, you can work out the rest if you care.
It also explains why he never produced anything on denpressure as the man is barely literate.
Wow...LOL....

I think you'll generally find that's because he's talking to posters like you who pop by, throw out a few insults, and repeat themselves rather than actually engage. I'm not at all interested in whether or not he's right, I'd just rather do something interesting and talk to someone rather than repeat the same few lines a hundred times in a row. What is it you think you're achieving with this utter refusal to engage in back-and-forth? How do you plan to make a good argument when you constantly refuse to deal with what you're arguing against?

You can never decelerate from a constant velocity.

It seems like this is the main departure from what's typically believed; normally acceleration is viewed as the accrued trait, rather than velocity, so even at a constant speed the object 'contains' the acceleration it took to get it to that point. Whereas for you it seems as though an object only really 'stores' velocity and needs a source of acceleration for acceleration/deceleration to be a factor.

Assuming my understanding was correct I think I've only got two questions.

The first might be basic, just worth checking. How real-world-applicable is the talk of travelling at a constant velocity? Most of the time there would be small variations, just due to the unpredictability of resistive forces, or imperfect sources of energy, trying to get both slightly-fluctuating causes of acceleration to balance perfectly seems nigh-impossible. When you talk about attaining a constant velocity, is it just getting close enough that the acceleration is negligible, or is there some factor that makes it easier to attain the constant velocity?

And second and lastly, if we go back to that iron ball that got thrown in the air via a springboard start, as it decelerates does it decelerate 'smoothly' or is there a point when even then it goes to a dead stop?
This is kind of related to the first question as far as trying to maintain a constant velocity goes. For a kind of illustration, if an object is travelling at some fast, constant velocity of 100m/s and then has its power cut off it would immediately drop to 0m/s. But then if there was a tiny boost in the engines for a split second and it travelled at 100m/s with an acceleration of 1m/s/s at the instant the engines were cut off, would it just decelerate to lose that 1m/s/s, or would it lose that and then decelerate through the 100m/s?

I am far too tired right now so there is a chance that made no sense at all. Apologies if so.

Quote from: Jonny B Smart on November 26, 2017, 10:48:48 AM

Objects do not remain at a constant speed due to acceleration.

We're abusing terminology a bit, but that part of what Scepti's saying is entirely correct. Technically he's talking about imparting acceleration, ie a force, and to go at a constant speed you need forces to be balanced.

Quote

Momentum is a product of mass and velocity. By “product,” I mean “result of multiplication.”

And that product is going to exist regardless of model. What matters is what it means.

Quote from: Nightsky on November 26, 2017, 12:00:41 PM

I have to say this is a classic post. Where to start is a difficult one. What it does demonstrate is that Sceptimatic hasnt the first clue about the laws of motion. Jane I hasten to add comes in a constant velocity second place. LoL possibly with a bit of deceleration.
Arguing the toss with Sceptimatic is a bit like chewing on a piece of cardboard, you can work out the rest if you care.
It also explains why he never produced anything on denpressure as the man is barely literate.
Wow...LOL....

I think you'll generally find that's because he's talking to posters like you who pop by, throw out a few insults, and repeat themselves rather than actually engage. I'm not at all interested in whether or not he's right, I'd just rather do something interesting and talk to someone rather than repeat the same few lines a hundred times in a row. What is it you think you're achieving with this utter refusal to engage in back-and-forth? How do you plan to make a good argument when you constantly refuse to deal with what you're arguing against?

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

You can never decelerate from a constant velocity.

It seems like this is the main departure from what's typically believed; normally acceleration is viewed as the accrued trait, rather than velocity, so even at a constant speed the object 'contains' the acceleration it took to get it to that point. Whereas for you it seems as though an object only really 'stores' velocity and needs a source of acceleration for acceleration/deceleration to be a factor.

Assuming my understanding was correct I think I've only got two questions.

The first might be basic, just worth checking. How real-world-applicable is the talk of travelling at a constant velocity? Most of the time there would be small variations, just due to the unpredictability of resistive forces, or imperfect sources of energy, trying to get both slightly-fluctuating causes of acceleration to balance perfectly seems nigh-impossible. When you talk about attaining a constant velocity, is it just getting close enough that the acceleration is negligible, or is there some factor that makes it easier to attain the constant velocity?

And second and lastly, if we go back to that iron ball that got thrown in the air via a springboard start, as it decelerates does it decelerate 'smoothly' or is there a point when even then it goes to a dead stop?
This is kind of related to the first question as far as trying to maintain a constant velocity goes. For a kind of illustration, if an object is travelling at some fast, constant velocity of 100m/s and then has its power cut off it would immediately drop to 0m/s. But then if there was a tiny boost in the engines for a split second and it travelled at 100m/s with an acceleration of 1m/s/s at the instant the engines were cut off, would it just decelerate to lose that 1m/s/s, or would it lose that and then decelerate through the 100m/s?

I am far too tired right now so there is a chance that made no sense at all. Apologies if so.

Simple rocket experiment:

1. Hold a small object in the palm of your hand at waist height, but don’t grip it.
2. Using force transmitted by your arm, accelerate your hand upward as quickly as you can, and then stop it at shoulder height.
3. Observer the object. Does it continue into the air above your hand?

If “yes,” then we know that objects launched vertically will continue upward once the force has been removed.

(Note: this has been attempted billions of times with the same result, but maybe Scepti knows something that all those people do not.)

Also, why would we not use common, scientific vocabulary?

“Cows are carnivores.”

“No, they eat grass.”

“Well, in my model, cows have claws and sharp teeth.”

“That’s a cat.”

“In your model, but the way I’m using the word ‘cow,’ I mean that small animal with sharp teeth and claws. And they ARE carnivores. My pet cow loves to sit on my lap, purr, and eat spinach.”

“You mean ‘tuna’?”

“Not in my model.”

"I'm not at all interested in whether or not he's right, I'd just rather do something interesting and talk to someone rather than repeat the same few lines a hundred times in a row. What is it you think you're achieving with this utter refusal to engage in back-and-forth"

I agree it's not about being right, But sceptimatic is just plain wrong and can't see it. Some of his comments such as not being able to decelerate from a constant velocity are just plain bonkers.  I get the feeling you like backing lost causes. As for me repeating things 100 times, that's just you resorting to exaggeration which is something I've told to at least a thousand times not to do. But as always who appointed you as the judge of the discussion? and as for the mythical moral mound you appeared go be perched on?

Just read your last post. Storing velocity, containing acceleration!
This debate is just too silly for words.

"I'm not at all interested in whether or not he's right, I'd just rather do something interesting and talk to someone rather than repeat the same few lines a hundred times in a row. What is it you think you're achieving with this utter refusal to engage in back-and-forth"

I agree it's not about being right, But sceptimatic is just plain wrong and can't see it. Some of his comments such as not being able to decelerate from a constant velocity are just plain bonkers.  I get the feeling you like backing lost causes. As for me repeating things 100 times, that's just you resorting to exaggeration which is something I've told to at least a thousand times not to do. But as always who appointed you as the judge of the discussion? and as for the mythical moral mound you appeared go be perched on?

Amazingly, this is how discussion works. One person says something about what the other person said. Pointing out a bad argument does not necessitate being made the judge of a discussion, all it means is that you're taking part in it.
It's bonkers? Great. Prove it. It shouldn't be hard once you move past that repitition nonsense and start responding to what Scepti's actually saying. No one is asking you to believe him automatically, but it's beyond tedious for everyone involved when all you do is repeat an argument directed against an initial impression and never alter it based on new information.

What do you think you're achieving? It's not logical to ignore someone's model while arguing against it, so you're not winning any debate. Do you want to object to people who treat FET as comparable to RET and dissuade people from taking it seriously? You're not managing that, anyone that is open to alternative models is going to spot the poor quality discussion. Are you not taking this site seriously and just trying to have fun? Great, except I sincerely doubt making posts you could basically copy and paste is fun for you.

Quote from: Nightsky on November 26, 2017, 02:53:33 PM

"I'm not at all interested in whether or not he's right, I'd just rather do something interesting and talk to someone rather than repeat the same few lines a hundred times in a row. What is it you think you're achieving with this utter refusal to engage in back-and-forth"

I agree it's not about being right, But sceptimatic is just plain wrong and can't see it. Some of his comments such as not being able to decelerate from a constant velocity are just plain bonkers.  I get the feeling you like backing lost causes. As for me repeating things 100 times, that's just you resorting to exaggeration which is something I've told to at least a thousand times not to do. But as always who appointed you as the judge of the discussion? and as for the mythical moral mound you appeared go be perched on?

Amazingly, this is how discussion works. One person says something about what the other person said. Pointing out a bad argument does not necessitate being made the judge of a discussion, all it means is that you're taking part in it.
It's bonkers? Great. Prove it. It shouldn't be hard once you move past that repitition nonsense and start responding to what Scepti's actually saying. No one is asking you to believe him automatically, but it's beyond tedious for everyone involved when all you do is repeat an argument directed against an initial impression and never alter it based on new information.

What do you think you're achieving? It's not logical to ignore someone's model while arguing against it, so you're not winning any debate. Do you want to object to people who treat FET as comparable to RET and dissuade people from taking it seriously? You're not managing that, anyone that is open to alternative models is going to spot the poor quality discussion. Are you not taking this site seriously and just trying to have fun? Great, except I sincerely doubt making posts you could basically copy and paste is fun for you.

“Open to alternative models” on this site means “prefer imagination to hundreds of years of accumulated scientific knowledge.”

This site is serious to the extent that it successfully damages public intelligence. Some of us have come here to do battle against misinformation. That is serious work (though exasperating). I don’t expect to convert the lost, but I hope inoculate the innocent by showing them that FE supporters’ logic and models ain’t worth their time.

“Open to alternative models” on this site means “prefer imagination to hundreds of years of accumulated scientific knowledge.”

This site is serious to the extent that it successfully damages public intelligence. Some of us have come here to do battle against misinformation. That is serious work (though exasperating). I don’t expect to convert the lost, but I hope inoculate the innocent by showing them that FE supporters’ logic and models ain’t worth their time.

How are you doing that? Leaving inside that pretentiousness, the only people not 'innoculated' are going to be more than capable of noticing the conspicuous absence of actual responses. It's only exasperating when you make it that way.
This should not be hard. If you are going to show that a model doesn't hold you have to take the time to understand it and see how it could work before responding. Launching straight into "You're wrong and everything you say is wrong," without trying to get clarification is just unutterably tedious for all involved, you Scepti and readers alike.

Also, why would we not use common, scientific vocabulary?

“Cows are carnivores.”

“No, they eat grass.”

“Well, in my model, cows have claws and sharp teeth.”

“That’s a cat.”

“In your model, but the way I’m using the word ‘cow,’ I mean that small animal with sharp teeth and claws. And they ARE carnivores. My pet cow loves to sit on my lap, purr, and eat spinach.”

“You mean ‘tuna’?”

“Not in my model.”

Gold.

However when you say your cow with its spinach breath sits in your lap, what you really mean is that it likes being pushed into your lap by layers of atmosphere caused by displacing its own dense mass of atmosphere. Sort of like having a gobstopper forced up its bum by a rocket springboard sort of effect, but in reverse. You just have to stop and think on that carefully.

Quote from: Jonny B Smart on November 26, 2017, 03:47:31 PM

“Open to alternative models” on this site means “prefer imagination to hundreds of years of accumulated scientific knowledge.”

This site is serious to the extent that it successfully damages public intelligence. Some of us have come here to do battle against misinformation. That is serious work (though exasperating). I don’t expect to convert the lost, but I hope inoculate the innocent by showing them that FE supporters’ logic and models ain’t worth their time.

How are you doing that? Leaving inside that pretentiousness, the only people not 'innoculated' are going to be more than capable of noticing the conspicuous absence of actual responses. It's only exasperating when you make it that way.
This should not be hard. If you are going to show that a model doesn't hold you have to take the time to understand it and see how it could work before responding. Launching straight into "You're wrong and everything you say is wrong," without trying to get clarification is just unutterably tedious for all involved, you Scepti and readers alike.

1. I have launched model rockets and watched them continue to fly up after the engine burns out, so I know that Scepti is wrong from personal observations. (I’ve also tossed many balls into the air. Try it—it’s fun!)
2. I have taken physics classes where we learned that f=ma is an algebraic formula and not just a general relationship. Each of the terms (force, mass, acceleration, velocity, momentum, speed...) have specific meanings and specific ways that they interact with each other that has been known and studied for hundreds of years. Now Scepti thinks he can just scrap all that? Our entire advanced technological society is built on Newton’s Three Laws. The entire aerospace industry has it wrong? Trains, planes, and automobiles work by some new set of rules that Scepti just popped out last month? Not a chance.
3. If Scepti doesn’t even know the meanings of the most basic scientific terms, can’t understand the easiest scientific equation around, and doesn’t seem to know that people can (or how they can) throw balls in the air, then how in the great, big, beautiful, round world is he going to come up with cutting-edge physics?!

As I’ve said before, FE folks need the ability to discern known science, plausible science, and science fiction. Scepti is science fiction.

2. I have taken physics classes where we learned that f=ma is an algebraic formula and not just a general relationship.

No, that is an equation, not a formula.  Are you sure you studied physics? 

Quote from: Jonny B Smart on November 26, 2017, 05:02:43 PM

2. I have taken physics classes where we learned that f=ma is an algebraic formula and not just a general relationship.

No, that is an equation, not a formula.  Are you sure you studied physics?

No, and yes I studied physics.

“What is a Formula?
A formula is a special type of equation that shows the relationship between different variables.

A formula will have more than one variable.

These are all equations, but only some are formulas:

x = 2y - 7   Formula (relating x and y)
a2 + b2 = c2   Formula (relating a, b and c)
x/2 + 7 = 0   Not a Formula (just an equation)”

https://www.mathsisfun.com/algebra/equation-formula.html

Quote from: Jonny B Smart on November 26, 2017, 05:02:43 PM

2. I have taken physics classes where we learned that f=ma is an algebraic formula and not just a general relationship.

No, that is an equation, not a formula.

Are you sure? Haven't you ever studied mathematics?

Are you sure you studied physics?

Smart Aleck!

What is the difference between equation and formula?
Sometimes equation and formula are used interchangeably, but I was wondering if there is a difference.

Simple example: Calculating your car's fuel efficiency mpg = distance traveled in miles / fuel used in gallon

Calculating the MPG above, is that an equation or formula?

Answer:
An equation is any expression with an equals sign, so your example is by definition an equation. Equations appear frequently in mathematics because mathematicians love to use equal signs.

A formula is a set of instructions for creating a desired result. Non-mathematical examples include such things as chemical formulas (two H and one O make H₂O), or the formula for Coca-Cola (which is just a list of ingredients). You can argue that these examples are not equations, in the sense that hydrogen and oxygen are not "equal" to water, yet you can use them to make water.

Mathematicians have long since realized that when it comes to numbers, certain formulas can be expressed most succinctly as equations. For example, the Pythagorean Theorem a²+b²=c²a²+b²=c² can be thought of as a formula for finding the length of the side of a right triangle, but it turns out that such a length is always equal to a combination of the other two lengths, so we can express the formula as an equation. The key idea is that the equation captures not just the ingredients of the formula, but also the relationship between the different ingredients.

In your case, "mpg = distance/gallons" is best understood as "a formula in the form of an equation", which means that in this instance the two words are interchangeable.

See the rest in: Mathematics Stack Exchange, What is the difference between equation and formula?

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

 You see, I class acceleration as one thing. I never use constant acceleration.

Acceleration can be constant. It is a rate of increase. If I drive from a stop to 10 km/h in one second, then that’s 10 km/h/s. If after two seconds I’m going 20 km/h, and after three seconds I’m going 30 km/h, that’s a constant acceleration of 10 km/h/s.

I disagree.
Acceleration can never be constant because of the nature of what it means. To advance in speed at every moment, or to CHANGE in speed at every moment, which means it is not a constant.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

Acceleration is simply just that. If you're accelerating you are never constant, because your speed is ever changing and this is the only real way you can decelerate.

You can never decelerate from a constant velocity.

Really? So if I’m traveling at 100 km/h for an hour, I can never slow down?

Of course you can slow down but you can never decelerate.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

As you accelerate vertically you have to add thrust to push the mass or you have to lose the mass of the object (not the fuel) to keep the acceleration. 

Why is rocket fuel not a part of the mass that needs thrusting? Burning fuel reduces the weight of the rocket. By f=ma, acceleration increases exactly as the expending of fuel reduces the mass of the rocket.

Rocket fuel is part of the mass and that burning mass does reduce the mass of the rocket.
However, the rocket is always at full thrust and that full thrust ensures the ejected burning mass matches the mass of the rocket to atmospheric push, so you're always getting a thrust to mass ratio that keeps the rocket at a constant velocity.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

If you can't lose the mass of the object then you have to increase the thrust.

If the thrust is already greater than the weight of the rocket, it will continue to accelerate. Decreasing the mass accelerates it faster.

Correct but this does not happen, except for initial springboard lift off to move a stationary mass into atmosphere at FULL thrust.
The only way to accelerate that rocket at full thrust would be to lose some of the rocket with each inch (for the sake of it) it moves vertically.
Dumping the burning mass of fuel will keep the mass of rocket constant.
Dumping masses of rocket will keep an acceleration.
The only time you'll ever see a rocket structural mass dump is when a section is jettisoned but it's counter productive to the vertical movement of that rocket because it immediately stops the initial constant velocity in favour of another springboard kickstart and back to constant velocity again by fuel burn.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

However, if you are already at full thrust then all you have left is to lose fuel mass to keep that thrust.

Thrust is a result of the potential chemical energy stored in the fuel and the efficiency of the engines. Losing mass doesn’t affect the amount of thrust (unless you burn all of your fuel).

Yes but that energy has to release from the rocket with fuel or as it (in terms of a bottle rocket or whatever).

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

The issue with this is, at best you keep a constant velocity and you can only keep it for each portion of atmosphere you plough through and push against.
A constant velocity cannot ever result in a deceleration because you are never accelerating, so a vertical deceleration that would allow the object an ever slowing forward vertical motion would not be relevant.

If you throttle back on a rocket engine until the thrust is less than the weight of the rocket, it will decelerate.

There's no such thing as throttling back on a rocket engine unless you are doing horizontal flight or water/ground based rocket propulsion.
Vertical flight requires no throttling down and would be absolutely counterproductive to the rocket flight.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

It all comes back to the rope climbing and snapping scenario.
If you were to jump on a rope and yank yourself up it and then it snaps, you will still advance a little due to your initial springboard start or accelerated start.

If after your springboard accelerated start you then start the climb and hit a constant velocity, you are required to use more energy with each movement just to keep that constant velocity, because you cannot lose your mass to aid you.
If that rope snaps at any time during any of that climb, you're going to stop dead and then accelerate downwards. You will never gain another inch UP.

That’s because rope climbing involves very short bursts of acceleration followed by short pauses. There’s always a jerkiness to it. Nobody climbs a rope at a steady speed.

If you want to be picky then just think of the elevator on a rope scenario.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

No matter what you're up against in whatever way vertically, you will never keep accelerating unless you lose the mass of the object and the fuel of it at the same time.
You will never do it by losing just fuel. All you can achieve is a constant velocity at best.

Rockets will travel at a constant velocity if their thrust equals their weight. If thrust is greater than their weight, then they will accelerate upward.

Correct and this is exactly what happens at springboard lift off and the only acceleration the rocket will encounter, unless it can shed its skin as it goes vertical.

The thrust always has to be greater than the mass of the rocket to move it.
The thrusting fuel will always be at maximum.
The rocket can never accelerate after initial springboard lift off by max thrust of fuel alone. It must shed its own mass to continue acceleration.

If it can't shed it's own mass as well as the max thrust of fuel, then no acceleration is ongoing, only constant velocity.

I have to say this is a classic post. Where to start is a difficult one. What it does demonstrate is that Sceptimatic hasnt the first clue about the laws of motion. Jane I hasten to add comes in a constant velocity second place. LoL possibly with a bit of deceleration.
Arguing the toss with Sceptimatic is a bit like chewing on a piece of cardboard, you can work out the rest if you care.
It also explains why he never produced anything on denpressure as the man is barely literate.
Wow...LOL....

"It all comes back to the rope climbing and snapping scenario"

It sure does...lol...What a fruitcake.

You have a lot to say without actually saying anything.

Quote from: sceptimatic on November 26, 2017, 10:25:18 AM

You can never decelerate from a constant velocity.

It seems like this is the main departure from what's typically believed; normally acceleration is viewed as the accrued trait, rather than velocity, so even at a constant speed the object 'contains' the acceleration it took to get it to that point. Whereas for you it seems as though an object only really 'stores' velocity and needs a source of acceleration for acceleration/deceleration to be a factor.

Assuming my understanding was correct I think I've only got two questions.

The first might be basic, just worth checking. How real-world-applicable is the talk of travelling at a constant velocity? Most of the time there would be small variations, just due to the unpredictability of resistive forces, or imperfect sources of energy, trying to get both slightly-fluctuating causes of acceleration to balance perfectly seems nigh-impossible. When you talk about attaining a constant velocity, is it just getting close enough that the acceleration is negligible, or is there some factor that makes it easier to attain the constant velocity?

Good question because you are correct in terms of not being able to keep that constant due to minor fluctuations on a horizontal.
However a much closer potential (although maybe not ultra perfect) would be the vertical rocket scenario in terms of max fuel thrust/burn to mass ratio after initial lift off.
Would any of it be perfect? I highly doubt it, because it would require a perfect burn and a perfect atmosphere, so all we can basically rely on is using all of it as as near as damn it.

And second and lastly, if we go back to that iron ball that got thrown in the air via a springboard start, as it decelerates does it decelerate 'smoothly' or is there a point when even then it goes to a dead stop?

If you're talking about the initial kick start or springboard start where acceleration ceases to be that, you would basically see it rise as it pushes through atmospheric resistance by it's own dense mass and the energy you applied to it.
Your ball deceleration as it moves up until it ceases to decelerate and then comes to a complete stop for a nano second and then accelerates back down, would be as perfect as you're going to get the smooth is the atmosphere you are in.

This is kind of related to the first question as far as trying to maintain a constant velocity goes. For a kind of illustration, if an object is travelling at some fast, constant velocity of 100m/s and then has its power cut off it would immediately drop to 0m/s. But then if there was a tiny boost in the engines for a split second and it travelled at 100m/s with an acceleration of 1m/s/s at the instant the engines were cut off, would it just decelerate to lose that 1m/s/s, or would it lose that and then decelerate through the 100m/s?

I am far too tired right now so there is a chance that made no sense at all. Apologies if so.

You seem to be suggesting a two stage rocket if I'm reading you correctly.

Let's assume you are.

Ok the rocket shuts down its first burn and jettisons its structural mass for that particular burn, it would cease to advance vertically up from that immediate burn out at constant velocity.
As it's shedding its mass it would immediately start to fall before any next burn can kick in, which would render the rocket a mess and basically an impossible happening.
However, I'm talking about the so called space rocket fantasy as we are shown it, so let's get back to what we do see.


What you really see in a two stage rocket (model) is a springboard acceleration to deceleration and then a separation and then a reburn.
If this happens fast then the rocket can still be decelerating (moving up) as the next stage kicks in and immediately kick in another springboard effect until it reaches constant velocity.

So what I'm basically saying is, you will never get a stage separation and reigniting booster on any rocket moving at constant velocity.

I'm not entirely sure if I've answered your query but if not feel free to try and get me to clarify.

His latest thoughts on motion  from Sceptimatic:-

"Of course you can slow down but you can never decelerate"

Acceleration can easily be a constant. You seem to be getting it confused with velocity. So let’s make this simple:

Velocity - the distance travelled over a set time
Acceleration - the change in velocity over a set time

If my velocity is 5 m/s to start and then I accelerate to 10 m/s over 5 seconds then my acceleration will have been a constant of 1 m/s/s.

The measure of a change can in itself be a constant.

Acceleration can easily be a constant. You seem to be getting it confused with velocity. So let’s make this simple:

Velocity - the distance travelled over a set time
Acceleration - the change in velocity over a set time

If my velocity is 5 m/s to start and then I accelerate to 10 m/s over 5 seconds then my acceleration will have been a constant of 1 m/s/s.

The measure of a change can in itself be a constant.

Acceleration can never be a constant.

Quote from: Mainframes on November 26, 2017, 11:49:22 PM

Acceleration can easily be a constant. You seem to be getting it confused with velocity. So let’s make this simple:

Velocity - the distance travelled over a set time
Acceleration - the change in velocity over a set time

If my velocity is 5 m/s to start and then I accelerate to 10 m/s over 5 seconds then my acceleration will have been a constant of 1 m/s/s.

The measure of a change can in itself be a constant.

Acceleration can never be a constant.

Latest quote from Sceptimatic on motion:-

"Acceleration can never be a constant."

How does he dream these Pearls of wisdom up?

Some advice Sceptimatic, why not go and read something on the subject :-
http://www.rocket-propulsion.info/resources/software/rpa/RPA_LiquidRocketEngineAnalysis.pdf

The whole problem with this discussion is the all contributors know very little when compared to an expert the field. The problem with Sceptimatic is that he knows even less but thinks he knows , sure recipe for disaster.

Question for Sceptimatic, what has given you believe that you know anything about rocket propulsion? How much of the contents of that PDF I posted are you familiar with? Before sounding off about as subject t it's always a good idea to have some knowledge on the subject.

Acceleration can never be a constant.

It is true that "Acceleration can never be a constant" for all time,
but acceleration can certainly be a constant for a limited period of time.

The whole problem with this discussion is the all contributors know very little when compared to an expert the field. The problem with Sceptimatic is that he knows even less but thinks he knows , sure recipe for disaster.

Question for Sceptimatic, what has given you believe that you know anything about rocket propulsion? How much of the contents of that PDF I posted are you familiar with? Before sounding off about as subject t it's always a good idea to have some knowledge on the subject.

Tell me how much you know about this.
http://www.popularmechanics.com/culture/movies/a21522/star-trek-beyond-uss-enterprise-cutaway/

Quote from: sceptimatic on November 26, 2017, 11:57:13 PM

Acceleration can never be a constant.

It is true that "Acceleration can never be a constant" for all time,
but acceleration can certainly be a constant for a limited period of time.

Acceleration can never be a constant, ever.

Acceleration is a gain in speed/mph. It is never a constant until it becomes a set speed/mph, in which it then ceases to be called, acceleration.

Quote from: Nightsky on November 27, 2017, 12:15:06 AM

The whole problem with this discussion is the all contributors know very little when compared to an expert the field. The problem with Sceptimatic is that he knows even less but thinks he knows , sure recipe for disaster.

Question for Sceptimatic, what has given you believe that you know anything about rocket propulsion? How much of the contents of that PDF I posted are you familiar with? Before sounding off about as subject t it's always a good idea to have some knowledge on the subject.

Tell me how much you know about this.
http://www.popularmechanics.com/culture/movies/a21522/star-trek-beyond-uss-enterprise-cutaway/

Are you serious? Like most of your posts irrelevant.
I give a scientific document on rocket propulsion, and you retort with trivia. Just about sums you up, and nails the level you operate at.

Quote from: rabinoz on November 27, 2017, 12:20:28 AM

Quote from: sceptimatic on November 26, 2017, 11:57:13 PM

Acceleration can never be a constant.

It is true that "Acceleration can never be a constant" for all time,
but acceleration can certainly be a constant for a limited period of time.

Acceleration can never be a constant, ever.

Acceleration is a gain in speed/mph. It is never a constant until it becomes a set speed/mph, in which it then ceases to be called, acceleration.

You don't know we what velocity is.
You don't know what speed is.
And you have no idea on acceleration which as most of us know can either be +ve or -ve.

For you to say that -ve acceleration does not exist just reveals the depth of your ignorance not just on this subject but in general. On the same topic, of course acceleration can be constant. Only in your head is it not possible!
Let remember that acceleration is just a rate of change of speed or velocity over a given time. If you accept that then you must accept the consequences.