Parachutes

Quote from: cbarnett97 on August 26, 2007, 12:16:26 AM

so what force counteracts the air resistance to produce a state of equalibrium so we achieve a terminal velocity? and therefore have a model that reflect reality

No force.  Once you reach terminal acceleration, the air cannot push and harder. 

that is not the same thing as what happens on the RE

Quote from: sokarul on August 26, 2007, 12:20:48 AM

Quote from: cbarnett97 on August 26, 2007, 12:16:26 AM

so what force counteracts the air resistance to produce a state of equalibrium so we achieve a terminal velocity? and therefore have a model that reflect reality

No force.  Once you reach terminal acceleration, the air cannot push and harder. 

that is not the same thing as what happens on the RE

Yeah, its not.  But you can't tell the difference. 

Ok, let's do this again, a little simpler.  I think sokarul has finally realized the reason as to why the EP still holds, like I said, in my first post.

You are in a plane, accelerating at the same rate as the earth.  You exit said plane and stop accelerating.  The earth continues to accelerate and accelerates the air along with it. You body experiences drag and starts to accelerate upwards.  As you accelerate (again after you open your chute) your apparent acceleration with the earth slows.  When your acceleration with the earth is at zero, or very close to it, you have reached your terminal velocity.  However, your overall velocity is less than that of the earth.  Therefore, you slowly approach the earth.

Ok. 
If you jump out of a plane in the FE you will loose your acceleration while the earth will not, right?

Yep.  I think I was getting everything way too convoluted.  Thanks for bringing the argument back on track.

Ok, so you see how your post was wrong?


Edit: Damn it!  Wrong button.  Sorry.

Ok, so you see how your post was wrong?

Yep.

Ok, let's do this again, a little simpler.  I think sokarul has finally realized the reason as to why the EP still holds, like I said, in my first post.

You are in a plane, accelerating at the same rate as the earth.  You exit said plane and stop accelerating.  The earth continues to accelerate and accelerates the air along with it. You body experiences drag and starts to accelerate upwards.  As you accelerate (again after you open your chute) your apparent acceleration with the earth slows.  When your acceleration with the earth is at zero, or very close to it, you have reached your terminal velocity.  However, your overall velocity is less than that of the earth.  Therefore, you slowly approach the earth.

but according to the FE model that is not what it states, there is a constant net force acting on the object so according to the FE model it is Impossible, so the only way you can be correct is you add elements of RE or introduce forces into the system that should not be there

Quote from: TheEngineer on August 26, 2007, 12:27:10 AM

Ok, let's do this again, a little simpler.  I think sokarul has finally realized the reason as to why the EP still holds, like I said, in my first post.

You are in a plane, accelerating at the same rate as the earth.  You exit said plane and stop accelerating.  The earth continues to accelerate and accelerates the air along with it. You body experiences drag and starts to accelerate upwards.  As you accelerate (again after you open your chute) your apparent acceleration with the earth slows.  When your acceleration with the earth is at zero, or very close to it, you have reached your terminal velocity.  However, your overall velocity is less than that of the earth.  Therefore, you slowly approach the earth.

but according to the FE model that is not what it states

That's exactly what it states.

using the FE model how would you calculate an objects terminal velocity? In RE we would use a variation of the original force Equation F=mg-R

RE =>  m*g = F_D

FE =>  m*a = F_D

Just solve for velocity from the drag equation.

nope you are wrong on both accounts

Uh, ok...show me.

In FE terminal velocity would be zero which fits into the FE model because you velocity is zero.
Now in RE model you just need to rework the equation to find the Acceleration of an object which will give you a=g-(DpA/2m)v² and then you set that to zero since that is what we want to find an then then equation goes to v_terminal=sqrt(2mg/DpA)

RE => Upwards force = downwards force  thus,  m*g=F_D

FE => Acceleration = Upwards force/mass thus, a=F_D/m

F_D= .5(area)(C_d)(rho)v^2

All is known except velocity.  Solve for velocity and you may be surprised at the equation you get.

In Fe what is accelerating the parachutist? Think before you answer because it is not the earth. In the FE model nothing can accelerate the guy, everything accelerates around him

In Fe what is accelerating the parachutist? Think before you answer because it is not the earth. In the FE model nothing can accelerate the guy, everything accelerates around him

Uh, the air is accelerating him.

Quote from: cbarnett97 on August 26, 2007, 01:19:32 AM

In Fe what is accelerating the parachutist? Think before you answer because it is not the earth. In the FE model nothing can accelerate the guy, everything accelerates around him

Uh, the air is accelerating him.

ok if the air is accelerating him what is his velocity? Not his velocity relative to the earth because in the case of the skydiver there is no velocity according to the FE model.

RE => Upwards force = downwards force  thus,  m*g=F_D

FE => Acceleration = Upwards force/mass thus, a=F_D/m

F_D= .5(area)(C_d)(rho)v^2

All is known except velocity.  Solve for velocity and you may be surprised at the equation you get.

also you are not looking for an equality you are looking for it to be zero

also you are not looking for an equality you are looking for it to be zero

You don't understand physics at all, do you?

ok if the air is accelerating him what is his velocity? .

It depends.

Not his velocity relative to the earth because in the case of the skydiver there is no velocity according to the FE model

Sure there is.

Quote from: cbarnett97 on August 26, 2007, 01:24:38 AM

also you are not looking for an equality you are looking for it to be zero

You don't understand physics at all, do you?

well since this is not really physics, just algebra. and also since you love to nitpick every little thing so much I figured that you would remember to set it to zero.

Quote from: cbarnett97 on August 26, 2007, 01:23:35 AM

ok if the air is accelerating him what is his velocity? .

It depends.

Quote

Not his velocity relative to the earth because in the case of the skydiver there is no velocity according to the FE model

Sure there is.

FE model states that he can hove no velocity. Remember we are talking about the different models and their ability to predict reality. so you can not add an acceleration into a system when the model tells you that you cannot

well since this is not really physics, just algebra. and also since you love to nitpick every little thing so much I figured that you would remember to set it to zero.

So you don't understand algebra, then?

Remember we are talking about the different models and their ability to predict reality. so you can not add an acceleration into a system when the model tells you that you cannot

Since when does the FE say you are not allowed to have an acceleration?

FE accelerates up to you, so in the case of the guy he sits there until the earth accelerate up to him hence he has no velocity. so like i said you cannot add elements to a system just to make sure you are correct.

But the air is accelerating him, remember?

But the air is accelerating him, remember?

...but the air isn't a solid and can't accelerate him at 9.8m/s² because air does get pushed out of the way so the earth catches the parachutist YAY WE'RE DONE!

FE accelerates up to you, so in the case of the guy he sits there until the earth accelerate up to him hence he has no velocity. so like i said you cannot add elements to a system just to make sure you are correct.

The air accelerates the parachutist, but not at the same rate as the Earth is accelerating, therefore the Earth catches up.

Quote from: TheEngineer on August 26, 2007, 01:43:53 AM

But the air is accelerating him, remember?

...but the air isn't a solid and can't accelerate him at 9.8m/s² because air does get pushed out of the way so the earth catches the parachutist YAY WE'RE DONE!

Yes it can.

The air accelerates the guy to 9.8m/s².  Then he pulls his chute and gets accelerated to some number above 9.8m/s².  Then as he is floating he then deaccelerates back to 9.8m/s²

Quote from: Mr. Ireland on August 26, 2007, 11:20:30 AM

Quote from: TheEngineer on August 26, 2007, 01:43:53 AM

But the air is accelerating him, remember?

...but the air isn't a solid and can't accelerate him at 9.8m/s² because air does get pushed out of the way so the earth catches the parachutist YAY WE'RE DONE!

Yes it can.

The air accelerates the guy to 9.8m/s².  Then he pulls his chute and gets accelerated to some number above 9.8m/s².  Then as he is floating he then deaccelerates back to 9.8m/s²

How the hell does pulling his chute get him above 9.8m/s²?

He never goes above the accleration of the earth he would just match it. Unless you want to be like the engineer and violate the Model and add an acceleration into the system that does not exist. and then claim in one post that the acceleration of the air can not recreate the acceleration of gravity and then later claim that it recreates it just fine

He never goes above the accleration of the earth he would just match it. Unless you want to be like the engineer and violate the Model and add an acceleration into the system that does not exist. and then claim in one post that the acceleration of the air can not recreate the acceleration of gravity and then later claim that it recreates it just fine

This thread has been going on for so long that I no longer understand your point. What is your point in all of this?

Edit: Also, the acceleration caused by air resistance will not match the acceleration of the Earth.