A linear acceleration of 4m/s^{2} means that every second your velocity is increasing by 4m/s. This is constant acceleration. If you start at a velocity of 0m/s after 4 seconds you are traveling at a velocity of 16m/s.

The acceleration is fine but it is not constant at all.

Non-constant acceleration would be if in the first second your velocity increased by 5m/s but in the second second it increased by 7m/s and in the third it only increased by 3m/s and in the fourth it increased by 1m/s. It has taken you 4 seconds to reach a velocity of 16m/s but it wasn't constant acceleration.

It's no different. Neither are constant.

Your first one can be construed as consistent acceleration and your other can be construed as haphazard acceleration.

Neither are constant and can never be,in reality.

In the first example

Velocity is changing by THE SAME AMOUNT each second.

Acceleration is THE SAME AMOUNT each second.

In the second example velocity is changing by DIFFERENT AMOUNTS each second

Accelerations is NOT THE SAME each second.

Constant acceleration - variable acceleration.

BUT in both cases velocity is changing

If something is changing it is never constant.

Constant acceleration cannot exist whether it's a measured build up or a variation. Neither are constant.

A speed can be constant and so can a velocity, but never acceleration....EVER.

Acceleration is a a RATE of change.

It could be positive, ie getting faster, or negative, getting slower or ZERO .

It can be constant or variable.

acceleration

əksɛləˈreɪʃ(ə)n/Submit

noun

a vehicle's capacity to gain speed.

"the three-litre model has spectacular acceleration"

synonyms: speeding up, increasing speed, increase in speed, gain in momentum, gathering speed, opening up; technicalrate of change of velocity

"the car's acceleration is sensational"

increase in speed or rate.

plural noun: accelerations

"the acceleration of the industrialization process"

synonyms: hastening, speeding up, quickening, stepping up, advancement, furthering, furtherance, forwarding, promotion, boosting, boost, stimulation, spur, aid, assistance, facilitation, easing, simplification, expedition, precipitation More

PHYSICS

the

*rate of change of velocity* per unit of time.

Please look at the bold bit

THE RATE OF CHANGE per unit of time.

Not the change per unit of time.