# Constant Velocity Question

I was recently asked this question:

If there is no net force on a system which is moving at a constant velocity, which of the following is also constant?

a) Acceleration
b) Momentum
c) Impulse
d) All of the above

My solution:
a) Acceleration must be constant via Newton's Second law. Since F=ma, a=F/m and with a force of 0, the acceleration must be constantly 0. By the definition of acceleration (change in velocity over time), there is no change in velocity, so the acceleration is 0.
b) Momentum is the measure of inertia that an object has due to it's motion, so with no change in the motion, momentum is constant (p=mv).
c) Impulse is a change in momentum, so since momentum is constant, impulse must remain at 0. Also, Impulse= Force * time so with no net force, impulse is zero.

Therefore, my answer is (d)... all of the above.

My instructor disagrees and seems to have a problem with acceleration being constantly zero. Apparently "nobody refers to acceleration as being constantly 0." His choice was just momentum, (b).

Can anybody support my answer or explain the issue more clearly?

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## Answers and Replies

kuruman
Science Advisor
Homework Helper
Gold Member
Your instructor is incorrect. Your instructor seems to think that zero is not a constant. Ask your instructor to prove that it is not, i.e. that zero changes with time.

Your instructor probably assumes that "constant" means "non zero", but that is not what the problem says.