# Collision Questions

## Homework Statement

A. Two objects collide in elastic collision (one is at rest). When is the greatest KE transferred?

B. Two objects w/unequal mass are at rest (frictionless surface) and have equal forces acting on them for equal amts of time. When these forces are remove, what will the object with greater mass experience?

## The Attempt at a Solution

A. Since masses are conserved, wouldn't the only variable change is the velocity? So shouldn't it be where the moving object is traveling very fast, or does this question involve mass?

B. I used equation F=p/t. If the forces are equal and so is time, wouldn't the momentum be the same for both? Considering the surface is frictionless.

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Doc Al
Mentor
A. Since masses are conserved, wouldn't the only variable change is the velocity? So shouldn't it be where the moving object is traveling very fast, or does this question involve mass?
I'd say that the question involves mass.

B. I used equation F=p/t. If the forces are equal and so is time, wouldn't the momentum be the same for both? Considering the surface is frictionless.
Yes, the momentum is the same. So what can you say about the object with greater mass?

I'd say that the question involves mass.

Yes, the momentum is the same. So what can you say about the object with greater mass?
For the first one though, since ke=1/2 m v^2, shouldn't the velocity have the greatest effect on KE since it's a squared variable?

And p=mv....so larger mass should have a smaller velocity.

Doc Al
Mentor
For the first one though, since ke=1/2 m v^2, shouldn't the velocity have the greatest effect on KE since it's a squared variable?
The question is vaguely worded. What I think they are looking for is under what conditions (consider the relative masses of the objects) will the greatest percentage of KE be transferred. That has a clearcut answer.

And p=mv....so larger mass should have a smaller velocity.
That's what I would say.

The question is vaguely worded. What I think they are looking for is under what conditions (consider the relative masses of the objects) will the greatest percentage of KE be transferred. That has a clearcut answer.

That's what I would say.
Sorry I'll try to reword it.

Two objects (A and B) have the same momentum. A has more kintetic energy than B if

1. it is heavier than B.
2. it has a higher speed than B.

My reasoning is that it would be #2 b/c of the squared factor I mentioned. Having a larger mass would mean a smaller speed for A and if u have a smaller speed it would be squared, resulting in a smaller KE than if u would be increasing the speed.

Doc Al
Mentor
Sorry I'll try to reword it.
I didn't mean that your question (in your last post) was vaguely worded, but that the question in the problem statement for A was.

What I gave is my guess as to what they are looking for.

One more question if you don't mind helping!

A object (A) has an elastic collision with another object (B) initially at rest. Greatest KE is transferred if:

A is traveling really fast.

A weighs a lot more than B.

Isn't the same thing...just look at the KE=1/2 m v^2? Having a faster velocity (squared value) is better than mass.

Doc Al
Mentor
I don't think that's what they are looking for. I suspect they want the conditions under which the greatest percentage of KE transferred, not the greatest amount. The velocity doesn't matter.

I don't think that's what they are looking for. I suspect they want the conditions under which the greatest percentage of KE transferred, not the greatest amount. The velocity doesn't matter.
I'm having difficulty understanding why velocity wouldn't matter. And yes you're right about the fraction/percentage.