# Elastic Collision Momentum Question

s8on95
Same question as this link except my teacher changed the numbers. 1996-B No.1 part C

http://lodischool.tripod.com/dovesol/DOVE96SOL.pdf

Velocity of Bock A=4v0

2. mv+mv=mv+mv

3. I understand how to do the question in the pdf file. However, I don't understand how the concept applies in my teacher's version. Solving for the velocity you get 2.5 m/s which is supposed to be the answer. However, the question says "the objects separate after collision". Wouldn't they not separate if they both travel 2.5 m/s after the collision? I don't understand how this is possible.

frogjg2003
Is the initial speed of block A the only thing the teacher changed? If so, then it is an inelastic collision.

s8on95
So would this problem be "impossible" to do then? If block B continues at 2.5, then it'd be inelastic but it says they separate. (I got this problem "wrong" on a test btw)

frogjg2003
Again, if the only thing the teacher changed is the initial velocity of block A from 3v0 to 4v0, then a speed of 2.5v0 for B implies a speed of 2.5v0 for A as well. You say you got that question wrong. Do you know what the right answers are?

s8on95
I got the problem wrong because I made up random numbers. What I did was wrong. The right answer was that block A continued at a speed of 2.5 V0. My dilemma is that would this not imply an inelastic collision if they continued at equal velocities?

frogjg2003
Nowhere in the problem was an elastic collision implied, either in the original, or the altered problem.

s8on95
Sorry, wrong wording. I mean to ask how is it possible they can separate when they go at the same velocity after collision?

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frogjg2003
They haven't separated. You're getting too caught up on the wording and not on the physics.

s8on95
The problem clearly says the blocks separate after collision in Part C.

frogjg2003
Your teacher copied the problem and didn't change the wording. No physics has been changed. You are just obsessing with the wording of a badly worded problem.

s8on95
But that is exactly my problem. Shouldn't it NOT BE POSSIBLE for the physics to be possible with the given wording? If I were given the same problem as the link, I could've done it with ease. I understand the physics concepts/math no problem. When I took the test though I just thought this was a weird scenario.

It is predicted an inelastic collision with 2.5 m/s of both of them together

What happens is they actually separate and still both go at 2.5 m/s?

During the test I calculated and was going to put 2.5 m/s but this would've implied they didn't separate. That is why I ended up becoming confused and doing a bunch of random weird stuff. I understand the mathematical physics but my problem is exactly with obsessing over the wording. It just shouldn't make sense!

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frogjg2003
I will state it again. THEY DID NOT SEPARATE. Your teacher changed the numbers but did not change the wording. This happens all the time, to the best and the worst teachers. The calculations you did show that it was going at 2.5v0 (not 2.5m/s, you were never given an actual speed), and I've shown you that your calculations are correct, so that is the speed it was moving with. If you had used 2.5m/s you would have gotten the question right. If you still have questions about the wording of the problem, you need to take it up with your teacher, not Physics Forums.

s8on95
Okay then. I shall consult my teacher (another student did and I think she was said to respond with something along the lines of "they just separate"). Thanks for the help!

frogjg2003
Let us know what your teacher says. I'm really curious.