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# M2 collisions question with kinetic energy- edexcel watch

1. Hi, I am having trouble with the following problem (small detail of the problem):

I understand the general argument and am confident I could set up such an argument myself. The problem I am having though concerns the marked area below:

So these to possible velocities for A and B have been found. Clearly only one velocity is possible and the solution says that y>x ( i.e. the speed of the 2kg sphere is greater than the speed of the 1kg sphere). But why is y>x?

Thanks for any help!
2. http://pmt.physicsandmathstutor.com/...hapter%204.pdf
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3. It's a bit early in the morning for me but is it just that sphere A is behind sphere B so can't possibly be going faster (because it would then have to overtake B in some way or would push it along and they'd both move at the same speed)?
4. (Original post by phys981)
It's a bit early in the morning for me but is it just that sphere A is behind sphere B so can't possibly be going faster (because it would then have to overtake B in some way or would push it along and they'd both move at the same speed)?
Thanks for your reply! I don't think that works in this case though...
I can't see that the question implies that A is behind B or that A and B can't collide a second time.
5. I can't see how it can NOT be behind B. They are in a line, right? How could it get past B?
6. here it is in a previous life https://www.thestudentroom.co.uk/sho....php?t=2302563
7. (Original post by phys981)
I can't see how it can NOT be behind B. They are in a line, right? How could it get past B?
Isn't it possible that they collide a second time? ( I do see that this is a bit absurd as they would have to collide instantaneously, after the first collision...)
8. They can only collide again if they'd actually lost contact - which they won't do if, as you suggest, A is going faster than B.
9. (Original post by phys981)
They can only collide again if they'd actually lost contact - which they won't do if, as you suggest, A is going faster than B.
Oh right! Makes sense... Thanks a lot for your help and patience.

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