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# Question on momentum watch

1. Attachment 700864700866

The answer is C. But why is option D incorrect? For elastic collision, isn't the relative speed of separation equal to relative speed of approach in D? Am I missing something?
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2. If you imagine say, runner 1 running at someone, say runner 2 whose running in the opposite direction at half the speed, it would make sense that the one who rebounds with a greater speed would be in the direction of the one running the fastest?
3. (Original post by BDunlop)
If you imagine say, runner 1 running at someone, say runner 2 whose running in the opposite direction at half the speed, it would make sense that the one who rebounds with a greater speed would be in the direction of the one running the fastest?
Does this always apply to all collisions with objects that move at opposite directions? So their final speeds will never exceed their initial speeds?
4. (Original post by sourmango1)
Attachment 700864700866

The answer is C. But why is option D incorrect? For elastic collision, isn't the relative speed of separation equal to relative speed of approach in D? Am I missing something?
I think you have made some mistakes in using the relative speed approach.

If you compute the overall "net" momentum before the collision, you should find 2m N s to the right where m is the mass of sphere.

From conservation of linear momentum, after the collision, the system should still have 2m N s to right.

Option D has a "net" momentum of 2m N s (to the left) after the collision.
5. (Original post by Eimmanuel)
I think you have made some mistakes in using the relative speed approach.

If you compute the overall "net" momentum before the collision, you should find 2m N s to the right where m is the mass of sphere.

From conservation of linear momentum, after the collision, the system should still have 2m N s to right.

Option D has a "net" momentum of 2m N s (to the left) after the collision.
Ah, I see. That explains everything now. I only took account of their magnitude. Thanks!

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