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# Change in momentum? watch

1. If change in momentum is mv-mu, and let's say we have a ball of mass 1 kg bouncing off a wall with u = 3 and v = -4 m/s. Now, change in momentum would be -4 - 3 = -7. But isn't this wrong? Shouldn't the change be positive?
2. (Original post by Kolasinac138)
If change in momentum is mv-mu, and let's say we have a ball of mass 1 kg bouncing off a wall with u = 3 and v = -4 m/s. Now, change in momentum would be -4 - 3 = -7. But isn't this wrong? Shouldn't the change be positive?
No. It had positive momentum and now has negative momentum so the change is negative.
3. (Original post by morgan8002)
No. It had positive momentum and now has negative momentum so the change is negative.
Ah, ok, a certain question confused me:

q2 here

http://theallpapers.com/papers/CIE/A...2_s09_qp_2.pdf
4. (Original post by Kolasinac138)
Ah, ok, a certain question confused me:

q2 here

http://theallpapers.com/papers/CIE/A...2_s09_qp_2.pdf
It's a similar situation. The ball is initially travelling in one direction and bounces back. You can calculate the momentum change in the same way as last time.
5. (Original post by morgan8002)
It's a similar situation. The ball is initially travelling in one direction and bounces back. You can calculate the momentum change in the same way as last time.
the MS says that the answer must be positive
6. (Original post by Kolasinac138)
the MS says that the answer must be positive
It asks for change in momentum not magnitude of change in momentum so I don't know where they got that from.

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