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# Momentum conservation watch

1. If a ball bounces on an object with a velocity V and then rebounds with a velocity of -V then its momentum has changed from +P to -P. How can this abide to the conservation of momentum?
2. (Original post by JakeRStudent)
If a ball bounces on an object with a velocity V and then rebounds with a velocity of -V then its momentum has changed from +P to -P. How can this abide to the conservation of momentum?
You've got to be somewhat careful about what you consider as being part of the system in which momentum is being conserved... the object the ball is bouncing off has mass and is subject to an impulse during the collision - so it's changing momentum too even if you can't notice any change in it's velocity.
3. (Original post by Joinedup)
You've got to be somewhat careful about what you consider as being part of the system in which momentum is being conserved... the object the ball is bouncing off has mass and is subject to an impulse during the collision - so it's changing momentum too even if you can't notice any change in it's velocity.
Ah I think I see. So considering the total momentum before must be equal to the total momentum aftef would the walls momentum be equal to 2mV after the collision?
4. (Original post by JakeRStudent)
Ah I think I see. So considering the total momentum before must be equal to the total momentum aftef would the walls momentum be equal to 2mV after the collision?
That's right. You just don't notice the wall moving down because it's connected to the planet.
5. (Original post by JakeRStudent)
Ah I think I see. So considering the total momentum before must be equal to the total momentum aftef would the walls momentum be equal to 2mV after the collision?
well yes but walls are probably not free to move & attached to the surface of the planet.

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