No conservation of momentum with bouncing ball?

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Discussion Overview

The discussion centers around the conservation of momentum in the context of a bouncing ball colliding with a wall. Participants explore the implications of elastic and inelastic collisions, the role of energy conversion, and the effects of mass and velocity on momentum conservation.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants suggest that when a handball is thrown at a wall, the wall exerts an equal normal force, leading to a change in the net momentum vector of the system.
  • Others argue that the ball is not perfectly elastic, as some energy is converted to heat during the bounce.
  • There is a proposal that in a perfectly elastic scenario, the ball would bounce back with equal momentum, although the direction of the momentum vector would change.
  • Some participants note that sound energy is also produced during the collision, contributing to energy loss.
  • A later reply emphasizes that the wall does move, albeit imperceptibly due to its connection to the Earth, suggesting that momentum would be conserved if the ball and wall/earth system were isolated.
  • Another participant mentions that the conservation law applies to the Earth as well, indicating that it would rotate slightly in response to the ball being thrown.

Areas of Agreement / Disagreement

Participants express differing views on the nature of momentum conservation in this scenario. While some agree that momentum is conserved in an isolated system, others highlight the complexities introduced by energy conversion and the apparent motion of the wall.

Contextual Notes

Participants discuss the implications of mass and velocity on momentum conservation, noting that the effects may be more pronounced in different environments, such as deep space.

nhmllr
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So I was thinking about the conservation of momentum. If you throw a handball at a wall, the wall will provide an equal normal force, thus sending the handball back at the same velocity (in a perfect scenario). The ball has a momentum vector, the wall never moves, and thus only has a zero-amplitude vector. But in this closed system, the net momentum vector changes! How?
 
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The ball is not perfectly elastic: some of the spring energy in the bounce is converted to heat.
 
russ_watters said:
The ball is not perfectly elastic: some of the spring energy in the bounce is converted to heat.

So what would happen if the ball was perfectly elastic? It would still bounce off, right?
 
It would bounce back with an equal momentum to what it started with.
 
Sound is also a form of energy in which the initial energy of the ball gets converted to, so it loses energy there. In a perfectly elastic system the momentum is conserved completely and none is wasted. meaning p = p' (momentum before = momentum after)
 
russ_watters said:
It would bounce back with an equal momentum to what it started with.

But the vector is in a completely different direction!
 
nhmllr said:
So I was thinking about the conservation of momentum. If you throw a handball at a wall, the wall will provide an equal normal force, thus sending the handball back at the same velocity (in a perfect scenario). The ball has a momentum vector, the wall never moves, and thus only has a zero-amplitude vector. But in this closed system, the net momentum vector changes! How?

the wall does move, except that it's connected to the ground (aka Earth) which means it only appears to not move. If you really could isolate the ball and wall/earth system, the momentum would be conserved. Of course, look at the masses you're talking about and you can understand why the wall seems to not move.
 
Pengwuino said:
the wall does move, except that it's connected to the ground (aka Earth) which means it only appears to not move. If you really could isolate the ball and wall/earth system, the momentum would be conserved. Of course, look at the masses you're talking about and you can understand why the wall seems to not move.

Ah- I see. little mass x big velocity = huge mass x tiny velocity. So I suppose in deep space the wall would actually start moving back, and the conservation would be more obvious.

Thanks
 
Don't forget that when you threw the ball, the conservation law also applied and the Earth rotated backwards a tiny bit.
The amount it moves forwards would be twice that value for a perfectly elastic collision and an equal value for a totally inelastic collision. All the Mv's add up to zero in every case.
 

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