Momentum of Wall: 2mv Explained

  • Context: High School 
  • Thread starter Thread starter pdpax
  • Start date Start date
  • Tags Tags
    Momentum Wall
Click For Summary

Discussion Overview

The discussion revolves around the concept of momentum during a collision between a ball and a stationary wall. Participants explore the implications of momentum conservation in elastic collisions, the nature of momentum in stationary bodies, and the transfer of momentum to larger structures such as buildings and the Earth.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that the momentum of the wall is 2mv due to the conservation of momentum during an elastic collision, where a ball with momentum +mv rebounds with momentum -mv.
  • Others argue that a stationary wall does not possess momentum, and when the ball strikes it, the wall exerts an equal and opposite reaction of -p momentum.
  • A participant suggests that while the wall does not move, the momentum is transferred to the entire structure and ultimately to the Earth, indicating that the wall's momentum is not isolated.
  • Some participants clarify that momentum is conserved, but the wall's lack of movement is due to its rigidity and the forces holding it in place, which does not imply that it has no momentum in the context of the entire system.
  • There is a contention regarding the definition of momentum for stationary bodies, with some stating that there is no such thing as a truly stationary body, as the Earth absorbs the momentum.
  • Participants discuss the change in velocity of the ball after the collision, emphasizing that while speed remains the same, the change in direction constitutes a change in velocity, affecting the momentum calculations.

Areas of Agreement / Disagreement

Participants express disagreement on the interpretation of momentum in stationary bodies and the implications of momentum conservation. There is no consensus on whether the wall's momentum can be considered in isolation or as part of a larger system.

Contextual Notes

The discussion highlights various assumptions about the nature of momentum, the definitions of stationary bodies, and the effects of external forces on momentum transfer, which remain unresolved.

pdpax
Messages
3
Reaction score
0
Hi everybody,
Any one can tell me why the momentum of the wall is 2mv while a ball collides it?
 
Physics news on Phys.org
pdpax said:
Any one can tell me why the momentum of the wall is 2mv while a ball collides it?
I'm not sure what you mean by 'why', but it's a consequence of momentum conservation. If a ball with momentum +mv collides elastically with a fixed wall and thus rebounds with momentum -mv, the wall (and all attached to it) must end up with an total momentum of +2mv.
 
Any one can tell me why the momentum of the wall is 2mv while a ball collides it?

wall is not moving.. a stationary body does not posses momentum.
When the ball strikes with +p momentum the wall gives out equal and opposite reaction of -p momentum.
 
Sakriya said:
wall is not moving.. a stationary body does not posses momentum.
When the ball strikes with +p momentum the wall gives out equal and opposite reaction of -p momentum.
Are you suggesting that momentum is not conserved during the collision?
 
suppose a ball is moving with initial velocity, v towards a wall, and it rebounds with velocity -v, then the change in momentum of the ball is -2mv, which is the same as the change in momentum of the wall, BUT the wall is held in place by the forces between it and the floor, roof etc. and thus it does not move. If there was no restraining force on the wall, it would have started to move backwards.

In other words the momentum of the wall is transferred to the whole structure (building,home etc.), and thus by extension to the earth
 
Are you suggesting that momentum is not conserved during the collision?

Of course it is conserved.

When ball hits the wall, the wall does not move because it is held at bottom and due to rigidity and elasticity of the material. The wall gives this back to the ball as reaction due to the action. That's why the velocity is same as it was when the ball was striking it. The wall doesn't have it's own momentum which it is imparted to the ball(it seems you suggested that), if so was true then velocity of the ball after striking would be different. That's what conservation of momentum is... final momentum equals initial
 
Sakriya said:
Of course it is conserved.

When ball hits the wall, the wall does not move because it is held at bottom and due to rigidity and elasticity of the material. The wall gives this back to the ball as reaction due to the action. That's why the velocity is same as it was when the ball was striking it. The wall doesn't have it's own momentum which it is imparted to the ball(it seems you suggested that), if so was true then velocity of the ball after striking would be different. That's what conservation of momentum is... final momentum equals initial
I think you are a little confused here. The velocity of the ball after striking the wall IS different! (Speed is the same, but velocity is a vector quantity so a change in direction is a change in velocity)

Because the velocity has changed the final momentum of the ball is NOT equal to the initial momentum of the ball. Therefore, for momentum to be conserved the wall's momentum must also change.
 
Sakriya said:
the wall does not move

Sure it does. It just takes the whole Earth with it. You might calculate the velocity change from that and convince yourself it's not a problem.
 
wall is not moving.. a stationary body does not posses momentum.
When the ball strikes with +p momentum the wall gives out equal and opposite reaction of -p momentum.

there is no such thing as a stationary body. if the house is firmly stuck to the ground, the Earth gets the extra momentum 2mv.
 

Similar threads

Graduate Wall made of Pendulum -- thought experiment
  • · Replies 7 ·
Replies
7
Views
1K
Graduate Time of Impact of Gas Molecules on the Inner Wall of a Pressure Vessel
  • · Replies 1 ·
Replies
1
Views
2K
High School How to characterize a rubber ball moving horizontally and bouncing off of a vertical wall?
  • · Replies 8 ·
Replies
8
Views
2K
High School Gun Recoil: Will Wall Impact Momentum & Velocity?
  • · Replies 30 ·
2
Replies
30
Views
4K
Undergrad Is Momentum Conserved in Elastic and Inelastic Collisions with Walls?
  • · Replies 25 ·
Replies
25
Views
13K
High School Need a bet settled about classical momentum
  • · Replies 7 ·
Replies
7
Views
1K
Undergrad Does the classical theory of angular momentum explain this video of a unicycle robot?
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad Kinetic theory of gases: rebound speed and force questions
  • · Replies 2 ·
Replies
2
Views
3K
High School Why Do Objects Bounce? Exploring Momentum and Energy Conservation
  • · Replies 6 ·
Replies
6
Views
2K
High School I don't get instantaneous impulse
  • · Replies 2 ·
Replies
2
Views
1K