Solving A Basic Physics Problem: Momentum Conservation and Ball-Wall Collision

[tennishaha]

Homework Statement

A very heavy wall moving at 60mph, a ball moving same direction at 120 mph.
What is direction and speed of ball after ball hit wall.

Homework Equations

I am thinking to use the momentum conservation: mass of ball*velocity of ball+mass of wall*velocity of wall don't change

The Attempt at a Solution

because the mass of wall is large, so the velocity of wall doesn't change after the hit, so does the velocity of ball (based on the conservation of momentum), so the ball still moves at 60mph? I feel i am wrong but i don't know how to prove myself wrong, thanks

 

[dacruick]

I would use an energy approach. Is it an elastic collision? because then energy would be conserved.

 

[gneill]

Homework Statement

A very heavy wall moving at 60mph, a ball moving same direction at 120 mph.
What is direction and speed of ball after ball hit wall.

Suppose you were sitting on the wall, moving with it, watching the collision. How fast would the ball be coming towards you? After it bounced, how fast and in what direction would it be moving with respect to you? How about in the original "ground" frame of reference?

 

[dacruick]

Suppose you were sitting on the wall, moving with it, watching the collision. How fast would the ball be coming towards you? After it bounced, how fast and in what direction would it be moving with respect to you? How about in the original "ground" frame of reference?

that is not the only problem. or the primary problem.

tennishaha doesn't have enough information.

 

[gneill]

that is not the only problem. or the primary problem.

tennishaha doesn't have enough information.

How so? What's missing?

 

[tennishaha]

I think it should be a reflection kind of problem. Before hitting, the velocity is u, and after hitting the velocity should be -u(same magnitude, but opposite direction), but I am not sure the u here is relative to ground or relative to the wall. Any ideas?

 

[gneill]

I think it should be a reflection kind of problem. Before hitting, the velocity is u, and after hitting the velocity should be -u(same magnitude, but opposite direction), but I am not sure the u here is relative to ground or relative to the wall. Any ideas?

Suppose you were sitting on the wall, moving with it... How would the situation look to you, sitting on the wall. Forget the ground for now. Just you, the wall, and the ball.

 

[dacruick]

How so? What's missing?

The type of collision is missing. The fact that the wall is very heavy just means that we don't have information. It means that depending on the collision, the wall could just absorb all of the momentum and not change its velocity, or it means that the wall could "reflect" the energy as tennishaha said.

 

[tennishaha]

I think the ball should be still relative to the ground after hitting the wall (velocity=0). Am I correct?

 

[gneill]

The type of collision is missing. The fact that the wall is very heavy just means that we don't have information. It means that depending on the collision, the wall could just absorb all of the momentum and not change its velocity, or it means that the wall could "reflect" the energy as tennishaha said.

Granted, the phrasing is imprecise, but I think that "the usual" conditions should be inferred. Very heavy ==> essentially infinite mass compared to other components, and a ball usually bounces perfectly unless otherwise specified.

 

[gneill]

I think the ball should be still relative to the ground after hitting the wall (velocity=0). Am I correct?

Sounds good. Now all you have to do is write out the argument and the smattering of math to accompany it.

 

[dacruick]

tennishaha seems to want to use momentum to solve this question. But the "usual conditions" that you've implied make this question only solvable with energy.