I have a conservation of momentum question. When a billiard ball hits the edge of the table, is momentum conserved? Why or why not? I have found 2 different answers to this and wonder which is true?

Question

vincent-lyon.fr · Answer

Momentum is always conserved in a collision.
In your case, you do not realise it because you do not see a change in the table's velocity.
This is because the billiard table is so massive with respect to the ball that its velocity is unnoticeable.

anonymous · Answer

The momentum changes from mv to -mv? The momentum given up by the ball is absorbed by the heavier billiard table? I can't see that any momentum is "given up". It just has a change of velocity direction? This is for an elastic collision.

anonymous · Answer

Where does the impulse come into it? That is $$\Delta p=F \Delta t$$? This is the force exerted according to Newton's Third Law? The ball strikes the table and the table strikes the ball with equal and opposite force?

anonymous · Answer

Momentum doesn't just change of its own accord, it has to be changed by a force.  The table exerts a force on the ball to change its momentum.  By Newton's 3rd Law, that same force is experienced by the table.  Therefore, the momentum of the ball goes from p to -p, and the momentum of the table goes from 0 to 2p.  One might say the momentum was transferred from the ball to the table. 

Vincent's point was that the final velocity of the table would be so small that it would not be noticeable.  This would be true if the table is on a frictionless surface.

If the table is NOT on a frictionless surface, then the friction between the table and the floor would keep the table from going anywhere.  In this case, momentum is not strictly conserved.

In general, the momentum of a system is conserved if the only forces acting on the system are between different parts of it.  For instance, if the ball and table were on ice and there was no friction, the only forces acting would be between the ball and the table when they collide.  In that case, the momentum of the ball and table would be conserved.

If there is friction between the table and the floor, then the momentum of the table and ball are NOT conserved because there is an added external force from the floor.

anonymous · Answer

imagine yourself standing in a container in space which is not accelerating and is not too massive with respect to you. if you jump on the floor of the container, the container would move in the reverse direction, away from your feet. 
Yes, the impulse during a collision is the action-reaction force. F*Delta T produces the change in momentum in both the objects in the collision pair.  If the other body is quite massive, then the change goes un-noticeable.

anonymous · Answer

Thank you to everyone. One more question. Why is the momentum of the table 2p? I understand the change in the momentum of the ball is from p to -p.

anonymous · Answer

initially, the total momentum of the ball table isolated system,  was the ball's momentum = p.  after collision, momentum of ball changes to -p. so the table's momentum would be 2p so that the total momentum of the ball-table system is still conserved.

anonymous · Answer

So.....p to 0 is a magnitude of p and then 0 to -p has another magnitude of p. Which makes 2p?

anonymous · Answer

yes