Answer to Question #118917 in Classical Mechanics for Lizwi

Question

A ball moving along the x axis bounces elastically off a horizontal wall that runs in along y-axis.
The magnitude of the momentum of the ball is the same before and after the collision.
Explain how we can conclude that the ball-wall system is closed but not isolated.

Answer 1

The wall runs in along y-axis, therefore it cannot change the x-component of the ball's momentum.

The x-component of the ball's momentum after the collision with with the wall is the same as before the collision:

$$P_x'=P_x$$

The ball-wall system can exchange energy (as heat or work) but not matter, with its surroundings. Thus, he ball-wall system is closed but not isolated.

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