Answer to Question #232626 in Classical Mechanics for nelly

Question

A ball moving along the x -axis bounces elastically off a horizonal 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

In the "wall - ball" system of bodies, the sum of impulses of all bodies does not change with time. According to the law of conservation of momentum, it follows that the vector sum of external forces is equal to zero. A closed system is a system of bodies for which the resultant of all external forces is equal to zero. According to this definition, the system of bodies "wall - ball" is closed.

An isolated system of bodies is called that other bodies do not act on. Assuming that the Earth's gravitational field acts on the "wall - ball" system of bodies, we get that this system is not isolated.