Question

Two metal balls with the same mass hang so that they are touching each other. The ball on the left is pulled to the side and released. What will happen when it swings back and hits the ball on the right?
A. The ball on the left will stop, and the ball on the right will remain still.
B. The ball on the right will swing to the right
C. Both balls will swing to the right.
D. Both balls will swing to the left.

Answer 1

B. The right ball will swing to the right.

Reason: For two equal masses in an (approximately) elastic collision, momentum and kinetic energy conservation give
v1' = (m1 − m2)/(m1 + m2) v1 = 0,
v2' = 2m1/(m1 + m2) v1 = v1.
So the left ball stops and the right ball leaves with the incoming velocity (Newton's cradle behavior).