Question

Two ***** are on a friction less horizontal tabletop. Ball X initially moves at 10 meters per second, as shown in Figure I. It then collides elastically with identical ball Y which is initially at rest. After the collision, ball X moves at 6 meters per second along a path at 53° to its original direction, as shown in Figure II above. Which of the following diagrams best represents the motion of ball Y after the collision?

Answer 1

Correct option

Since there is no y momentum before, there cannot be any net y momentum after. The ***** have equal masses so you need the y velocities of each ball to be equal after to cancel out the momenta. By inspection, looking at the given velocities and angles and without doing any math, the only one that could possibly make equal y velocities is choice D.