Lock X of mass M slides across a horizontal surface where friction is negligible. Block X collides with block Y of mass 2M that is initially at rest, as shown in Figure 1. After the collision, both blocks slide together with a speed vs , as shown in Figure 2. What is the kinetic energy of the two-block system before the collision?

answer would be letter choice (a),   a ball resting on a shelf. would be another example of potential energy   >   stored energy > not moving.

a ball spinning on a finger   >   kinetic energy > moving energy

a ball rolling on the ground   >   kinetic energy   > moving energy

a   ball flying through the air   > kinetic energy   >   moving energy

hope that                                         : )

not too sure thinking all accelerations are pointed down