To show the idea of mass or how much energy an objects has in your animations, consider using the squash (squish) and stretch technique. Squash and stretch is the deformation of an object. The amount of deformation depends on what the material it is made from, which ranges from rigid to soft materials. Rigid materials don't deform much whereas softer materials like rubber would deform considerably.

The Bouncing Ball

Consider the squash and stretch of a bouncing ball. The bouncing rubber ball would start to deform as it is released from the hand into the air. As it travels up it starts slowing down due to the resistance of the air and gravity. At the highest point of the arc, the point of equilibrium, it is at the its slowest speed and it is where it will resume its natural shape. As the ball falls it will start to deform again as the speed increases until it hits the the ground, where it will deform or get squash the most. As it begins to move up again it will return to its natural state. When doing squash and stretch, remember to maintain the correct volume, otherwise the believability would be gone, since it would not look like the same object.

The Squash and Stretch of an Elastic Ball