A man lets go of a bowling ball. A short while later, he is rushed to the hospital.
A physics professor is demonstrating conservation of energy by suspending a bowling ball from a piece of rope. He pulls the ball back until it's right in front of his nose, then lets go. It is supposed to swing away from him, then back at him, stopping just in front of his nose. Unfortunately, he gave the ball a slight push, resulting in the ball crashing into his nose upon its return.