Problem: The fool fox is out chasing the roadrunner again! As usual, the roadrunner makes one of his physically impossible right angle turns immediately before a cliff, and the fox can't stop! Suppose the fox is moving with a horizontal speed of 12 m/s as he goes off the 30 m high cliff. What is his speed when hits the bottom?

Solution: The fox becomes a projectile. If you were to do this by the methods of module 4, you'd have to find separate the x and y motions, find the final y-velocity, and use the Pythagorean Law to get the final speed. Energy is easier!

The fox has an initial gravitational potential energy (taking the base level for potential energy at the bottom of the cliff). He also has an initial kinetic energy. At the moment he hits the ground -poor fox - he has only kinetic energy. Using the energy balance method:

This method is useful for finding speed, but not forces or accelerations.