1. The problem statement, all variables and given/known data A 2100 kg pile driver is used to drive a steel I-beam into the ground. The pile driver falls 5 m before coming into contact with the top of the beam. Then it driver the beam 12 cm farther into the ground as it comes to rest. Using energy considerations, calculate the average force the beam exerts on the pile driver while the pile driver is brought to rest. 2. Relevant equations 3. The attempt at a solution http://img238.imageshack.us/img238/7935/beamdi0.jpg [Broken] http://g.imageshack.us/img238/beamdi0.jpg/1/ [Broken] I don't understand why they use cos 0 and cos 180 here? Can someone please explain it to me?