Guns Firing Under Water - a Lesson in Energy Efficiency

Check out high-speed cameras catching two guns firing under water - a conventional revolver, and a semi-automatic pistol. Watch the waste of energy in the revolver, shown by a burst of gases coming out in the rear of the gun. The semiautomatic has much less energy loss in the gases that escape prior to the barrel. Also check out the complicated turbulent patterns in the water.

Spring Problems and the use of Energy

Springs provide a nice example of NON-constant force. We know Hooke's law, F = -kx, where x is the displacement of the spring from its equilibrium position. Keep in mind the - sign is for direction: the force always tries to bring a spring back to equilibrium, which is in the opposite direction relative to the direction of the displacement (a point of stable equilibrium). Here are examples of using energy conservation to solve spring problems.