Pendulums(2)

　　Velocity　　Calculating the velocity of the pendulum bob at the equilibrium position requires that we arrange our coordinate system so that the height of the bob at the equilibrium position is zero. Then the total mechanical energy is equal to the kinetic energy at the equilibrium point where U = 0. The total mechanical energy is also equal to the total potential energy at　where KE = 0. Putting these equalities together, we get

　　But what is h?

　　From the figure, we see that . If we plug that value into the equation above, we can solve for v:　　Don’t let a big equation frighten you. Just register what it conveys: the longer the string and the 美国GREater the angle, the faster the pendulum bob will move.　　How This Knowledge Will Be Tested　　Again, don’t worry too much about memorizing equations: most of the questions on pendulum motion will be qualitative. There may be a question asking you at what point the tension in the rope is 美国GREatest (at the equilibrium position) or where the bob’s potential energy is maximized (at ). It’s highly unlikely that you’ll be asked to give a specific number.

（责任编辑：申月月）