The physics that caused the swaying of the Millennium Bridge has to do with oscillations, or back-and-forth motion. 1:04 More specifically, it has to do with simple harmonic motion: 1:07 where oscillations follow a particular, consistent pattern. 1:10 But before we had the Millennium Bridge as a real-life example, 1:13 physicists often described simple harmonic motion in terms of a ball attached to a horizontal spring, lying on a table. 1:20 While it's lying there, at rest, it's in equilibrium. 1:23 And when you move the ball so that it stretches the spring, then let go, 1:26 the ball keeps moving back and forth forever... in a frictionless world. Energy and Velocity 1:30 That back-and-forth motion caused by the force of the spring, is simple harmonic motion. 1:34 Now, we want to know two things about this oscillating ball: 1:37 What kinds of energy does it have? 1:39 And, what's its maximum velocity? 1:41 To better understand what's happening to the ball, let's start with its energy. 1:44 As the ball compresses and stretches the spring, both 'kinetic energy' and 'potential energy' come into play. 1:49 Kinetic energy is the energy of motion, and as the ball moves, there are two points -- 1:54 the turning points -- where it's NOT moving: 1:56 One point is where the spring is compressed all the way, and the other is where it's stretched all the way. 2:00 And the distance between either of these two points, and the equilibrium point, is called the 'amplitude'. 2:05 At those two