Rotation Introduction Introduction

Sometime towards the end of the Stone Age—after mammoth hunts were the pastime du jour but before the Flintstones—mankind stumbled upon the best invention since sliced bread and "30 Rock" reruns: the wheel. Along with enabling agriculture and transportation and construction and pretty much every single thing that makes the modern world the modern world, the wheel opened up a whole new branch of physics in the form of rotational motion.

In the rotational world, our previous study of linear motion is extended to angular analogs: position becomes angle, velocity becomes angular velocity, and acceleration becomes angular acceleration. Much of the physics remains the same, but new ideas like moment of inertia (how objects resist rotation) or torque (what forces do to objects when they try to spin, instead of push, them) let us fully describe rotational motion, the way mass and forces let us describe linear motion.

As with any good chunk of physics, rotation also introduces a new conservation law: the law of conservation of angular momentum. Like the linear law of conservation of momentum, this conservation law lets us solve rotational problems by declaring that the total amount of angular momentum in a system must remain constant.

We can put all of these ideas together to examine rolling objects—wheels, tubes, stones, whatever. Rolling objects are moving and spinning, and so have linear kinetic energy as well as rotational kinetic energy.

And if that's all starting to make your head spin, well, good. Get ready to spin some more.