Bouncy balls

The feature we'll implement here is, when a ball drops from mid-air and hits the ground, it bounces back up and down, and up again, diminished over time.

We are going to start simply with a new scene that consists of a ground plane and a sphere. Then, we'll add physics to it, a bit at a time, as follows:

1. Create a new scene by navigating to File | New Scene.
2. Then, navigate to File | Save Scene As... and name it BallsFromHeaven.
3. Create a new plane by navigating to GameObject | 3D Object | Plane and reset its transform using the Transform component's gear icon | Reset.
4. Create a new sphere by navigating to GameObject | 3D Object | Sphere and rename it BouncyBall.
5. Set its Scale to (0.5, 0.5, 0.5) and Position to (0, 5,0) so that it's above the center of the plane.
6. Drag the Red material from Project Assets (created in Chapter 2, Content, Objects and Scale) onto it so that it looks like a bouncy ball.

The new Unity scene defaults come with Directional Light and Main Camera. It's OK to use...