Jumping and falling

- make it jump, fall, and collide with the platforms
-add a Rigidbody component to Runner
-create a physic material with no friction
-use two of Unity's collision event methods – OnCollisionEnter and
OnCollisionExit – to detect when we touch or leave a platform.
-assign a physic material to our platforms
-acceleration and regular platform
-detect the player's input
- Jump input configuration
-Runner jumping