6. Slow In and Slow Out

The movement of the human body, and most other objects, needs time to accelerate and slow down. For this reason, an animation looks more realistic if it has more frames near the beginning and end of a movement, and fewer in the middle.This principle goes for characters moving between two extreme poses, such as sitting down and standing up, but also for inanimate, moving objects and bouncing ball.

Bouncing ball and pendulum might be the best eg for explaining this principle.When the ball is going up its got more energy so its going up with speed but soon it starts losing its energy and it starts to slow down because earth's gravity is pulling it down finally it loses all its energy starts to fall slowly then starts to increase its speed downwards till it hit the ground again.