Slow In and Slow Out

In reallife, a moving object doesn't instantly move from stationary or stop instantly after travelling at speed. For a stationary object to move, a force is required to overcome the force of friction and gravity that is holding it stationary, and that takes a bit of time. To stop a moving object, a force must act against it opposite the direction its going, like stationary object object or friction, and that also takes a bit of time.

For example, a car doesn't go 0-60 in a instant, it takes a bit of time to build up momentum to reach high speed. This is easily seen in the following video of a drag race.

http://www.youtube.com/watch?v=Xdxo8zmVak0

Here's a simple Flash animation showing this principle.

In the screen shot below, you'll see that in the inital frames of the movie, the sucessive images of the car are very close together, simulating the inital build up of momentum. In the later frames, the sucessive images of the car are far apart, simulating the burst of speed.

The principle is also applied to objects slowing down to a stop. Here's a Flash animation of a car stopping naturally.

As seen in the screenshot below, the sucessive images of the car get closer together near the end of the moive, creating the illusion of slowing down.

Sometimes, this principle is ignored for comedic effect. The most famous example of this is when Road Runner stops instantly.