The applet shows the motion of a car which is being driven along a straight road. You can control the acceleration of the car using the scrollbar. The speed increases or decreases (up to a maximum safe speed for the car) depending on the acceleration you give it.

In the centre position, the acceleration is zero. Above the centre position the acceleration is directed in front of the car (positive direction). If the scrollbar is below the centre position then the acceleration is directed backwards (negative direction).

Once the car starts to move then the velocity and acceleration are shown as vectors in the side view of the car at the top left. Note that both these quantities are vectors, and so direction is important. The speedometer will also show you the speed of the car, which is a scalar, and so does not depend on direction. Note the conditions under which the speed increases, and those when the speed decreases.