The driver reaches another junction, where he must stop. He applies the brakes. This provides another force to slow down the car . The car is moving forwards, but the force needed to make it decelerate is directed backwards. If the driver wants to stop in a hurry, a bigger force is needed. He must press hard on the brake pedal, and the car’s deceleration will be greater.

Finally, the driver wants to turn a corner. He turns the steering wheel. This produces a sideways force on the car (Figure 3.6c), so that the car changes direction.

To summaries, we have seen several things about forces:

• They can be represented by arrows. A force has a direction, shown by the direction of the arrow.

• A force can make an object change speed. A forward force makes it speed up (accelerate), while a backward force makes it slow down (decelerate).

• A force can change the direction in which an object is moving.

• A body will remain at rest or will move at a constant speed in a straight line unless acted upon by a resultant force. There are alternative ways of saying the same thing.

• For example, a resultant force will change the speed or direction of a body. However, the problem with this statement is that some people forget to include starting and stopping as changes in speed.

• Another alternative is to say that a resultant force will change the velocity of a body, but as velocity is a vector, a resultant force can change the direction as well as the speed of a body. A resultant force can change both speed and direction at the same time.