An important property of force is that it can change the state of motion of a body. Hitting a tennis ball with a racquet changes its speed and direction of motion. A race car can be accelerated to a high speed in a short period because its powerful engine can deliver a very large force. Wherever the state of motion of a body is changed by a net force, acceleration is produced.

Fig. 4-1 A race car has a greater acceleration than an ordinary car. A person in the race car would experience a greater force during acceleration.

Taking a ride in a car may also give you a feeling of the relation between force and acceleration. When the car just departs, you will experience a pushing force from the seat. The force causes a forward acceleration on your body. If you are riding a race car, you will experience a greater force and hence a greater acceleration (Fig. 4-1). In fact, the acceleration on a body is proportional to net force acting on it. Thus we may write

(for constant mass )