Acceleration is the rate of change or increase in the **velocity **of an object due to an increasing net force being exerted on that object. Velocity is the speed of an object in a particular direction so the rate of change in the velocity, or acceleration, refers either to a change in the speed or a change in the direction of the moving object. Therefore, if a car travelling in an easterly direction speeds up it is said to be accelerating. Likewise, if the car maintains its speed but goes round a corner it is also said to be accelerating. The opposite of this, an object slowing down, is called deceleration. Manufacturers often describe a car as being able to increase its speed from 0 to 50m/s in 10 seconds. The acceleration of the car can be calculated as:

Acceleration = __change of speed__ = change in speed per unit time

time taken

The car’s acceleration therefore = __(50-0)m/s__ = __5m/s__

10s s

Since acceleration is measured in metres per second per second, the speed of the car increases by 5 metres per second every second (5m/s^{2}).

Newton’s second law of motion is concerned with acceleration and states that when a force acts upon an object the object will start to move, speed up, slow down or change direction. The greater the force, the greater the change of movement.