For an object moving in a straight line where there is no change of direction the acceleration is defined as the rate of change of velocity with time.
It is given by the following equation.
Two cars could be travelling with the same speed of 100 km/h on a motorway. However, by stating one car is travelling 100 km/h north and the other 100 km/h south do we realise they are travelling in opposite directions, thus the velocity gives a better indication of the motion.
The average speed of an object can be determined from the equation below:
In the case where an object is slowing down (decreasing velocity) the acceleration is in the opposite direction to the moving object. This is referred to as negative acceleration or retardation or deceleration.