In the previous section, the concepts of Speed and Velocity were explained. However, only constant Speed and Velocity was considered. In this section changing Speed will be considered. 

When an object changes speed it is Accelerating. The sign of the acceleration depends on the change in speed:-

1. Object gets faster - positive Acceleration

2. Object gets slower - negative Acceleration 

The Acceleration of an object doesn't just depend on the change in Speed, it also depends on the time it takes to make the change. A short time period for a change in Speed will give a large Acceleration. 

Definition of Acceleration

Acceleration is the change in Speed  of an object over the time of  1 second. The Acceleration can be found using the following formula:-


V  =  Final Speed (ms-1)

u  =  Initial Speed (ms-1)

t  =  Time (s) 

a  =  Acceleration (ms-2)

Measuring Acceleration Experimentally 

The diagram below shows the experimental set-up for the measurement of Acceleration:- 

By measuring the length of the single mask (in meters) and the time taken to break the light beam (in seconds), the instantaneous Speed can be found at the two light bridges. By timing how long the trolley takes to travel between the bridges, the Acceleration can then be found using the above formula. 

Example 1 -

A car is travelling at 15ms-1 and then accelerates for 30 s to reach an new speed of 30ms-1. What is the acceleration of the car?

a  =  ?

v  =  30 ms-1

u  =  15 ms-1

t  =  30 s  

a  =  (30 - 15 )/30 

a  =  0.5 ms-2

Example 2 -

A car is travelling at 13 ms-1 when a child runs into the road. If the driver takes 1.3 seconds to stop, what is the Acceleration experienced by the driver? 

a  =  ?

v  =  0 ms-1

u =  13 ms-1 

t  =  1.3 s

a  =  ( 0 - 13 ) / 1.3

a  =  -10 ms-2