Example A car accelerates, its velocity changing from 20 m s-1
calculated as follows: Acceleration =
Change in velocity Time taken
=
10 m s-1 5 s
to 30 m s-1 = 2 m s-2
in 5 seconds. The acceleration is (m/s2 or metres per second squared)
The change in velocity is calculated by subtracting the initial velocity from the final velocity: Final velocity – Initial velocity = 30 m s-1 to 0 m s-1
– 20 m s-1
So, if a cyclist changes velocity from 6 m s-1 as follows:
Deceleration =
Change in velocity Time taken
= = 10 m s-1 in 2 seconds the deceleration is calculated
6 m s-1 2 s
takes 12 seconds then the deceleration will be as follows: = 1
6 m s-1 12 s
/2 m s-2 A Formula 1 racing car can accelerate at 15 m s-2 .
This means that after only 3 seconds after the start of a race the velocity would be 45 m s-1
. Formula
1 racing cars can decelerate even faster, at up to 50 m s-2
a force that is equal to five times their own body weight.
Fig. 8 A Formula 1 racing car.
Speed versus Time graphs Downhill skiers can reach speeds of up to 40 m s-1
. Skiers can carry GPS
tracking devices to monitor their speed. The table below shows the speed of a skier recorded at regular time intervals during a downhill journey.
Time (s) Speed (m s-1 )
0 0
3 6
6 12 9 18
12 24
15 30
18 36
This information can be used to draw a graph of speed against time, putting time on the horizontal (x) axis.
310
Fig. 9 A skier accelerates going downhill.
. At this rate of deceleration the drivers feel = 3 m s-2
If the change in velocity takes longer, the deceleration will be slower. If the change from 6 m s-1 to 0 m s-1
