Everything Science Grade 10

21.7 CHAPTER 21. MOTION IN ONE DIMENSION

driver sees childdriver hits brakes

constant v negative acceleration

child is here

Abb Bb Cb

50 m

0,5 s

We need to know the following:

• What distance the driver covers before hitting the brakes.


• How long it takes the truck to stop after hitting the brakes.


• What total distance the truck covers to stop.

Step 2:Calculate the distance AB

Before the driver hits the brakes, the truck is travelling at constant ve-

locity. There is no acceleration and therefore the equations of motion

are not used. To find the distance travelled, we use:

v = dt

10 = 0 d, 5

d = 5m

The truck covers 5 m before the driver hits the brakes.

Step 3:Calculate the time BC

We have the following for the motion between B and C:

~vi = 10m·s−^1

~vf = 0m·s−^1

a = − 1 , 25 m·s−^2

t =?

We can use Equation 21.1

~vf = ~vi+at

0 = 10m·s−^1 + (− 1 , 25 m·s−^2 )t

− 10 m·s−^1 = (− 1 , 25 m·s−^2 )t

t = 8s

Step 4:Calculate the distance BC

For the distance we can use Equation 21.2 or Equation 21.3. We will

434 Physics: Mechanics