#### Question

A car is travelling at 20 m/s along a road. A child runs out into the road 50 m ahead and the car driver steps on the brake pedal. What must the car’s deceleration be if the car is to stop just before it reaches the child ?

#### Solution

We have to find the deceleration. We have the following information given,

Initial velocity, (u) = 20 m/s

Final velocity, (v) = 0 m/s

Distance travelled, (s) = 50 m

Let the deceleration for the entire journey be (a)

We can calculate acceleration by using the 3^{rd} equation of motion,

`a = (v^2-u^2)/(2s)`

(s) - Displacement

(u) - Initial velocity

(a) - Acceleration

(v) - Final velocity

Put the values in above equation to find the deceleration,

`a = [(0-400)/(2(50)]] "m/s"^2`

= `(-400/100)` `"m/s"^2`

= -4 `"m/s"^2`

Hence, deceleration is 4 m/s^{2}.