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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 ?
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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 3rd 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/s2.
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