Question

A car moving at 40 km/hr is to be stopped by applying brakes in the next 4 m. If the car weighs 2000 kg, what average force must be applied to stop it?

Answer 1

Given:
Initial speed of the car, u = 40 km/hr $=\frac{4000}{3600}=11.11\mathrm{m}/\mathrm{s}$
Final speed of the car, v = 0
Mass of the car, m = 2000 kg
Distance to be travelled by the car before coming to rest, s = 4m
Acceleration, $a=\frac{v^2-u^2}{2\mathrm{s}}$
$\Rightarrow a=\frac{0^2-{\left(11.11\right)}^2}{2\times 4}=\frac{-123.43}{8}=-15.42\mathrm{m}/{\mathrm{s}}^2$
∴ Average force to be applied to stop the car, F = ma
⇒ F = 2000 × 15.42 ≈ 3.1 × 10⁴ N