A car of mass 1000 kg traveling at 20 m/s brakes to a complete stop over a distance of 40 metres. What average braking force was exerted on the car?

Answer and explanation

Correct answer: C. 5000 N

  1. Step 1: Initial kinetic energy E_k = 0.5 m v^2 = 0.5 1000 20^2 = 0.5 1000 400 = 200,000 J. Step 2. By work-energy theorem, work done by brakes equals kinetic energy lost: W = F d => F 40 = 200,000 => F = 5000 N. Answer: 5000 N. Why not the others: Alternatively: v^2 = u^2 + 2ad => 0 = 400 + 2a(40) => a = -5 m/s^2; F = ma = 1000 * 5 = 5000 N.

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