A 1200 kg car travelling at 20 m/s brakes to a stop. The brakes provide a constant frictional force of 6000 N against the motion. How far does the car travel while stopping?

1. All the kinetic energy of the car is removed by the work done by friction. 2. Start with the kinetic energy, which is half the mass multiplied by the square of the speed. 3. That is 0.5 multiplied by 1200 kilograms multiplied by 20 metres per second squared. 4. Twenty squared is 400 metres squared per second squared, so the kinetic energy is 240 000 joules. 5. The work done by friction is the force multiplied by the distance, and it must remove all 240 000 joules. 6. So the stopping distance is 240 000 joules divided by 6000 newtons, which is 40 metres. 7. The 20 metre answer traps anyone who uses the speed rather than the square of the speed. 8. The 80 metre answer leaves out the factor of one half in the kinetic energy, and the 10 metre answer divides by the force twice. _Source: Siyavula Physical Sciences Grade 12 (Everything Science, CC BY 4.0), Chapter 5: Work, energy and power_