Intelligent assessment and revision tools for OCR A Physics

On a dry test track the same car makes two emergency stops using the same brakes so the braking force is effectively constant. From 15 m/s it comes to rest in 20 m (braking distance only). From 30 m/s, what is the braking distance, and why?

• A 20 m, because friction sets the distance and initial speed doesn’t matter.
• B 40 m, because doubling the speed doubles the distance when the force is the same.
• C 60 m, because the time to stop doubles and the average speed during braking is roughly the initial speed.
• D 80 m, because the energy to remove increases with the square of speed while the braking force is the same.