AP Physics 1 Question 170: Answer and Explanation
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10. A car of mass m initially travels at speed v. The car brakes to a stop on a road that slants downhill, such that the car’s center of mass ends up a vertical height h below its position at the start of braking. Which of the following is a correct expression for the increase in the internal energy of the road-car system during the braking process?
- A. mv – mgh
- B. mv2
- C. 0
- D. mv2 + mgh
Correct Answer: D
D-"Increase in the internal energy of the road-car system" is a fancy way of saying "work done by the car's brakes to stop the car." Without the brakes, the car would have gained mgh of mechanical energy in dropping the height h, giving it a total mechanical energy of ½mv2 + mgh. The brakes convert all of that mechanical energy to internal energy.