SAT Subject Physics Practice Question 645: Answer and Explanation
11. Two high school students attempt to push a car uphill. The car rolls downhill against them for a distance of 10 m before they can bring it to a stop. If both students pushed on the car with a force of 1000N while it rolled downhill, how much work did they do?
B. 1000 J
C. -1000 J
D. 10,000 J
E. -10,000 J
Correct Answer: E
The correct answer is (E). Work can be either a positive quantity or a negative quantity. When a force is applied in one direction and the displacement is in the opposite direction, the work equation becomes F·s·csine 180° The cosine of 180° is -1, therefore any work done under those conditions is negative work. W = -F·S = -(1000N)(10 m) = -10,000 J