A ball is thrown straight upward. How does the sign of the work done by gravity while the ball is traveling upward compare with the sign of the work done by gravity while the ball is traveling downward? A. Work done by gravity is negative while the ball is traveling upward and positive while the ball is traveling downward. B. Work done by gravity is positive while the ball is traveling both upward and downward. C. Work done by gravity is positive while the ball is traveling upward and negative while the ball is traveling downward. D. Work done by gravity is negative while the ball is traveling both upward and downward.

A.

Explanation:

According to the work energy theorem, the work done by external forces on a mass, must equal to the change in kinetic energy. When thrown upward, the change in kinetic energy is negative, due to it must have some initial velocity, which it will diminishing while going upward, as gravity is slowing down it. So, in this part of the trajectory, work done by gravity (only force acting on the ball if we neglect air resistance) must be negative. In the downward part of the trajectory, as the change in kinetic energy is positive (the ball is speeding up when falling), the work done by gravity must be positive too. So, the option A is the one that is true.