A projectile is fired upward with an initial speed v0 on an air filled world. A short time later, it comes back down and has a final speed v < v0 (air resistance is not negligible). What was the sign of the work done by friction during the flight?

(A) W fric > 0
(B) Wfric=0
(C) Wfric < 0

(A) W fric > 0

Explanation:

According to the law of energy conservation, mechanical energy stays always constant. So if the ball come back to the same position (same potential energy) but with reduced speed (kinetic energy), then the kinetic energy has been affected by friction work created by air resistance. This work is non 0 and larger than 0. So the total of this friction work and kinetic energy after the fall = kinetic energy before the fall.

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