drag force question

[nkafeel]

Drag is a force that acts on any object moving through a fluid. The drag force is always directed opposite the velocity of the object. Suppose a ball is thrown straight up into the air and a few seconds later it is caught. From the moment the ball leaves the thrower's hand to the instant before it is caught, what can be said definitively about the work done by drag?

(A)
Wdrag > 0

(B)
Wdrag = 0

(C)
Wdrag < 0

Correct Answer

(D)
The work done by drag over the flight is equal to the negative of the work done by gravity.

Why is work negative and not zero since displacement is zero?

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[sazerac]

Drag (really, friction against air) is not a conservative force. You can't just examine the start and end points and draw a conclusion because "displacement is zero".

On the way up, the force of drag works against (i.e. negative) the upward displacement. On the way down, the force of drag works against (i.e. negative) the now downward displacement.