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May 1, 2016

Homework Help: Physics (thx drwls, bobpursley)

Posted by Matt on Tuesday, March 13, 2007 at 10:34am.

After reaching terminal velocity, a raindrop of m=3.06E-4kg falls vertically down a distance of 500m.

a)what is the work done by the force of gravity on the raindrop?

sol) since the displacement is in the same direction as the force, the work done is Fcos90 = 0. (correct?)

b) what is the work done by the force of air resistance on the raindrop?

sol) again, 0. I have big doubts on both answers because it moves in same direction. please help.

Another interesting question: in terms of unit vectors i or j. I know that the normal force of a particular object is mgcostheta (i hat) - fsintheta (j hat). My question is, since the normal force is always vertical, should I only report -fsintheta (j hat) as the only normal force acting on a skier in terms of unit vectors?

work done by gravity on the raindrop is mgh.

work done by air resistance=Force*distance, but the force is equal to mg, in the upward direction, so work done by friction is
-mgh

Notice the net work (gravity +friction) is zero, that is why the KE does not increase.

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