A weight lifter grabs a 100 kg dumbbell that\'s sitting on the floor and lifts i

Question

A weight lifter grabs a 100 kg dumbbell that's sitting on the floor and lifts it 2.0 meters to a point over her head. Then she holds the dumbbell over her head, motionless, for several seconds. a) How much work did she do on the dumbbell while lifting it? Show your reasoning. b) How much work did she do on the dumbbell while holding it over her head? Explain. c) The weightlifter says, "Don't even try to tell me I'm not doing work while holding the dumbbell over my head. I'm working hard - look at me sweat, and look how tired I get! " Reconcile your part b answer with the weightlifter's feeling that she's doing work on the dumbbell while holding it motionless. In other words, explain how it's the case that the weightlifter gets tired even though she's not doing work on the dumbbell.

Explanation / Answer

1)

A)

Work done by gravity while lifting = -mgh = -100*9.8*2 = -1960 J

Note : -ve sign for work is due to force of gravity acting opposite to the direction of motion

Now, Work done by lifter + work done by gravity = 0 <----- as final kinetic energy = 0

So, Wok done by lifter = - (-1960) = 1960 J <------answer

B)

While holding it overhead work done = 0 < ------- as displacement is 0

C)

She does 0 work on the dumbbell. But to stand still she does work on her muscles. As muscles consume energy to produce the force needed to do work

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