Work done by weight lifter

  • #1

Main Question or Discussion Point

Hi -

Question: A person trying to lose weight (dieter) lifts a 10 kg mass, one thousand times, to a
height of 0.5 m each time. Assume that the potential energy lost each time she
lowers the mass is dissipated. (a) How much work does she do against the gravitational
force ? (b) Fat supplies 3.8 × 107J of energy per kilogram which is converted to
mechanical energy with a 20% efficiency rate. How much fat will the dieter use up?

What is the significance of the following line in this question?
"Assume that the potential energy lost each time she lowers the mass is dissipated."

Thanks
Puneet
 

Answers and Replies

  • #2
Doc Al
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What is the significance of the following line in this question?
"Assume that the potential energy lost each time she lowers the mass is dissipated."
It means to ignore the work done by gravity as the weight is lowered. Just count the positive work she does as she lifts the weight.
 
  • #3
It means to ignore the work done by gravity as the weight is lowered. Just count the positive work she does as she lifts the weight.
Is this the right way of looking at it: since the loss of PE is dissipated, she does not have to do any negative work against gravity on the way down?
 
  • #4
Doc Al
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Is this the right way of looking at it: since the loss of PE is dissipated, she does not have to do any negative work against gravity on the way down?
Yes, just ignore the negative work done when the weight is lowered.
 
  • #5
russ_watters
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Is this the right way of looking at it: since the loss of PE is dissipated, she does not have to do any negative work against gravity on the way down?
I agree with @Doc Al that that's intended interpretation of the question, but a couple of other ways to look at it since you seem to be trying to connect this to reality:

1. If a machine like an elevator were doing this work, it might be equipped with a way to recover that energy by charging a battery as it lowers the weight.
2. Humans are very inefficient machines and use energy to generate a force, even when moving backwards. So in reality you actually are still expending energy even when lowering the weight.
 

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