The concept of a force lifting a weight and the work done

  • #26
Mister T
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If F is the force and D is the relative distance the surfaces move against one another, the sum of the two is given by -FD.
You can't answer the questions I asked in Post #24?
 
  • #27
jbriggs444
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You can't answer the questions I asked in Post #24?
I did. You quoted the answer. This is not complicated. You multiply force by distance to get the work done by a force. If you want the work done by two forces you compute the work for each separately and then add.
 
  • #28
Mister T
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It means that the net of the work done by block on table and of table on block comes to -10J.
Okay. And the work done by the hand on the block is +10 J. This means that the internal energy of the block-table system increases by 10 J. According to the 1st Law of Thermodynamics, this makes sense. 10 J of energy is put into the system by the hand, and the internal energy of the system increases by 10 J. That explains where the energy went.

I see no room in this scheme for the -10 J of work done by friction.

I can provide references if you want to delve further. This is a well-known issue.
 
  • #29
jbriggs444
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I see no room in this scheme for the -10 J of work done by friction.
10 J of mechanical energy was lost. There is your -10 J of work.
 
  • #30
A.T.
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...friction in the real world is always a positive force ...
The sign depends on the coordinate system convention and has no significance.
 
  • #31
A.T.
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Note that if one insists that the friction force does work,
You just apply the definition of work to both bodies (A, B) and the two equal but opposite friction forces acting on them.

it is not possible to explain where the energy is going.
For kinetic friction the work done by A on B is of different magnitude than the work done by B on A. That difference represents the energy dissipated as heat.
 
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