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

Mister T
Gold Member
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.

jbriggs444
Homework Helper
2019 Award
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.

Mister T
Gold Member
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.

jbriggs444
Homework Helper
2019 Award
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.

A.T.
...friction in the real world is always a positive force ...
The sign depends on the coordinate system convention and has no significance.

A.T.