I think I have a bit of a misunderstanding that I can't resolve... Let's say I exert a force on a block of wood that rests on a table with friction. The force I'm exerting has the same magnitude as the friction(but in opposite direction) so that the net force is zero, and the block of wood moves with constant velocity.

My question here is; the work done by the force I exert would be the same as the work done by friction(but one will be plus and the other will be minus) therefore work done by the force I exert and the work done by friction would cancel, resulting in the net work being zero.

But the thing is; by friction, the block of wood gets heat.

Then does this mean that even though the net work is zero, thermal energy is produced?? What am I not thinking correctly here?? Can anyone please help?

  • $\begingroup$ Work has no associated direction. For example, if I rub my hand in a circle across the wood, the work done is the friction force times the circumference of the circle. By you reasoning, work would be zero. $\endgroup$ – James Sep 22 '16 at 17:51

To begin, the block moves with costant velocity, not force, but you probably know that. To answer your question, yes you are not creating energy from nothing; the force you apply gives some energy that is converted into kinetic energy and heat, or if the block is already moving, just heat. You can see it this way: the block is moving, and friction is trying to suck its kinetic energy converting it into heat, so the net energy remains the same but is changing its form. When you push it, you are increasing the net energy of the system, because you are "replenishing" the kinetic energy while friction is diminishing it, converting it.

The net work is not zero if you don't take in account the body of the person pushing the block. It is if you consider it.

Hope that helps!

  • $\begingroup$ Your answer sure helped me a lot!! $\endgroup$ – Danny Han Sep 22 '16 at 15:58
  • $\begingroup$ But the last part that the net work does not equal to zero, is a part I can't fully understand $\endgroup$ – Danny Han Sep 22 '16 at 15:59
  • $\begingroup$ Could you please explain to me about that part?? $\endgroup$ – Danny Han Sep 22 '16 at 16:00
  • $\begingroup$ I can't reply to James' comment, he's not quite right. Sure the work on a circular path is not zero, but only because the force he is applying has the same direction of the movement, every istant. the friction force instead makes negative work beacuse its direction is always opposite to movement. Work therefore can have a sign. Wait a moment for your reply $\endgroup$ – Andrea Leo Sep 22 '16 at 18:28
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    $\begingroup$ If for example the friction heated only the table and not the block, you could say that if your system is only the block the net work is zero, given that its energy is not increasing nor decreasing, but if you consider also the table, there's a change in the amount of energy (thermal). That also turns out to be zero if you consider also the person doing the action, as the increased energy in the table comes from the chemical energy in its muscles! $\endgroup$ – Andrea Leo Sep 22 '16 at 20:01

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