A confusion about the conception of work

  1. I've learned that a force is said to do work when it acts on a body and causes a displacement in the same direction of the force and that W = F.s ( when theta equals zero)

    But I have a problem understanding the concept. On Pushing a brick wall, carrying a bag heavy brick at a constant height, it is said that zero work is done, according to W = F.s Cos theta, these previous examples can be explained, but equations are not everything about physics, I have a hard time to believe that one can spend hours pushing a wall, he sweats, his face becomes red and he eventually falls unconscious, but after all of that it is said that the work done is Zero because s = zero.

    I'm not saying W = F.s isn't correct, I'm just saying that there's is something I'm missing causing this misunderstanding, So I would like some clarification, thanks in advance.
    Last edited: Jun 19, 2014
  2. jcsd
  3. Doc Al

    Staff: Mentor

    The mechanical work done on the stationary brick wall is zero. You have transferred no energy to the wall. Same for holding the heavy bag of bricks at a fixed height.

    Of course, internally your muscles are expending energy, tensing and relaxing to maintain that muscular tension. But none of that energy goes into the wall or the bag of bricks. You could get the same effect by leaning something against the wall or by placing the bag of bricks on a table. The fact that we have to expend energy just to go nowhere does not change the fact that no work is done on the wall or bag.
  4. You are missing efficiency. A human body is very inefficient at exerting static forces, to be precise, the efficiency is 0%. Energy is used, but no work is done and all of the energy used goes to waste heat.

    We can devise idealized machines that do the same thing (carrying bags, pushing walls) without using any energy, and we can make real machines that are pretty close to those ideal machines.
  5. Nugatory

    Staff: Mentor

    What DocAl said...

    The only thing I will add is that our bodies are very inefficient machines, and especially inefficient at maintaining constant forces. You are right that a human will rapidly become tired holding a heavy bag of bricks at a constant height... But a steel table can support a heavy bag of bricks pretty much forever without any energy input and without ever getting tired or otherwise weakening.
  6. jtbell

    Staff: Mentor

    In this situation, we do indeed expend energy, but it doesn't go into work done on the brick or wall. It ends up as thermal energy. We become not only sweaty and tired, we become hot (or at least a bit warmer).

    [Added: I see now that this simply repeats DaleSpam's comments in different words.]
  7. Is the same thing with Objects in circular motion?
  8. Nugatory

    Staff: Mentor

    Pretty much, yes. You can easily exhaust yourself swinging a weight on the end of a string... But a wheel doesn't get tired and if it weren't for friction would spin forever without doing any work or needing any energy input.
  9. Here's how I think of it:
    When work is done, the guy must leave the wall in a different way than he has found it, otherwise he has done nothing. When he leaves, the wall is still exactly the same as when he found it.

    If you worked at McDonald's and cleaned the same 1 sq foot section of the floor all day long, you may be very, very tired from scrubbing the ground for 10 hours, and you'll have done hard physical labor, but your manager will think you haven't done much work, and you'll be fired.
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