Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why there is no work done when one is holding weight?

  1. May 9, 2013 #1
    why there is no work done when one is holding weight?
    there isno work done when moving while carrying a school bag?
     
  2. jcsd
  3. May 9, 2013 #2

    Jano L.

    User Avatar
    Gold Member

    That refers to technical meaning of the term work, which is product of component of force along the direction of displacement times displacement of the body. As you hold the weight, you exert a force on it, but there is no displacement, since the bag does not move, so the work is zero.

    That does not mean that you do not lose energy - exerting force by skeletal muscles requires that they spend energy, and you will get tired. I think there are other kinds of muscles, which can exert force without such great exhaustion, perhaps like those of seashells.
     
  4. May 9, 2013 #3

    Danger

    User Avatar
    Gold Member

    The other kind is "smooth" muscle. (The 3rd, "cardiac", appears only in the heart, so is of no consequence here.) Any differences in exhaustion rates or efficiency are due to external influences such as leverage and oxygenation methods. That's getting off topic, though.
     
  5. May 9, 2013 #4
    You can keep a toy helicopter at a certain height by leaving it on the table or letting it hover next to the table. Hovering needs energy from the helicopters batteries but the table isn't going to run out of juice. This shows that it is not fundamentally necessary to do work do keep something at a certain height, but depending on what holds it up some energy might be constantly lost. In the case of the hovering helicopter it continuously accelerates air particles and does work on them and the movement of the air eventually dissipates into heat. So the hovering helicopter wastes energy to heat, not to holding it up.

    The problem is how the muscles work, they cant stay contracted for long periods of time without requiring extra energy. Also when walking you bounce up and down and your arm moves up and down when holding a weight. Skeletal muscles can't store elastic energy efficiently, so you have to do work while rising up. Smooth muscles are better at storing tension for longer periods of time without a lot of extra energy.

    So the answer is basically that there is no work done on the weight or the schoolbag, but you still have to spend energy simply because skeletal muscles waste energy and aren't capable of doing it better.
     
  6. May 9, 2013 #5

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Work done 'ON' is not the same as Work done 'BY'. It's all in the definition.
     
  7. May 9, 2013 #6

    Jano L.

    User Avatar
    Gold Member

    I am not sure what do you mean. Work done on X and work done by X have the same magnitude and opposite signs. In our case, they should both be zero...
     
  8. May 9, 2013 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Not really. The work done BY your muscles is not the work done ON the weight you lift. There is a small matter of efficiency, which can often be zero.
     
  9. May 9, 2013 #8

    Jano L.

    User Avatar
    Gold Member

    On what is this work done then?
     
  10. May 9, 2013 #9

    russ_watters

    User Avatar

    Staff: Mentor

    I agree with Jano L. The muscles are doing no work here. The work "on" and "by" are the same thing, just with opposite signs.

    The input to your muscles is [chemical] energy (not work), the output is mechanical work.
     
  11. May 9, 2013 #10

    Office_Shredder

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    An analogy I like is that holding something and getting tired is similar (in basic physics, not in actual chemistry/on the molecular level) to tying a rubber band to the ceiling and suspending something heavy from it, then watching the rubber band slowly stretch and break.

    You wouldn't accuse the rubber band of doing work on the object even though it experienced material fatigue and eventually broke (actually it does do a negative amount if it stretches before it breaks), so you shouldn't say that you are doing work when your muscles experience fatigue either.
     
  12. May 10, 2013 #11

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You can't support the principle of Energy Conservation yet say there is no difference between what happens in the muscles (which metabolise food) and what happens to the stationary weight. That is unless you want a paradox in a most basic bit of Physics. The difference between your arm and a table top is that there are constant movements in the muscle fibres, there must be hysteresis involved in this movement. These give a force times distance inside the muscles - which resolves any paradox.
     
  13. May 11, 2013 #12

    Jano L.

    User Avatar
    Gold Member

    That is true, there may be microscopic work done on fibres, conserving or not conserving energy. However, don't you think this is irrelevant for the original question? As far as there is no macroscopic work, it is OK to say just there is no work, because that is what textbook rightly say and what the original question was about.
     
  14. May 11, 2013 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    This question is hardly worth discussing unless there is an introduction of some apparent paradox, imo (it's just a matter of accepting the definition. The only reason there is some suggestion of actual work being done by muscles is because you get tired (whereas a table doesn't) so I think the microscopic events are highly relevant.
     
  15. May 11, 2013 #14

    Office_Shredder

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If you hung up a box on a string and the string broke, would you say "wow lots of work being done by that string, so much it broke!" If anything it did less work than a rope that didn't break and just held the object up the whole time
     
  16. May 12, 2013 #15
    I think work was done when you lifted the weight from ground and this work is now stored as potential energy.work cannot be quantified if there is no displacement so its all about how we define work like if you appear for an exam it doesn't matter how hard you study if you don't secure minimum passing marks you won't pass same like this it doesn't matter how much force you exert if there is no displacement then no work.
     
  17. May 12, 2013 #16
    table top

    wouldn't all those atoms in the table moving in reaction to the force of an object "resting" on the table be the distance component of F*D?
     
  18. May 12, 2013 #17

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    What do you mean by "moving in reaction to"? They may be in thermal motion but, after the system has settled down, once it's been lifted, there won't be any motion due to the lifting action.
     
  19. May 12, 2013 #18

    russ_watters

    User Avatar

    Staff: Mentor

    Are your muscles constantly twitching in a way that helps them to their job? I'm not sure, but either way you look at it, there is no paradox here: Your muscles are simply a zero efficiency machine.

    I'm sure you can think of other devices that have an input of energy but no output of work.
     
  20. May 13, 2013 #19

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Afaiaa, muscle fibres cannot just stay contracted. Other fibres need to take over in order to maintain a lift force. This involves a constant process of force times distance with successive parts of the muscles. Animals that need to stand for a long time will use the skeleton in an equilibrium position (straight legs etc). to reduce the muscle power needed. Some will even 'lock' in position.
    I mentioned the concept of "efficiency" earlier on.
     
  21. May 13, 2013 #20

    russ_watters

    User Avatar

    Staff: Mentor

    But they are just fighting each other and not actually making the book move, right? Or another way; even if the book is vibrating ever so slightly, it doesn't, over time, change its elevation. Zero work being done on the book. You asserted an apparent paradox in that. I see none.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why there is no work done when one is holding weight?
Loading...