1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Loss of energy?

Tags:
  1. Mar 2, 2017 #1
    Hello, I have been learning about energy and work in my 100 level physics class and I got to thinking:
    W = F*d
    But I'm wondering how this works in the case of static friction. For instance, if I take a really heavy object and apply a force to it that isn't enough to move it, the distance it moves is 0, which would in turn make the work done by me on the system of the object 0. However, I am still using energy to constantly apply that force to the block, whether it moves or not. According to the energy principle, the total energy must be conserved, so where is that energy from my applied force going if not to the object?
     
  2. jcsd
  3. Mar 2, 2017 #2

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No energy is required from letting a book lie on a shelf. If you get tired from holding up the book yourself instead of letting the shelf do that, that does not mean you are passing on energy to the book...
    Same with not moving a wall horizontally, even if you push as hard as you can.

    Good thinking though!
    Funny world physics lives in :smile:
     
  4. Mar 2, 2017 #3

    russ_watters

    User Avatar

    Staff: Mentor

    Heat. Like a car with its engine running and the parking brake on.
     
  5. Mar 2, 2017 #4

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Making things difficult eh ? Clutch engaged and burning up ? Or you have an automatic ? :smile:
     
  6. Mar 2, 2017 #5

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I suggest an experiment. For example,if you have access to a gym, try moving some weights and see how long it takes your muscles to tire. Then, push on a wall (or anything that doesn't move) and try to tire your muscles.
     
  7. Mar 2, 2017 #6
    The way I like to think about work is as if you're giving (or taking) energy from a system. You're not transferring any energy into the system: the object is not changing its kinetic energy nor its potential energy, etc, so your work is indeed zero. You are definitely using energy to attempt to push the object but it is lost as heat. Now consider if the object is moving at a constant velocity. It's not gaining energy, because your work is being counteracted by the work done by friction so your NET work is zero: F(d)cosx+(-F)dcosx=0 (the cos(x) comes from the dot product). You are doing work while it's moving, but so is friction, in the opposing direction therefore the NET work on the system is zero. Now what do we know happens with friction? We get energy as heat! The same as if the object was not moving. I hope that helped to prove it goes to heat for you. :)
     
  8. Mar 3, 2017 #7

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Kinetic friction does work and generates heat, but not static friction. If you push something and it doesn't move, then the force you apply is acting through a zero distance, so no work is done; static friction acts through a zero distance, so no work is done, and the objects doesn't heat up.

    In order to generate the force, your muscles must move somewhat, so you will tire, but much more slowly than if the object started to move. If a heavy object starts to move, you can exhaust your muscles in seconds. But, if you push on a heavy object that doesn't move, it will take a long time to create muscle fatigue.
     
  9. Mar 3, 2017 #8

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Agree with PerOK, but holding a heavy book with an arm stretched out does no work in the physics sense either, but it sure is tiring !
     
  10. Mar 3, 2017 #9

    Nidum

    User Avatar
    Science Advisor
    Gold Member

  11. Mar 3, 2017 #10

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

  12. Mar 3, 2017 #11
    Brother in this case of pushing or pulling a heavy body which doesn't move has two different aspect In which you can see it.

    (1)The first aspect is WORK DONE BY THE STATIC FRICTION.
    I.e here is a force of static friction by which there is no displacement so the work done by static friction is Zero.

    (2) The second aspect is WORK DONE BY THE MAN.
    But in this case you are applying a force to the heavy body which is not moving . Since you are applying force with hand in doing so your muscles of whole hand get contract in order to remain contracted they require more oxygen to respire (or to produce energy). blood pressure also increase in order to fulfill the demand so heart also need to pump more blood to vessels.
    Hence all your energy used up in all these biological processes.
    Hence the work done by you is positive and you get tired after sometime.
    hoping that you may get your answer from my way of thinking....
     
  13. Mar 3, 2017 #12

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That's not really what happens if you push in a position where your muscles are locked - you don't get tired like you do when the block moves. And your heart rate doesn't go up the way you describe.

    Holding a book out is different, as your muscles have to work to stay in that position. But, pushing in a position where your muscles are comfortable requires very little energy and you will tire only slowly.

    For example, if you are at the gym and use a very heavy weight that you cannot move, you can barely exercise at all, let alone be able to exercise fully.
     
  14. Mar 3, 2017 #13
    Then what will you say about when you hold 20 kg of iron peice for 30 minutes you will definitely suffer pain and strain ...It is true that blood pressure will not increase upto that much but upto some extent it rise..
     
  15. Mar 3, 2017 #14

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It depends how you hold it. If you hold it so that your muscles and joints are loaded in a natural position, then you won't get tired. E.g. balanced on your head! But, your muscles are not rigid and have to work internally to stay rigid. In fact, you wouldn't even be able to hold 20kg at arm's length.

    And if you held it on a hanging arm, your shoulder and fingers would tire, rather than your arm muscles, which would actually be stretched. In fact, pulling on something that doesn't move stretches your muscles rather than tires them. Although, stretching can be slightly tiring.
     
  16. Mar 3, 2017 #15

    jbriggs444

    User Avatar
    Science Advisor

    The work done by the man on the heavy body is zero.

    The questions of whether the man's muscles tire, whether ATP is consumed, whether blood pumps and lungs inflate and deflate are interesting, but have nothing to do with the work done across the interface between man's hand and rock's face.
     
  17. Mar 3, 2017 #16
    Compare 1. heavy weights by a weight lifter for suppose one minute in static position.
    2 Now replace weight lifter with a wooden stand .
    Weight lifter will get tired and cannot hold for more than a limited time whereas stand can hold it for as much time as you want.
    Now weight lifter will afterwards need more diet for energy spent but wooden stand does not require any energy.
    This is the paradox.
    We can explain energy needed by weightlifter but why stand going same work doesn't require energy.
     
  18. Mar 3, 2017 #17

    jbriggs444

    User Avatar
    Science Advisor

    Go ahead. Explain the energy needed by the weightlifter. Then see whether the same explanation applies to the stand.
     
  19. Mar 3, 2017 #18
    The total energy is conserved within the system.
    The system has slightly more mass when energy is being applied than when it is not.
    A charged battery has more mass than one that has no charge.A spring under compression is slightly more massive than one that is not.
    Some of the mass will be lost from the system as heat.
     
  20. Mar 3, 2017 #19

    jbriggs444

    User Avatar
    Science Advisor

    Please, let us stay purely in the realm of classical physics. Mass-energy equivalence is not relevant here. It does not resolve the poster's conundrum.
     
  21. Mar 3, 2017 #20
    Stand keeps holding the weights without any chemical reactions happening inside it.So it does not require any energy to be supplied to it.
    But still paradox remains i.e. What difference between weightlifter and stand.
    These are two systems doing same action(static) on weights.One needs energy and other doesn't.
    Any way I know that weghtlifters muscles continue to do dynamic action and hence chemical reactions in his body enable his muscles to hold the weights.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Loss of energy?
Loading...