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B Will the work done be infinite?

  1. Aug 20, 2016 #1
    If I displace an object at rest in space by giving it a force F in X direction and the object tends to move forever, will the work done be infinite?
    Knowing that work done = force*displacement . Since the space has no external resistance (unbalanced force) to stop the object from moving , making the displacement infinitely large.Let us assume that it stays clear of gravitational influences of other bodies in space.
     
  2. jcsd
  3. Aug 20, 2016 #2

    Dale

    Staff: Mentor

    Hi Vijay, welcome to PF!

    The displacement is the displacement during the application of the force. So if you continue to apply the force forever then yes the work will be infinite. If, at some time, you stop applying the force then there will be no more work done, regardless of how long the object continues to drift with no force.
     
  4. Aug 20, 2016 #3
    Thank you Dale , I am kind of new to this Forum .I am not sure how this forum works yet , so apologies for any inconvenience caused .Coming back to my question , This is what i understood in my high school about work done
    Example when you push a wall of the room or when you stand in the room for a long time you feel exhausted and you think a lot of work is done, But in Physics the word Work Done has a definite meaning and is used in a restricted sense. Work is said to be done only if the point of application of force moves. For work do be done they must satisfy the following two criteria

    1. Force should be applied
    2. The Force produces displacement
    So , irrespective of whether the force is instantaneous or continued , isnt work defined the same ?
     
  5. Aug 20, 2016 #4

    Dale

    Staff: Mentor

    Yes, work is defined the same. The place where you are going wrong is determining what displacement to use. The work is always calculated using the displacement during the application of the force.

    I am not sure if you are familiar with calculus, but for a time-varying force it is easier to think in terms of power, which is the derivative of work. ##P=f\cdot v = dW/dt##
     
  6. Aug 20, 2016 #5
    Yes , I Am familiar with the basics of calculus. That does indeed clarify my doubt , thank you :)
     
  7. Aug 20, 2016 #6
    What if the force is applied only for an instant like an impact force where the two bodies are in contact only for an instant of time ?
     
  8. Aug 20, 2016 #7
    What Dale is trying to say is for time varying force like that where it is difficult to find the actual displacement (it might be just few x units, but thanks to newtons law of inertia it will keep on moving ) because the object will move forever thanks to no other force acting on it . It is easier if we studied the objects power instead of work done. where dt is 0 . We know the force and we know the velocity.
     
    Last edited: Aug 20, 2016
  9. Aug 20, 2016 #8
    Yes it is easier but I want the work only
    So ##P.dt## = ##dW## if dt=0 then
    ##dW =0##

    after integration

    W= constant (not infinite)

    How do you find the value of this constant ?
    By work-energy principle
    Work done = change in kinetic energy
    Since the initial velocity is zero.
    Work done W = (1/2)mv2 ??
     
    Last edited: Aug 20, 2016
  10. Aug 20, 2016 #9

    PeroK

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    You cannot easily tire yourself out by pushing on an immovable object. Certainly not compared with how quickly you tire if you move a large object.
     
  11. Aug 20, 2016 #10
    Yes it indeed looks that way , this was my thought initially where you use w=F*d when work is done against an opposing force otherwise the change in kinetic energy would be more appropriate.
     
  12. Aug 20, 2016 #11
    I am not sure I get your point
     
  13. Aug 20, 2016 #12

    PeroK

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    Try it. Push against a wall as hard as you can and see how long it takes your muscles to tire out. It would be a long time/

    Then, if you can, try bench-pressing some heavy weight where you can only do 5-10 reps. Your muscles will be exhausted within 30 seconds.
     
  14. Aug 20, 2016 #13
    ha ha i got that ....what exactly is your point ? what has that got to do with my question asked? I just said what we speak of work in general is different from the work done in physics.
     
  15. Aug 20, 2016 #14

    PeroK

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    You're the one who said that if you push against a wall you get exhausted. Reread your post #3.

    I was just pointing out politely that you were wrong.
     
  16. Aug 20, 2016 #15

    Dale

    Staff: Mentor

    If a finite force is applied for an instant then the displacement is indeed 0 and the work is 0. The only way to have a non zero work over an instant of time is to have an infinite force.

    Then ##W=F \cdot d= \infty 0## which is undefined. So you must use some other method to find the work.
     
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