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Homework Help: How to calculate the potential energy of a spring?

  1. Apr 25, 2016 #1
    Hi
    I want to calculate the potential energy of the following figure in function of x

    upload_2016-4-25_18-49-26.png

    Here is my attempt :

    mgx + 1/2*k*(sqrt(x^2+L^2)-L)=1/2*m*v^2

    Is that correct ?

    thanks
     
    Last edited by a moderator: Apr 25, 2016
  2. jcsd
  3. Apr 25, 2016 #2
    I really suck at physics, but I think that the forces exerted by the two springs should cancel each other out. So, the only thing which comes into play is gravitational potential energy. Someone please correct me if I'm wrong.

    Edit: I think have taken only the mass m into account.
     
  4. Apr 25, 2016 #3

    berkeman

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    Welcome to the PF.

    Not quite right. Please do not deleted the Homework Help Template that you are provided when starting a schoolwork thread here. It helps to organize your thoughts, especially showing the Relevant Equations.

    Your "equation" does not make sense in the context of the question you posed. The potential energy does not depend on velocity (why is that on the righthand side of the equation?)

    Can you list the Relevant Equations for this problem? If you want to find the PE(x), you will add the gravitational PE to the PE of the stretching springs. Please draw a FBD of the mass in the middle, showing all of the forces on it.
    No, you are incorrect.
     
  5. Apr 25, 2016 #4
    here is my draw :

    upload_2016-4-25_21-41-21.png

    Is it correct?
    thanks
     
  6. Apr 25, 2016 #5

    berkeman

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    Closer :smile:

    You have the gravitational PE correct at mgx.

    The PE from the stretched springs has to do with the force in the vertical direction, not in the directions of the springs. Take the vertical component of the spring force(s) to get the PE contribution from the stretched springs.
     
  7. Apr 25, 2016 #6
    mgx = 1/2*k*(x/sin(a) - L)^2 + 1/2*k*(x/sin(a) - L)^2

    upload_2016-4-25_22-1-33.png

    Is it correct?
    thanks
     
  8. Apr 25, 2016 #7

    berkeman

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    Closer still, but you should be adding mgx to those spring terms. You want an equation for PE(x) = ____________________

    I'm also not real clear why you are dividing by the sine of the angle a...
     
  9. Apr 25, 2016 #8
    PE(x) = mgx -1/2*k*(x/sin(a) - L)^2 - 1/2*k*(x/sin(a) - L)^2
    (x/sin(a) - L) means that the distance of the stretched spring ... So x/sin(a) is total new distance and L is old distance .. So the difference x/sin(a)-L is the distance of the stretched spring ..
    upload_2016-4-25_22-42-49.png

    thanks
     
  10. Apr 25, 2016 #9

    berkeman

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    Ah, I get the distance thing now with the x/sin(a)-L. But that just gives you the PE from the force of the spring in the direction of the spring, not in the vertical direction. The horizontal forces of the two springs on the mass cancel out.
     
  11. Apr 25, 2016 #10
    I do not know how to do ?
     
  12. Apr 25, 2016 #11

    haruspex

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    No, energy is a scalar. The PE in the springs relates to their extensions. It does not have components.
     
  13. Apr 25, 2016 #12
    Do you have any idea how to build the equation of the potential energy for the system
    thanks
     
  14. Apr 25, 2016 #13

    haruspex

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    Nearly right, but in my experience stretching a spring increases its potential energy.
     
  15. Apr 25, 2016 #14
    Can you correct my formula ?
    thanks
     
  16. Apr 25, 2016 #15

    haruspex

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    I can, but you are going to. The equation you wrote would have the springs' contribution to the total PE decreasing when the springs are stretched. What do you think must be wrong?
     
  17. Apr 25, 2016 #16
    is it the signs ?
    PE(x) = mgx +1/2*k*(x/sin(a) - L)^2 + 1/2*k*(x/sin(a) - L)^2
     
  18. Apr 25, 2016 #17

    haruspex

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    Yes.
     
  19. Apr 25, 2016 #18
    So now it is correct formula ?
     
  20. Apr 25, 2016 #19
    thank you very much
     
  21. Apr 25, 2016 #20

    SammyS

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    It strikes me that from that figure, the block may be moving on a horizontal surface.
     
  22. Apr 26, 2016 #21
    Is that the force of the block in function of x is : F(x) = k*(x/sin(a) - L) + k*(x/sin(a) - L)
    and the kinetic energy in function of x is : KE(x)= 1/2*m*v^2 + 1/2*m*v^2 (I cannot express it in x)
    Is that correct ?
    thanks
     
  23. Apr 26, 2016 #22

    haruspex

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    Why the two 1\2 mv2 terms?
    You probably need to express it in terms of dx/dt.
     
  24. Apr 26, 2016 #23
    So you mean that this F(x) = k*(x/sin(a) - L) + k*(x/sin(a) - L) is correct !!
    and KE(x)= 1/2*m*v^2 + 1/2*m*v^2 = KE(x)= 1/2*m*(dx/dt)^2 + 1/2*m*(dx/dt)^2
    Is it like that?
    thank you very much
     
  25. Apr 26, 2016 #24
    Why didn't you use vectors if you want the net force on the object?

    I think he means, Why do you have two 0.5 m v^2? What does the second one represent?

    I guess I know what the mentor meant by getting its components.. Because the vertical is doing positive work and the horizontal should have a work of 0 J because it doesn't move horizontally( Springs forces cancel each other if they have the same spring constant).

    So If you use the vertical component but you have a variable that changes over time which is theta but both ways should yield in the same result right?
     
    Last edited: Apr 26, 2016
  26. Apr 26, 2016 #25
    We have two springs so the kinetic energy should be double as much ,, is not that?
    thanks
     
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