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Work done by a spring & its potential energy

  1. Nov 14, 2012 #1
    According to work - mechanical energy theorem ,
    W = K(final) - K(initial) + U(final) - U(initial) . . . . (1)
    as we define Potential energy as negative of work done by conservative force and assuming that the only force in this situation is Spring force then ,
    W(spring) = K(final) - K(initial)
    As work done is calculated by finding component of spring force in direction of displacement. How can we say that U(final) - U(initial) applies for all possible conditions of extension of spring as displacement may not be in direction of force ?
    Spring force = 0.5kx2
  2. jcsd
  3. Nov 14, 2012 #2

    Andrew Mason

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    Homework Helper

    Welcome to PF!

    First of all, your equation (1) defines the external work done by/on a system. If no energy is added or lost (Wext = 0), Kf + Uf = Ki + Ui.

    Second, your question is not clear. What do you mean when you say U(final) - U(initial) applies? U(final) - U(initial) is not a mathematical statement.

    Finally, your statement: Spring force = 0.5kx2 is not correct. F = -kx.

  4. Nov 14, 2012 #3
    In case this was a simple slip, the formula

    [tex]W = \frac{1}{2}k{e^2}[/tex]

    W = work, e = extension, k = spring constant

    Refers to the work done in extending a spring = potential energy stored in that spring on extension.
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