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    E = elastic potential energy.
    x = extension.

    I know how to derive these, it's from a graph of F-x.

    E = 1/2Fx, and E = 1/2kx^2.

    And these mean: E ∝ x or E ∝ x^2.

    But how do I know when to use each one.
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    (Original post by BrainJuice)
    E = elastic potential energy.
    x = extension.

    I know how to derive these, it's from a graph of F-x.

    E = 1/2Fx, and E = 1/2kx^2.

    And these mean: E ∝ x or E ∝ x^2.

    But how do I know when to use each one.
    E = 1/2Fx means energy is proportional to the product of both force and extension... but a given amount of extension implies a given force (Hookes law). force and extension are not independent of each other.
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    (Original post by Joinedup)
    E = 1/2Fx means energy is proportional to the product of both force and extension... but a given amount of extension implies a given force (Hookes law). force and extension are not independent of each other.
    Thanks.

    So you're saying that force is not a constant so E is not directly proportional to x?
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    I think E is directly proportional to x, but only when F is constant.
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    (Original post by BrainJuice)
    I think E is directly proportional to x, but only when F is constant.
    Constant F means constant x... you cannot vary them independently
    -----
    F=kx [Hooke's law]

    E=1/2 Fx
    [sub in Hookes law for F]
    E=1/2 (kx) x

    E=1/2 kx2
 
 
 
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