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Bathroom scale compression problem

  1. Dec 15, 2004 #1
    if you stand on a bathroom scale, the spring inside the scale compresses .50 mm, and it tells you your weight is 700 N. Now if you jump on the scale from a height of 1.4 m, what does the scale read at this peak.

    i found an equation to use, i just can't figure out what i'm solving for. i'm using
    mgh = 1/2kx^2

    i have m (71.4) g (9.8) and h (1.4). what i don't know is k and x. can any one help please?
  2. jcsd
  3. Dec 15, 2004 #2

    Doc Al

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    Staff: Mentor

    This is a statement of conservation of mechanical energy: the initial gravitational PE is transformed into spring PE.

    k is the spring constant, x is the displacement of the spring from its uncompressed position. You can figure out the spring constant from Hooke's law (using the data supplied in the problem set up): F = -kx.
  4. Dec 16, 2004 #3
    Yeah k can be determined from the given data.

    One more thing!

    When you jump from a ht. of 1.4 m, the gravitational PE lost by you is definitely stored as PE in spring. But You are not actually falling 1.4 m but you are falling 1.4 m PLUS the distance contracted bythe spring.

    In this case, it wouldn't make much of a difference(why?)
    Anyway, you should know the concept! :smile:
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