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Homework Help: Spring Elastic Potential Energy! Help!

  1. Nov 16, 2009 #1
    I feel so confused right now, I'm probably over thinking this though. Please help!
    1) A slingshot consists of a light leather cup, containing a stone, that is pulled back against two parallel rubber bands. It takes a force of 11 N to stretch either one of these bands 1.1 cm.

    (a) What is the potential energy stored in the two bands together when a stone is placed in the cup and pulled back 0.19 m from the equilibrium position?

    Round your answer to two significant figures.

    (b) With what speed does the stone leave the slingshot?

    Round your answer to two significant figures.

    2) A child's toy consists of a piece of plastic attached to a spring in the following figure. The spring is compressed against the floor a distance of 2.05 cm, and the toy is released. If the toy has a mass of and rises to a maximum height of 57.0 cm, estimate the force constant of the spring.

    Note: the height of the toy in a compressed state is negligible in comparison with the maximum height.

    Round your answer to three significant figures. Take the free fall acceleration to be .
    I've already tried using the equations F= kx and Us=1/2kx^2 but I'm not getting the right answer. I filled F in with 11 N and x with 0.011 m to find k, I got 1000. Then I plugged 1000 into the other equation and got 1/2(1000)(0.19^2). I got 18.05 and rounded to two sig figs 18 but it is still not right. What am I doing wrong? Please help!
  2. jcsd
  3. Nov 16, 2009 #2


    User Avatar

    Staff: Mentor

    Welcome to the PF. You need to show us the relevant equations and show us your attempt at a solution before we can be of much tutorial help.

    What are the main equations that we use for spring questions? The first relates the force F to the sprink constant k and the distance the spring is pushed/pulled from equilibrium.

    The second relates the energy E in a pushed/pulled spring to its spring constant and the distance it is pushed/pulled.
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