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Homework Help: Conservation of Mechanical energy

  1. Oct 19, 2015 #1
    1. The problem statement, all variables and given/known data
    Attached figrue shows an 8.00 kg stone at rest on a spring.The spring is compressed 10.0 cm by the stone.
    (a) What is the spring constant?
    (b) The stone is pushed down an additional 30.0 cm and released.What is the elastic potential energy of the compressed spring just before that release?
    (c) What is the change in the gravitational potential energy of the stone–Earth system when the stone moves from the release point to its maximum height?
    (d) What is that maximum height, measured from the release point?

    2. Relevant equations
    F = mg
    F = -kx
    W = 1/2 (kx²)

    3. The attempt at a solution
    m = 8kg
    x1 = - 0.1m
    x2= - 0.1m - 0.3m = -0.4m

    Don't worry about a) and b), I've worked them out here just in case of reference
    a) F=mg=-kx1 ⇒k=784 n/m
    b) kΔx²/2= (784 n/m)(-0.1m-0.3m)²/2 = 62.72 J

    What I have trouble with is c), can someone explain how I should deal with this question? (p.s, You are awesomeee if you can accompany your explanation with drawing) http://imgur.com/t4BkCe1

    Much thanks!
  2. jcsd
  3. Oct 19, 2015 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The title of your thread is a big hint! Mechanical energy is conserved. At the lowest point, what is the total mechanical energy? (Measure gravitational PE from that point.) At the highest point?
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