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Homework Help: Potential energy incline problem

  1. Nov 11, 2009 #1
    1. The problem statement, all variables and given/known data
    A 20.0 kg block is connected to a 30.0 kg block by a string that passes over a light frictionless pulley. The 30.0 kg block is connected to a spring that has negligible mass and a force constant of 300 N/m, as shown in Figure P7.53. The spring is unstretched when the system is as shown in the figure, and the incline is frictionless. The 20.0 kg block is pulled 18.0 cm down the incline (so that the 30.0 kg block is 38.0 cm above the floor) and released from rest. Find the speed of each block when the 30.0 kg block is 20.0 cm above the floor (that is, when the spring is unstretched).
    http://www.webassign.net/sj/p7-45.gif" [Broken]

    2. Relevant equations
    Ki + Ui = Kf +Uf

    3. The attempt at a solution
    0 + (30)g(.18m) = 1/2(M + m) v^2 + (20)(g)(.18)(sin40)

    I solved for v but the answer was not correct. I am not sure what I did wrong. I did not include any spring potential force because it was asking for the speed right when the 30kg block hits the spring so the spring has not yet been compressed so there is no spring potential energy.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 11, 2009 #2


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    In your attempt, you have not used spring constant.
    What are the forces acting on the 30 kg block is the stretched position?
    What are the forces acting on the 20 kg during that time?
  4. Nov 11, 2009 #3
    For me, the problem can be interpreted in two ways.
    1. Calculating the speed right when it touches the spring so there is no spring compression yet.
    2. the block compresses the spring resulting in a spring potential force.
    Okay I understand now thank you rl.bhat
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