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Help in energy questions

  1. Dec 18, 2009 #1
    1. The problem statement, all variables and given/known data
    When mass M is at the position shown, it is sliding down the inclined part of a slide at a speed of 2.07 m/s. The mass stops a distance S2 = 2.5 m along the level part of the slide. The distance S1 = 1.13 m and the angle q = 30.50°. Calculate the coefficient of kinetic friction for the mass on the surface.


    2. Relevant equations
    http://capa2.cc.huji.ac.il/res/msu/physicslib/msuphysicslib/13_EnergyConservation/graphics/prob27a_MechEnWFriction.gif


    3. The attempt at a solution
    I tried solving this question by:
    H=1.13sin30.5
    mgH+1/2mv^2-(s1+s2)u*mgcosa=0
    9.8*0.5735+1/2*(2.07^2)-3.63u*9.8cos30.5=0
    5.6203+2.14245=30.651u
    u=0.2532
    I get wrong answer all the time
    can someone tell me where i went wrong?

    I have another question which I have no clue hot to solve it:

    The left side of the figure shows a light (`massless') spring of length 0.290 m in its relaxed position. It is compressed to 71.0 percent of its relaxed length, and a mass M= 0.250,kg is placed on top and released from rest (shown on the right).
    http://capa2.cc.huji.ac.il/res/msu/physicslib/msuphysicslib/13_EnergyConservation/graphics/prob24_CompSpring.gif
    The mass then travels vertically and it takes 1.50 s for the mass to reach the top of its trajectory. Calculate the spring constant, in N/m. (Use g=9.81 m/s2). Assume that the time required for the spring to reach its full extension is negligible.

    Can someone tell me how to do this question?
     
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Dec 18, 2009 #2

    PhanthomJay

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    why sure, in that last term you have s1(u)mgcos theta, which is fine, but then the friction force over the 's2' length is over a level surface, so there's no cos part, its just s2(u)(mg)
    Someone can help if you show an attempt at a solution, but beyond that, please post separate questions in separate posts, to avoid the chaos that develops when multiple questions on different topics are posted together.
     
    Last edited by a moderator: Apr 24, 2017
  4. Dec 18, 2009 #3
    I tried to solve the question ne more time as you told me,but am still getting a wrong answer

    mgH+1/2mv^2-s1umgcosa-s2umg=0

    is the equation is correct?
     
  5. Dec 18, 2009 #4

    PhanthomJay

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    Looks good, if i'm interpreting the problem correctly. So I get 5.62+2.14 -u(9.54 + 24.5)= 0
    Solve u = 0.23, did you have a math error?
     
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