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Finding the velocity of an inclined mass

  1. May 14, 2016 #1
    1. The problem statement, all variables and given/known data
    As shown in the figure, ( a mass sliding down an incline of 60°) a car of 1000 kg mass start from the rest, it's velocity after 5s is? ( the kinetic fraction coefficient 0.1)

    2. Relevant equations
    E1=E2
    K1+U1=K2+U2
    E1+Wf=E2

    3. The attempt at a solution
    -Fk= mg cos 60 (0.1) = -490
    K1+U1 + Wf = K2 + U2
    We should cancel K1 and U2 because they equal zero
    Then
    Mgh -Fk d = 1/2 mv^2
    Cancelling both m from each side
    gh -Fk (vt) = 1/2 v^2
    9.8*h+490(5v) = 1/2 v^2

    Then I don't know what to do
     
  2. jcsd
  3. May 14, 2016 #2

    Doc Al

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    Staff: Mentor

    Since you want the speed at a given time, why not find the acceleration and then use kinematics. Start with Newton's 2nd law.
     
  4. May 14, 2016 #3
    I was doing the wrong law for this equation.

    mg sin 60 - Fk = ma
    1000*9.8*sin60 - ( 0.1 * 1000*9.8*cos60)= 1000a

    8487.07-490=1000a
    7997.04/1000
    = 40m/s
     
  5. May 14, 2016 #4

    haruspex

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    Good.
     
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