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Homework Help: Two boxes sliding down a 20° ramp.

  1. Feb 20, 2012 #1
    °1. The problem statement, all variables and given/known data
    The figure shows two boxes, A and B, on a ramp. The ramp forms a 20° angle with the horizontal. Box A is in front of Box B and they are touching. Box A is 2 meters from the bottom of the ramp. The mass of A is 3 kg and the mass of B is 10.5 kg. The coefficient of kinetic friction for box A is .210 and the coefficient of kinetic friction for box B is 0.170.

    How long will it take for A to reach the bottom of the ramp?

    2. Relevant equations
    F=ma
    The kinematic equations
    Force of Kinetic Friction=μ*n

    No help needed! I solved it!


    3. The attempt at a solution
    I know that B is acting on A and A is acting on B. My problem is finding the net force between the two in the x direction. And then once I find that, I am having trouble finding an equation to use that will help me find how long it takes for A to reach the bottom.

    I think that I might have found the correct net force but I'm not sure and I don't know what to do with that net force.

    I've also drawn the two free body diagrams for the problem.

    The Force that gravity exerts in the x direction on:
    A=10.0554 N
    B=35.1939 N
    The force that the kinetic friction exerts in the x direction on:
    A=-5.80166
    B=-16.43804
     
    Last edited: Feb 20, 2012
  2. jcsd
  3. Feb 20, 2012 #2
    Can you show us your diagrams and work?
     
  4. Feb 20, 2012 #3

    ehild

    User Avatar
    Homework Helper

    You can not know the force of interaction between the boxes, but you know that the force A exerts on B and the force B exerts on A are equal and opposite. Write the equation F=ma for both bodies, including the force of interaction between them (just use a symbol for it). Show the equations, please.

    ehild
     
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