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Homework Help: Motion on an inclined plane

  1. Oct 18, 2012 #1
    1.A block of mass 10 kg is resting on a frictionless ramp. The ramp is free to slide on a horizontal, frictionless table and has a mass of 40 kg. The angle of the ramp is 37 degrees.

    A. Draw a free body diagram for the block and for the ramp. Clearly labeling all forces. Write down the equations of motion.

    Hint: A coordinate system attached to the ramp is non-intertial. Velocities and accelerations in such a system need to be related by the relative velocity formula to an inertial system attached to the table.

    Hint 2: Don't forget Newton's Third Law!

    Hint 3: You should end up with seven equations and seven unknowns. The seven unknowns are the magnitudes of two normal forces and six components of acceleration, one of which is zero. The seven equations are: Two from the FBD of the block. Two from the FBD of the ramp. Two from the relative acceleration formula and one additional constraint (the acceleration of the block relative to the ramp is parallel to the ramp.)

    B. What is the acceleration of the ramp?

    C. What are the components of the acceleration of the block in a coordinate system that is attached to the table?

    2. Relevant equations:
    ar = acceleration of ramp ab = acceleration of block m = mass of block M = mass of ramp

    Fn = Normal Force g = 9.8 m/s^2 Fn1 = Normal force of block towards incline plane Fn2 = Normal force of flat surface on ramp

    Components of Block:

    Fx = m*ar*cos(37) + g*sin(37)

    ab = ar*cos(37) + g*sin(37)

    Fy = Fn1 + m*ar*sin(37) = mg*cos(37)

    Components of Ramp:

    Fx = Fn1*sin(37) = m*ar

    Fy = Fn2 = Fn1*cos(37) + Mg

    3. The attempt at a solution:
    Part A is attached.

    Part B: Fn1 + m*ar*sin(37) = mg*cos(37) Plug in Fn1 = m*ar/sin(37)

    m*ar/sin(37) + m*ar*sin(37) = mg*cos(37)

    ar = [mg*sin(37)*cos(37)] / [M + m*sin^2(37)]

    ar = 1.08 m/s^2

    Need help with Part C please.

    Attached Files:

    Last edited: Oct 18, 2012
  2. jcsd
  3. Oct 18, 2012 #2


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    welcome to pf!

    hi mtwain! welcome to pf! :smile:
    you know the forces on the block, so where's the difficulty?

    (alternatively, you could use conservation of energy or conservation of momentum :wink:)
  4. Oct 18, 2012 #3
    Mr. Tiny-Tim,

    Apparently I need to reformulate my equation for acceleration due to the fact that the plane attached to the ramp is non-interial. I need to use a plane attached to the flat surface and use relative velocity formula to relate to acceleration.

    So, any good ideas?

    As far as Part C. I'll worry about it when I finish Part B.
  5. Oct 18, 2012 #4


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    hi mtwain! :smile:

    personally, i'd start by calling the height h, and the position of the plane x, and then use conservation of momentum to find the relation between h and x

    (and then that'll give you the relation between h' and x', and between h'' and x'', and you can have a stab at conservation of energy)
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