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Homework Help: Friction and Moving Blocks

  1. Feb 3, 2010 #1
    1. The problem statement, all variables and given/known data
    A block of mass m1 = 280 g is at rest on a plane that makes an angle θ = 30° above the horizontal. The coefficient of kinetic friction between the block and the plane is μk = 0.10. The block is attached to a second block of mass m2 = 220 g that hangs freely by a string that passes over a frictionless and massless pulley. Find its speed when the second block has fallen 30.0 cm.

    2. The attempt at a solution

    Mass 1 Mass 2
    Fground=280*9.81=2746.8 N Fm2 ground=220*9.81=2158.2 N
    =433.714 N
    =250.405 N
    =25.04 N
    Fx= 250.405-25.04= 225.36N

    ax=.805 m/s2

    It would be greatly appreciated if someone could please help!

    Thank you!
  2. jcsd
  3. Feb 3, 2010 #2
    friction = normal force x coefficient of friction, and the normal force is not equal to the force on x-direction.

    There are total 3 forces on the x-direction. You are one less
  4. Feb 3, 2010 #3
    So in the x direction I have...

    For the second mass the Fg=2158.2N

    and on the first mass Fgx=250.405 N
    and friction which= 433.714*.1=43.37 N

    Would it be:



    or is "m" both of the masses and acceleration is squared?
  5. Feb 4, 2010 #4
    Hi dragonladies

    You have to convert the mass to kg first.

    The second mass doesn't have component on x-direction, only y-direction

    Don't know how you got his value..

    After you find the force components and friction, set the equation using Newton's second law then find acceleration. Finally,use kinematics to find the speed.
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