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Homework Help: I need help with hw (my book hasnt came in yet)

  1. Jun 27, 2010 #1
    1.A mass m = 20 kg is moving along a surface with coefficient of kinetic friction mk = 0.3.

    A)What is the force of kinetic friction on the mass?


    2.A wooden block of mass m = 9 kg starts from rest on an inclined plane sloped at an angle q from the horizontal. The block is originally located 5m from the bottom of the plane.

    a)If the block, undergoing constant acceleration down the ramp, slides to the bottom in t = 2 s, and q = 30°, what is the magnitude of the kinetic frictional force on the block?

    3A block of mass m1 = 2 kg and a block of mass m2 = 3 kg are tied together and are pulled from rest across the floor by a force of Fp = 21 N. The coefficient of friction of the blocks with the floor is µ = 0.13.
    a) What is the acceleration of the two blocks?

    a = ?m/s2
    b) What is the tension in the string between the blocks?

    T = N ?

    4 A block of mass mblock = 1.9 kg rests on a cart of mass mcart = 3.1 kg, which moves without friction on a horizontal surface. The coefficient of static friction between the block and the cart is µ = 0.76. The cart is accelerating to the right.

    a) What is the maximum acceleration such that the block does not slide on the cart?

    amax = ? m/s2
    b) What is the magnitude of the force on the cart required to provide the acceleration amax?

    F = N ?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jun 27, 2010 #2

    Uku

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  4. Jun 27, 2010 #3
    im on #2 right now
     
  5. Jun 27, 2010 #4
    still need help tho
     
  6. Jun 27, 2010 #5

    Uku

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    Well show what you have going, don't know where to help.
     
  7. Jun 27, 2010 #6
    whats your email, ill send you the graph
     
  8. Jun 27, 2010 #7

    Uku

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    Just put the graph here using the image function...
     
  9. Jun 27, 2010 #8
    this is for #3
     

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  10. Jun 27, 2010 #9

    Uku

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    So how far are you on that nr. 3? The golden rule of all homework forums, no-one is going to solve the problem for you.
    Hint: write out Newtons second law for every body involved.
     
  11. Jun 27, 2010 #10
    Oh alright im on B, the acceleration is 2.94 and i cant find the tension in between.
     
  12. Jun 27, 2010 #11
    2.92
     
  13. Jun 27, 2010 #12

    Uku

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    So you mean you are on exercise 3). For these kind of tension problems there are two powerful tools:
    a) Make a good drawing of the setup, put all the forces on the diagram and also add a coordinate axis (in pulley problems you simply bend the axis around the pulley). The axis will give you a clear view on the signs of things.
    b) Write out Newtons second law for every body included, then solve the equation system you get.
     
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