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Homework Help: Help on tis Pulley question.

  1. Oct 14, 2007 #1
    1. The problem statement, all variables and given/known data
    A 4kg Ball and a 3kg block of wood were placed in an arrangement as shown below. When the ball was released it fell through the distance of 5m pulling the block of wood up the ramp. If the pulleys were well oiled and the surface of the ramp exerts a frictional force of 2N. What is the final speed of the sphere after falling through the 5m?

    2. Relevant equations
    work done= Distance moved x force
    1/2mv² =K.E

    3. The attempt at a solution

    P.E= MGH = 4x10x5 =200 J
    energy used to raised the block = 3x3x10+ (2x4) =98J
    K.E of sphere after 5 m fall = 200-98 =102J
    1/2mv² =102J

    But then answer is 5.34m/s

    How to solve tis question. I have attached the diagram below

    http://img400.imageshack.us/img400/3519/77494753vj1.jpg [Broken]
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 14, 2007 #2
    it seems you have forgotten about the frictional force. You should take the work done by the frictional force into consideration.
    Last edited: Oct 14, 2007
  4. Oct 14, 2007 #3
    You will also have to consider the addition of both masses in the kinematic equations at the end
  5. Oct 14, 2007 #4
    What do u mean by addition of both masses. I have taken the frictional force into account in the (2x4)
    Last edited: Oct 14, 2007
  6. Oct 14, 2007 #5
    Is tis correct i still have to minus the resultant force times the total distance travelled by both of them like root 3² +4² =5 then 5x9=45

    1/2mv² =102-45
    v = 5.34m/s

    but i do not understand why i have to minus away the 45
  7. Oct 14, 2007 #6
    The 3kg load moves through a distance of 5m, not 4m. The work done by the frictional force shoulb be 2x5.

    there are 2 methods of doing this, if we use the conversation of energy method like you tried, shouldnt the equation be:

    P.E. lost by 4kg mass=W.D. by frictional force+P.E. gained by 3kg mass+K.E. gained by both masses

    Now you need only to find out the K.E. gained by both masses.

    Notice that the 2 objects move through the same distance and have the same final velocity. From there, you should be able to find the final velocity.
    Last edited: Oct 14, 2007
  8. Oct 14, 2007 #7
    What i dun understand is why do they have the same final velocity?
  9. Oct 15, 2007 #8
    Let's imagine 2 boxes laid to rest on a frictionless surface. They are tied together by a string. If pull the first box along with a force of 2 N, wouldnt both box accelerate at the same speed and have the same final velocity? Its the same for this question. The 4kg ball is pulling the box along.
  10. Oct 15, 2007 #9
    But F=MA so if the mass is heavier won't it accelerate less than the lighter mass? so won't the final velocity be different?
    Last edited: Oct 16, 2007
  11. Oct 16, 2007 #10
    So won't the velocity be different?? since both mass are diff
  12. Oct 16, 2007 #11
    nope, the velocities and acceleration of both objects will be the same. In fact, this is the condition to solve such "pulling a box tied to a box" kind of question. The driving force per object will thus be proportionate to the mass of the object since acceleration is the same for both objects given that there is no friction.
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