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Work and energy for a lowered block

  1. Oct 10, 2016 #1
    1. The problem statement, all variables and given/known data
    A cord is used to vertically lower an initially stationary block of mass M = 35 kg at a constant downward acceleration of g/5. When the block has fallen a distance d = 6.5 m, find the work done by the cord's force on the block.
    Find the work done by the weight of the block.
    Find the kinetic energy of the block.
    Find the speed of the block.
    2. Relevant equations
    W = F*d
    F = m*a

    3. The attempt at a solution
    So I tried finding the first part of the question where I multiplied the mass, 35 kg, and multiplied it by the acceleration, 9.80/5= 1.96, to get F= 198N and then I multiplied the force by the distance to get the work and got 445.9 J but it is wrong.
     
  2. jcsd
  3. Oct 10, 2016 #2

    gneill

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    Staff: Mentor

    Note: Thread title changed to describe the question subject matter.

    Have you drawn a the free body diagram for the situation? What forces are operating on the block? Is the force due to the cord the same as the net force on the block?
     
  4. Oct 10, 2016 #3
    No, I do not think it is. So I drew out the free body diagram and I got that the Tension force of the cord, T - mg = ma. So T = ma + mg
    For this I got 411.5 N
    To solve for work I multiplied it by 6.5m and got 2675.4J which is wrong. I am not sure what I am doing.
     
  5. Oct 10, 2016 #4

    gneill

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    Staff: Mentor

    Your equation describes a mass accelerating upwards with acceleration a. Your free body diagram should help you to sort out the directions of the vectors and the signs of the terms in your equation.
     
  6. Oct 10, 2016 #5
    So the Tension force would be equal to T= M*g - M*a and that is equal to 274.4N
     
  7. Oct 10, 2016 #6

    gneill

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    Staff: Mentor

    That looks reasonable. Be sure to keep track of the directions of the forces acting versus the direction of the motion of the block.
     
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