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Homework Help: Help needed with Work Done, Power & Kinematics

  1. Jul 6, 2010 #1
    1. The problem statement, all variables and given/known data

    A large box of mass 50kg rests on a horizontal floor. A man moves the box by pushing it with a horizontal force of 200N. The box moves at a constant speed of 0.25m/s.

    a) Calculate the work done on the box each second by the man.
    b) State the rate at which work is done on the box, i.e the power.
    c) State the value of the force of friction between the floor and the box.

    2. Relevant equations

    Work done = Force x Direction
    Power = Work done / Time taken/

    3. The attempt at a solution

    a) Work Done = 200 x 0.25 = 50J
    b) Power = Work Done / Time Taken = 50J / 1 = 50W
    c) * i need help here. Heres my attempted solution * Since the box moves at a constant speed, acceleration = 0 , WHY is resultant force ZERO then ? can someone help me with the thinking process ??

    Thanks in advance.
  2. jcsd
  3. Jul 6, 2010 #2


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    Homework Helper

    Force= mass times acceleration. Zero acceleration means zero resultant force. What is the problem?
  4. Jul 6, 2010 #3
    Oh i just discovered this formula as we are doing energy,work and power before kinematics. Sorry, newbie here. :shy:

    But why is frictional force 200N when resultant force is 0 ?
    It it because in the free body diagram, both sides must minus off to produce 0 resultant force?
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