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Homework Help: Power problem

  1. Dec 15, 2009 #1
    1. The problem statement, all variables and given/known data

    A box of mass M= 1 kg rests at the bottom ofa frictionless plane inclined at an angel theta= 20 degrees. The box is attached to a string that pulls with a constant tension T=5 N.

    What is he power in Watts being delivered by the tension T as the block reaches x=50 cm

    2. Relevant equations

    P= dW/dt
    w=Fd
    F=ma

    3. The attempt at a solution

    w= (5N)(.5m)=2.5 J

    Then to find the change in time I used kinematics, and to get acceleration I used newton's second law.

    Net force= T-mgsin(theta)= 5N-(1kg)(9.81 m/s^2)sin(20)=1.644782394 N

    F/m=a

    m=1

    a=1.644782394
    v=at
    r=.5at^2

    then I set the position equation equal to .5m to get t

    .5=.5at^2

    sqrt(1/a)=t =.7797327501 seconds

    the from here I just put W/t

    W/t=3.206226748=P

    The correct answer is 6.41

    Could someone please help me get going in the right direction.

    Thank you.
     
  2. jcsd
  3. Dec 16, 2009 #2

    CompuChip

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    Science Advisor
    Homework Helper

    Note that the distance traveled by the box, is not 0.5 m.
    The box is pulled along the inclined plane, so a horizontal distance of 0.5 m corresponds to a greater distance along the plane.
     
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