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Work and Power of pulling student

  1. Dec 28, 2009 #1
    1. The problem statement, all variables and given/known data
    Using a force of 12.0 Newtons, a student pulls a 50.0 Newton weight along the table top for a distance of 10.0 meters in 4.0 seconds. Compute the power developed by the student.


    2. Relevant equations
    W=Fcos(theta)d
    P=W/t

    3. The attempt at a solution
    W=(12.0)(cos(50))(10)
    W=115.8 J

    P=115.8/4.0
    P=28.95 J/s
     
  2. jcsd
  3. Dec 28, 2009 #2

    Doc Al

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    Staff: Mentor

    Where did you get the angle = 50°?
     
  4. Dec 28, 2009 #3
    I thought since the weight is 50, that was the angle. So it should just be (12.0 N)(10.0 m) which equals 120. J? Then the power will be 120.J/4.0s which will equal 30. J/s or watts, I presume?
     
  5. Dec 28, 2009 #4

    Doc Al

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    Staff: Mentor

    No, you cannot draw any such conclusion. The angle in question is that between the force and the displacement. Assuming the force is exerted in the direction of the displacement (parallel to the table top) then the angle will be 0.
    Right, assuming an angle of 0° as discussed above.
     
  6. Dec 28, 2009 #5
    Like Doc Al had pointed out, your confusion is with the concept of weight.

    Weight is simply the gravitational force applied to an object with a given mass.

    If I told you I had an object whos weight was 29.4N then it's mass would simply be 3kg.

    Can you see how I was able to solve for the mass?

    You must have a clear understanding of the difference between weight and mass and the properties they contain before moving on so hammer this stuff into your head!
     
  7. Dec 28, 2009 #6
    I'm sorry, but I dont understand how you were able to solve for mass. How did you do that?:uhh:
     
  8. Dec 28, 2009 #7

    Doc Al

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    Weight = mass*g

    Given the weight of an object, you can solve for its mass and vice versa.
     
  9. Dec 28, 2009 #8
    Thank you for all clarification!!!! :smile:
     
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