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Thread: wasted energy

  1. #1 wasted energy 
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    If I pull/push an object which weighs 100N on a track at an angle of 45 and the weight moves along the track by 1 metre, I've done work 100J.


    If I had pulled the weight along the track I suppose the dislocation of the object would have been 1/cos45 = 1.41 m at the same cost of 100J. Can we conclude that I wasted/burned calories equivalent to 41J? If this is not a valid calculation, what is the right one?


    Last edited by Password; September 14th, 2014 at 01:00 AM.
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  3. #2  
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    Quote Originally Posted by Password View Post
    If I pull/push an object on a track at an angle of 45 with a force of 100 N and the weight moves along the track by 1 metre, I've done work 100J.


    If I had pulled the weight along the track I suppose the dislocation of the object would have been 1/cos45 = 1.41 m at the same cost of 100J. Can we concluded that I wasted/burned calories equivalent to 41J? If this is not a valid calculation, what is the right one?
    Seriously?

    45 what?
    In what time?


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    Come to think of it, you are probably mixing the usage describing torque vs work done.
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  5. #4  
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    Quote Originally Posted by Password View Post
    If I pull/push an object which weighs 100N on a track at an angle of 45 and the weight moves along the track by 1 metre, I've done work 100J.
    You forgot to tell us what the coefficient of sliding friction is between the object and the track. The force on the track due to the weight, while being 100N is not parallel to the displacement. The force in the direction of displacement is 100N u cos (45) where u is the coefficient of sliding friction.
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  6. #5  
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    Quote Originally Posted by physicist View Post
    Quote Originally Posted by Password View Post
    If I pull/push an object which weighs 100N on a track at an angle of 45 and the weight moves along the track by 1 metre, I've done work 100J.
    You forgot to tell us what the coefficient of sliding friction is between the object and the track. The force on the track due to the weight, while being 100N is not parallel to the displacement. The force in the direction of displacement is 100N u cos (45) where u is the coefficient of sliding friction.
    frictionless
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  7. #6  
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    Quote Originally Posted by Password View Post
    If I pull/push an object which weighs 100N on a track at an angle of 45 and the weight moves along the track by 1 metre, I've done work 100J.


    If I had pulled the weight along the track I suppose the dislocation of the object would have been 1/cos45 = 1.41 m at the same cost of 100J. Can we conclude that I wasted/burned calories equivalent to 41J? If this is not a valid calculation, what is the right one?
    There is insufficient information here to solve the problem. The force of gravity does no work since it's perpendicular to the direction of motion and therefore the work done by gravity is zero. Since we don't know the force in the direction of motion we cannot tell you how much work is done. We could have exerted a large force for a short time or a short force for a short time. In any case all we have to do is to is get it to move. We don't care what the end speed is. The work done on a particle is the value of the kinetic energy put into the object which is unspecified and we are allowed to be what we want.
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  8. #7  
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    Quote Originally Posted by Password View Post
    If I pull/push an object which weighs 100N on a track at an angle of 45 and the weight moves along the track by 1 metre, I've done work 100J.
    Incorrect. If the weight is 100N and the track is inclined at an angle of 45 degrees to the horizontal, the force applied is 100 sin (45), the distance is 1 meter and the work is 70.7 J.

    From a potential energy standpoint, you have raised the object by a height of 1 meter*sin (45 degrees)=.707 meters, so it has gained 100N*.707=70.7J of potential energy.
    Last edited by Harold14370; September 14th, 2014 at 06:45 AM.
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