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Don't mechanical advantage disagree with the law of nature ?

  1. Aug 14, 2008 #1
    Don't "mechanical advantage" disagree with the law of nature ?

    hello

    I just can't swallow the idea of mechanical advantage.

    * if you have a 10 KG mass , you need , say , 100 N to left it up
    * if your friend helps you , each one of you will exert only 50 N
    * but if you are alone , and you have two pulleys in a distinct
    configuration , then - again - you need only 50 N

    I just want to ask :
    in the third case , who is the "friend" that exerts the
    other 50 N ?? from where did that energy came from ??

    isn't energy "cannot be created" ??
     
    Last edited: Aug 14, 2008
  2. jcsd
  3. Aug 14, 2008 #2
    Re: Don't "mechanical advantage" disagree with the law of nature ?

    Your not creating energy from anything, your just trading force for displacement.

    Work = Force x Displacement

    With a mechanical advantage, like a lever or a pulley, your making the displacement part of the equation bigger and making the force part smaller, yielding the same amount of work.
     
  4. Aug 14, 2008 #3

    tiny-tim

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    Welcome to PF!

    Hi ali171! Welcome to PF! :smile:

    Work and energy are the same thing.

    And work = force x distance.

    So the same work is done if you halve the force but double the distance.

    With the pulley system, one man has to pull the rope twice as far (2 feet of pull for only 1 foot of raising the weight).

    So he has only half the force, but the same amount of work. :smile:
     
  5. Aug 14, 2008 #4

    Doc Al

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    Re: Don't "mechanical advantage" disagree with the law of nature ?

    Depending on how you arranged the pulley, it's the ceiling that exerts the other half of the force.
    As has been explained, it's you doing all the work. (That ceiling isn't much of a "friend"--it can't move and doesn't do any work.)
     
  6. Aug 14, 2008 #5
    Re: Don't "mechanical advantage" disagree with the law of nature ?

    That and he's always looking down on you!
     
  7. Aug 14, 2008 #6
    Re: Don't "mechanical advantage" disagree with the law of nature ?

    Think of an incline, such as a ramp. If you have a short ramp that goes 1m left and 1m up, and you are pushing 1kg (assume no friction), you push 1kg up to 1m = 10J of energy used.

    Next case, ramp goes up 1m, but left 5m. So again you lift it up to 1m, so it's 10J of energy used. But this time the ramp isn't as steep, so it's easier to push, but you have to push longer.

    With pulleys, you'll use 50N instead of 100N, but you'll have to pull twice as much rope to get the weight up.
     
  8. Aug 15, 2008 #7
    Re: Don't "mechanical advantage" disagree with the law of nature ?

    first , thanks for the 'welcome' :)

    all the replies were just great . now i understand it...
    i should though of the work relation with displacement and force

    anyway , thanks again .
     
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