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How can I lift more than my weight?

  1. Sep 30, 2011 #1
    There is a tricep machine which is basically a pulley, with some rope connected to a stack of weights and a bar for you to push down on using your triceps on the other end of the rope.

    How is it possible that if I'm 140 pounds, I can lift like 180 or more on that machine? If I want to want the weight of 180 pounds, then I must apply more than 180 pounds of force on the bar. But if I only weigh 140 pounds, won't I just be lifted up off the floor? I thought that I could resolve this by thinking that maybe I was just transferring the gravity of my body onto the bar, but my muscles are providing extra force, but isn't that just an internal force?
     
  2. jcsd
  3. Sep 30, 2011 #2
    Usually machines don't have you apply the amount of force that gets lifted up. There's levers and pulleys (as you mentioned) which greatly reduce the force you need to lift the weight.

    Just placing the pulley on top of the weights would half the force you need to apply. And there's probably other mechanical advantages as well.

    180lbs is ALOT of weight to lift with just your triceps.

    Can you find a picture of a similar machine by any chance?
     
  4. Sep 30, 2011 #3

    russ_watters

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    Staff: Mentor

    For a tricep machine, there is a torque around your elbow, not a vertical lifting force applied at your center of gravity (unlike the lat machine) -- and the torque is in the wrong direction to lift you off your seat!
     
    Last edited: Sep 30, 2011
  5. Sep 30, 2011 #4

    rcgldr

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    There are two of these machines at local cetner I go to. One uses a pulley at the weight stack, and the force you apply is 1/2 the weight selected. The other is connected directly to the weight stack and the force you apply is equal to the weight selected. There is a second pulley overhead, but that doesn't reduce the force. Since you'd be lifted off the ground trying to push down 180 lbs, it must be the type where you apply 1/2 the force of the selected weight stack.

    That would be a bicep machine (pulling up instead of pushing down).
     
    Last edited: Sep 30, 2011
  6. Sep 30, 2011 #5

    fluidistic

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    I also found this strange, 1 week ago I tried to lift 70 kg (my mass is 75 kg) with the triceps machine and I could do it easily. I thought that with all the friction (I can hear it, I guess the machine needs some oil somewhere) and I don't feel like my mass is only 5 kg when I lift the weight, I was really astonished.
    Can someone explain me how a pulley can help you to lift up to twice your mass? I know it looks like a problem of introductory physics but I'm still surprised of the result. Anyway that would explain why it's so easy to lift up a lot of weight with this machine rather than with an iron bar.
     
  7. Sep 30, 2011 #6

    rcgldr

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    You generate a tension on the cable by pushing down on the bar connected to the cable, and that tension remains the same through out the cable back to it's connection point (ignoring friction). At the weight stack with the pulley, there are two runs of the cable going to the weight stack, down from the connection point to the pulley, then back up to the upper pulley. Since there are two runs of the cable lifting the weight, the upwards force from the cable is double the tension. If the weight stack isn't accelerating, then the tension is 1/2 the weight of the weight stack.
     
    Last edited: Sep 30, 2011
  8. Sep 30, 2011 #7
    Work is force times distance. If you increase the distance over which you apply a force, you can do the same work with a smaller force. This is the principle behind pulleys, levers, ramps, screws, and gears. You use less force but over a longer time to accomplish the same work. It's called "Mechanical Advantage". Archimedes was one of the first ones to exploit this. He famously said, "Give me a place to stand and with a lever I will move the whole world." He also purportedly stunned King Hiero of Syracuse, by lifting a three mast ship out of the water by himself using ropes and pulleys.
     
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