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I don't understand how torque works

  1. Jul 26, 2013 #1
    Is there any good pictorial representations of how torque, or a pulley, actually makes it easier to do work (less force required). I always hear that it spreads the work over a larger distance, hence why torque= distance*force, but that explanation just reiterates the mathematical equation. So why does a further distance from the center require less force to do the same amount of work? (an actual physical description)

  2. jcsd
  3. Jul 26, 2013 #2
    First open a door with your hand on the handle, as usual. Then try to open the same door putting your hand two inches from the hinge. See what you feel.
  4. Jul 26, 2013 #3
    Ya but why is it easier to open it near the handle? I mean if you opened the door 30 degrees near the hinge and then 30 degrees near the handle, opening near the handle would require you to push it a further distance to achieve the same angle. So by that reasoning it should be harder to push it near the handle. Where am I going wrong here?
  5. Jul 26, 2013 #4


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    That is a confusing explanation. The distance in work is parallel to the force. The distance in torque is perpendicular to the force.
  6. Jul 26, 2013 #5


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    Opening the door, the work is the same. But if the knob is 3 feet from the hinges, it takes less force than if the knob was 3 inches from the hinges.

    Have you ever played on a seesaw? Have you ever used a crowbar? How about turning a nut with a wrench?
  7. Jul 26, 2013 #6


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  8. Jul 26, 2013 #7
    One thing every car mechanic knows is that rusty old screws come loose only when you put a metal pipe on a wrench (usually) as in that way the end result torque is much higher even though the force your arms deliver is pretty much the same.

    this also is the thing behind gears and gear ratios.
    Have you ever seen one of those old diesel tractors, now they don't have that strong of an engine yet the gear ratios are so small that the torque on the wheels is kinda large so it is useful as a tractor , you could never plow a field with a passenger cars gearbox even if the engine would be very strong.
  9. Jul 26, 2013 #8
    I think one way to look at is consider the work done. Move the door 30 Deg. - it does not matter how it gets done is is the same amount of work. So .... one basic formula for work (energy) is force x distance.... from this you should be able to see that the handle side of the door - where you apply the force travels farther - so the force required is reduced proportionally.
  10. Jul 26, 2013 #9


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    Fixed it for you.
  11. Jul 27, 2013 #10
    Let's see if we can understand a rope. In the picture below force F is pulling a horizontal rope downwards. That causes the rope to pull the two vertical poles A and B with a large force, for some unknown reason.

    Same kind of thing would happen if we cooled the rope using liquid nitrogen.

    Code (Text):

    A                            B
    |                            |
    |               |            |
    |               F                  
  12. Jul 27, 2013 #11


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    torque = force x distance (well, perpendicular force, but you know that)
    So if the distance is more, and the torque is the same, then the force can be less.
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