Moments, gears and levers

  • Created by: Andrew
  • Created on: 22-02-19 13:25


A moment is the turning effect of a force applied to pivot

Eg. a door handle:

  • The pivot is the bit that goes in the door
  • You apply a downwards force to the handle
  • This causes a turning effect that moves the handle clockwise

In the example, you create a moment when you push the handle down, this will have to be greater than another moment that is pushing the handle the other way

The equation for a moment is: Moment = force x distance (M=Fd)

You can increase a moment by increasing the force applied or by increasing the distance to the pivot

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For an object to be at equilibrium, there must be no resultant force on it. 

Circular equilibrium:

  • Moments can affect circular equilibrium (whether an object rotate)
  • Eg. a beam suspended by a string will turn
    • Although the forces on both sides are the same (gravity)
    • The string will likely not be in the centre
    • So the distance to the pivot (string) is not equal for both the forces
    • Therefore one moment will be larger than the other
    • The beam will turn
  • Increasing the force applied or the distance to the pivot can make the moments equal and put the object in equalibrium
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Moments with levers

Levers use moments to move things, they have a pivot and you apply a force to one end to move the object, it is easier to move the objects the further you are from the pivot beacuse it increases the moment with less force needed.

The object on the other end of the lever also exerts a force on the lever, when you move the object you are actually just creating a larger moment on your side of the lever, it goes out of equilibrium and turns.

This allows you to lift things that you normally couldn't, eg. an object exerts 100N of force 1m from the pivot, meaning it has a moment of 100Nm, if you exert 25N on the lever but you are 5m from the pivot, you create a moment of 125Nm, as your moment is larger the lever starts to rotate in the direction you apply force to.

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Moments with gears

Gears can transfer the rotational effect of a moment (if something rotates them they can make something else rotate).

Because of how gears work, the rotation is reversed when passed to the next gear

When turning a larger gear, it turns more slowly, but has a larger moment as the distance to the pivot (the centre of the gear) is more.

The opposite is true when turning a smaller gear, it turns faster, but has a smaller moment.

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