Torque and Moment

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Torque (moment of a force)

Which of these three forces is most likely

Torque (moment of a force) Which of these three forces is most
to loosen the tight bolt?
Why?

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Torque (moment of a force)

The turning effect of a force F about

Torque (moment of a force) The turning effect of a force F
a given axis is called its torque. A torque (also called ‘moment’ of a force) is a ‘turning force’. It gives a measure of how much ‘turning effect’ a force has about a given axis.

Torque = force x perpendicular distance from line of action of force to the pivot

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Example 1. ‘Opening doors’

A boy pulls on a door handle.
A) Calculate

Example 1. ‘Opening doors’ A boy pulls on a door handle. A)
the torque about the hinge.
B) If he pulls at an angle as shown, what is the new torque?

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Principle of moments

The principle of moments states that for a body to

Principle of moments The principle of moments states that for a body
be in equilibrium:
The sum of the external torques on a rigid object must be equal to zero

F1d1 = F2d2 +F3d3

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Example 2. Family on seesaw

A young girl wants to sit still with

Example 2. Family on seesaw A young girl wants to sit still
her mother and father on a seesaw. Her father, 70 kg, sits only on one side 2.5 m from the pivot. On the other side, her mother, 50 kg, sits 3 m from the pivot. The girl sits on the same side as her mother. At what distance should the girl, who weighs 20 kg, sit from her mother?

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Example 3. Equating moments

A hinged trap-door of mass 15 kg and length

Example 3. Equating moments A hinged trap-door of mass 15 kg and
1.0 m is to be opened by applying a force F at an angle of 45º.
Find F.

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Example 4

Find the moment of F about X in the diagram shown.
Hint:

Example 4 Find the moment of F about X in the diagram
redraw the diagram so that the line of action of F is clear.
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