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Slide 1

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5.1.4 Force and moments
You should be able to:
(a) understand that the weight of a body may be taken as
acting at a single point known as its centre of gravity.
(b) understand a couple as a pair of equal parallel forces
tending to produce rotation only.
(c) define and use the moment of a force and the torque of
a couple.
(d) show an understanding that, when there is no resultant
force and no resultant torque, a system is in equilibrium.
(e) apply the principle of moments to solve problems
involving forces acting in two dimensions.…read more

Slide 2

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The centre of mass is the point where all of the mass of
the object is concentrated. When an object is supported
at its centre of mass it will remain in equilibrium.
If the object is uniform, for example a meter stick, the
center of mass will be at the exact geometric center; if
the object is irregular in shape the center of mass will be
closer to the heavier end.
An easy way to determine the location of the center of
mass of a rigid pole is to support the pole on one finger
from each hand. Gently slide your fingers together. When
your fingers meet, you will be at the centre of mass.
Try it with a bat or a broom.…read more

Slide 3

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To find the center of mass of an planar object use a plumb
Suspend the mass from each vertex and trace the plumb
line's location. Since the center of mass will fall below the
suspension point the center of mass will be at the
intersection of all of the plumb lines.…read more

Slide 4

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Where is the centre of mass here ?…read more

Slide 5

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The Moment of a Force (also called torque)
The moment of a force is a measure of its turning
The moment can be calculate using the following
Moment = Force × Perpendicular distance of force from
"pivot"…read more

Slide 6

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Consider a person trying to open a door, by
applying a force, of magnitude, F, as shown below.
The perpendicular distance, d, between the line of action of
the force and the pivot (the hinge of the door) is rsin
Therefore, the moment of the force is given by
Moment = Frsin…read more

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