Moments

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Forging new generations of engineers
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Moments
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Moments
A moment of a force is a measure of its tendency
to cause a body to rotate about a point or axis.
It is the same as torque. A moment (M) is
calculated using the formula Moment Force
Distance M F D
Always use the perpendicular distance between
the force and the point!
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Moments
What distance would you use here?
Moment Force Distance M F
D
F 20 lb
D 2 ft Find the moment about point P
M F D M (20 lb) (2 ft) M 40 ft-lb
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Moments
Moment Force Distance M F
D
Can you visualize the result of the force
acting on the beam?
The beam has a tendency to rotate clockwise about
point P.
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Moments
Moment Force Distance M F
D
F 20 lb
D 0.5 ft Find the moment about point P
M F D M (20 lb) (0.5 ft) M 10 ft-lb
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Moments
Can you visualize the result of the force
acting on the beam?
Moment Force Distance M F
D
The beam has a tendency to rotate clockwise about
point P.
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Moments
You have seen how a see-saw will balance on its
fulcrum if equal weight is applied at both
ends. If two people of unequal weight are on the
see-saw, it will rotate about its fulcrum to the
side with the heavier person.
Fulcrum
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Moments
A see-saw can also be balanced by moving the
smaller person further from the fulcrum. If a
45-pound child sat on one end 3 ft from the
fulcrum, then how far from the fulcrum would a
30-pound child have to sit in order for the
see-saw to be balanced?
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Moments
M1 M2
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Moments

• Typically it is assumed
• A moment with a tendency to rotate counter
  clockwise (CCW) is considered to be a positive
  moment.
• A moment with a tendency to rotate clockwise (CW)
  is considered to be a negative moment.