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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!

Moment = Force * Distance

M = F * D

What distance would you use here?

2 ft

F = 20 lb

D = 2 ft

Find the moment about point P:

M = F * D

M = (20 lb) * (2 ft)

M = 40 ft-lb

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.

Moment = Force * Distance

M = F * D

2 ft

0.5 ft

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

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.

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

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?

3 ft

45 lbs 30 lbs

x

45 lbs * 3 ft = 30 lbs * x

135 ft-lbs = 30 lbs * x

30 lbs 30 lbs

4.5 ft = x

3 ft

45 lbs 30 lbs

x

MM11 = = MM22

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.