F

d

In addition to the size of a force and its direction of

c

action, the location of the force is important. For

example, if two persons of the same weight sit on

opposite ends of a seesaw, equally distant from the

support (fig. 12-5), the seesaw will balance. However,

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if one person moves, the seesaw will no longer remain

balanced. The person farthest away from the support

will move down because the effect of the force of

his/her weight is greater.

A special case of moments occurs when two equal

and opposite forces not in the same line rotate a body.

This system of two forces, as shown in figure 12-7, is

termed a *COUPLE*. The moment of the couple is the

product of one of the forces times the distance between

them (fig. 12-8).

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The effect of the location of a force is known as the

MOMENT OF FORCE. It is equal to the force

multiplied by the distance from an axis about which

Calculation of the moment of the couple, as shown

you want to find its effect. The moment of a force is the

in figure 12-8, is as follows:

tendency of the force to produce rotation or to move the

object around an axis. Since the force is expressed in

terms of weight units, such as tons or pounds, and the

Therefore, the moment of the couple is 50 feet

moment is force times distance, the units for moment

times 12 pounds that equals 600 foot-pounds.

are expressed as foot-tons, foot-pounds, or

inch-ounces.

F = 50 LBS.

In figure 12-6 the moment of force (F) about the

axis at point a is F times d; d being called the moment

arm. The moment of a force can be measured about any

point or axis; however, the moment differs according to

12 FT

the length of the moment arm. It should be noted that

the moment of a force tends to produce rotary motion.

In figure 12-6, for example, the force F produces a

clockwise rotation. If, at the same time, an equal and

F = 50 LBS.

opposite force produces a counterclockwise rotation,

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there will be no rotation; and the body is in

12-4