Lesson 4, Topic 3
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# Moment of a Force

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When taps are opened, a turning effect of force is experienced, likewise, when doors are opened, the applied force brings about a turning effect about a point or hinges attached to the wall of the door.

The turning effect experienced in each case is called the moment of a force.

The moment of a force about a point (or axis )O, is the turning effect of the force about that point. It is equal to the product of the force and the perpendicular distance from the line of action to the point or pivot.

Moment = Force x Perpendicular distance of pivot to the line of action of the force

= Newton x Metre

Its unit is Newton metre (Nm), hence, it is a vector quantity.

If the force is inclined at an angle θ.

Moment = Fdsinθ

The magnitude of moments depends on:

i) The Force applied

iI) The perpendicular distance from the pivot to the line of action of the force.

### Resultant Moments:

When more than two forces act on a body, the resultant moment on the body about any point can be obtained using algebraic moments using the clockwise moment and anticlockwise moments about the same point.

If the clockwise moment is taken as positive and the anticlockwise moments are negative.

$$\scriptsize F_1 \: \times \: X_1 = F_2 \: \times \: X_2$$
$$\scriptsize F_1 \: \times \: X_1 \: -\: F_2 \: \times \: X_2 = 0$$