The Moment of a Force (also called torque)

One force on its own isn't much use to us. We normally look at situations where turning effects are balanced (or not!). The turning effect of a force about a pivot is known as its moment about that point.

A moment is NOT a period of time. Nor is it the same as momentum.

The moment of a force is a measure of its turning effect.

Moment = Force × Perpendicular distance of force from "pivot"

Moment =F x d

Where,

F is the force (in N)

d is the perpendicular distance from the line of action of the force to the pivot (in m)

The SI unit is the newton - metre (Nm)

Moment of force is a vector(has magnitude and direction).The direction of the moment can be either clockwise or anticlockwise.

When presenting answers, we need to state:

1.Magnitude in Nm

2.Direction (either clockwise or anti-clockwise)

Some Examples:

When there is more than one force , there is more than one moment about the pivot,

The resultant moment is computed by adding all the moments in the clockwise

direction and subtracting the moments in the anticlockwise direction.

Question 1

A wheel nut is tightened to a moment of 100 Nm. A motorist has to undo the nut with a wheel wrench which is 0.40 m long. What force must he apply?

Answer

Question 2

How can the force applied be reduced?

Answer

Consider a person trying to open a door, by applying a force, of magnitude, F, as shown below.

The two obvious changes the person could make in order to open the door more easily are

i) he/she could increase the distance "r"

ii) he/she could push at 90° to the door.

If the angle between the line of action of the force and the door is 90°, we have

and it is clear that this is the maximum value of the turning effect for a given force.

Why are door knobs located at the far edge of the door?

It is easier to close a heavy door by applying a force as far away as possible from the hinges, hence is larger and the moment, Fd is greater.

Why are hammers so long?

You just require a small force to pull out a nail when you use a hammer with a long handle. Again the principle of generating a large moment with a large value of d, the distance of the force from the pivot is large, even though the force is small.

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