Moment of a force is also called the measure of the turning effect of a force.

Moment of a force is a product of a force and the perpendicular distance of the line of action of the force from the fulcrum (pivot).

image 234

SI unit of moment is a newton meter (Nm). Moment of a force is a vector quantity.

Examples of the turning effects of a force;
Opening or closing a door
Children playing on a see-saw
Bending of the fore arm of a hand

A force of 12N is applied to open a door handle, which is 0.8m from the hinges of the door. Calculate the moment of the force produced.

image 235

Taking moments about the Hinge; π‘€π‘œπ‘šπ‘’π‘›π‘‘ π‘œπ‘“ πΉπ‘œπ‘Ÿπ‘π‘’ =πΉπ‘œπ‘Ÿπ‘π‘’,πΉΓ—π‘ƒπ‘’π‘Ÿπ‘.π‘‘π‘–π‘ π‘‘π‘Žπ‘›π‘π‘’ πΆπ‘™π‘œπ‘π‘˜π‘€π‘–π‘ π‘’ π‘šπ‘œπ‘šπ‘’π‘›π‘‘=12Γ—0.8 πΆπ‘™π‘œπ‘π‘˜π‘€π‘–π‘ π‘’ π‘šπ‘œπ‘šπ‘’π‘›π‘‘=9.6π‘π‘š

The moment of a force is 4Nm in the clockwise direction when the lid of a tin is opened. Calculate the vertical force applied, if the perpendicular distance from the hinges is 40cm.


image 236

Taking moments about the Hinge; Moment of Force =Force,FΓ—Perp.distance 4 =FΓ—0.4 F =40/4N F =10N
Thus the vertical force applied is 10N

From above, it can be noted that:

The greater turning effect of a force occurs when the force acts on an object at a right angle.

It is easier to close the door by pushing it at a point as far away from the hinges as possible. Because the force applied can easily balance with the reaction at the hinges.