Back to: O level physics notes uganda
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).
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
Example;1
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.
Solution
Taking moments about the Hinge; 𝑀𝑜𝑚𝑒𝑛𝑡 𝑜𝑓 𝐹𝑜𝑟𝑐𝑒 =𝐹𝑜𝑟𝑐𝑒,𝐹×𝑃𝑒𝑟𝑝.𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝐶𝑙𝑜𝑐𝑘𝑤𝑖𝑠𝑒 𝑚𝑜𝑚𝑒𝑛𝑡=12×0.8 𝐶𝑙𝑜𝑐𝑘𝑤𝑖𝑠𝑒 𝑚𝑜𝑚𝑒𝑛𝑡=9.6𝑁𝑚
Example;2
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.
Solution
Taking moments about the Hinge; Moment of Force =Force,F×Perp.distance 4 =F×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.