The moment of a force is the tendency of that force to cause rotational acceleration rather than linear acceleration.
The moment or torque of a force is calculated like this:
T = F × d
Where F = magnitude of force;
d = perpendicular distance from point of rotation
Torque is measured in Nm.
For example: A girl pulls on door open using the handle on one side of the door. If she opens the door with a force of 3N perpendicular to the door and the handle is at 1.3m from the hinges, what is the torque on the door about the hinges?
T = F × d
= 3N × 1.3m
= 3.9 Nm
Principle of Moments
Moments can cause a an object to turn clockwise or anticlockwise. So, if a clockwise turn is a positive moment, and an anticlockwise turn is negative, then the total moment acting on an object is the sum of both the clockwise and anticlockwise moments.
When an object is in equilibrium or is completely balanced, the total moment acting on it is zero. So, the principle of moments states that the sum of all clockwise moments (positive) must be equal to the sum of all anticlockwise moments (negative). This is written like this:
Applying the principle of moments:
Two children are balanced on a see-saw. The one on the left has a mass of 60 kg and sits 1.5 m from the pivot. If the other one sits 1m away from the pivot, what is their mass? Use gravity to be 10 N/kg.
Let's first find the force exerted by the child on the left by calculating his weight:
W = mg
= 60 kg × 10 N/kg
= 600N
We can then use this information to draw a diagram to help us understand:
This is a problem involving an object in balance. So, the total anticlockwise and clockwise moments must be equal. Thus, we can calculate the anticlockwise moment and equate it to the clockwise moment:
Therefore, the child on the right has a mass of 90 kg.