Law of Moments & Stability (Oxford AQA IGCSE Physics)
Revision Note
Written by: Leander Oates
Reviewed by: Caroline Carroll
Law of Moments
The law of moments states that
For an object that is not turning, the total clockwise moment must be equally balanced by the total anticlockwise moment about any pivot
A moment is clockwise if the force applied causes the object to move in a clockwise rotation and vice versa
Clockwise and anticlockwise moments
In the example below, the forces and distances of the objects on the beam are different, but they are arranged in a way that balances the whole system
Using the law of moments
In the above diagram:
Force causes an anticlockwise moment of about the pivot
Force causes a clockwise moment of about the pivot
Force causes an anticlockwise moment of about the pivot
Collecting the clockwise and anticlockwise moments:
Sum of the clockwise moments =
Sum of the anticlockwise moments =
Using the principle of moments, the beam is balanced when:
Sum of the clockwise moments = Sum of the anticlockwise moments
Worked Example
A parent and child are at opposite ends of a playground see-saw. The parent weighs 690 N and the child weighs 140 N. The adult sits 0.3 m from the pivot.
Calculate the distance the child must sit from the pivot for the see-saw to be balanced.
Answer:
Step 1: List the known quantities
Clockwise force (child),
Anticlockwise force (adult),
Distance of adult from the pivot,
Step 2: Write down the relevant equation
Moments are calculated using:
For the see-saw to balance, the principle of moments states that
Total clockwise moments = Total anticlockwise moments
Step 3: Calculate the total clockwise moments
The clockwise moment is from the child
Step 4: Calculate the total anticlockwise moments
The anticlockwise moment is from the adult
Step 5: Substitute into the principle of moments equation
Moment of child (clockwise) = Moment of adult (anticlockwise)
Step 6: Rearrange for the distance of the child from the pivot
The child must sit 1.5 m from the pivot to balance the see-saw
Examiner Tips and Tricks
Make sure that all the distances are in the same units and that you’re considering the correct forces as clockwise or anticlockwise.
Moments & Stability
If the line of action of the weight of an object lies outside the base of the object, there will be a resultant moment and the body will topple
Car and bus on varying inclines
Tall objects with a narrow base will topple easily
Ten-pin bowling pins are designed specifically to topple easily
The stability of objects can be increased by widening the base
High chairs are designed with a wide base so that they do not topple
Bunsen burners have a wide base to ensure they do not topple
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