What is the principle of moments and how do I solve lever problems in ICSE Physics?
GradeHunt_TeamGradeHunt Team · · ICSE 10th
ICSE Class 10 Physics Force chapter: how to apply the principle of moments to a metre rule balanced on a knife edge? I keep making mistakes with distances.
principle of momentsmoment of forceleversicse physics
1 answer
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Moment of force = Force x perpendicular distance of its line of action from the pivot.
SI unit: newton metre (N m).
Principle of moments:
When a body is in equilibrium under several forces, the sum of anticlockwise moments about the pivot equals the sum of clockwise moments.
Steps for a metre rule problem:
1. Mark the pivot (knife edge) clearly.
2. Measure every distance from the pivot, not from the zero end of the rule.
3. Decide for each force whether it turns the rule clockwise or anticlockwise.
4. Include the weight of the rule acting at its centre of gravity (the 50 cm mark for a uniform rule) if it is not supported there.
5. The upward reaction of the knife edge acts at the pivot, so its moment about the pivot is zero; leave it out of the equation.
6. Equate the two sums.
Example:
A uniform metre rule is balanced at the 40 cm mark when a 20 gf weight hangs at the 10 cm mark. Find the mass of the rule.
- Pivot at 40 cm.
- 20 gf at 10 cm: distance 30 cm, anticlockwise moment = 20 x 30 = 600 gf cm.
- Weight of rule W at 50 cm: distance 10 cm, clockwise moment = 10W.
- 10W = 600, so W = 60 gf. Mass of the rule = 60 g.
Common mistakes:
1. Using the scale reading (10 cm) instead of the distance from the pivot (30 cm).
2. Forgetting the rule's own weight when the pivot is not at 50 cm.
Exam tip: draw the rule with the pivot, forces and distances marked before writing any equation. A neat diagram helps you avoid most distance mistakes, and it keeps your method clear to the examiner even if the arithmetic slips.
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