Force — Part 2: Equilibrium, centre of gravity and circular motion: visual lesson, ICSE Class 10 Physics (Science Paper 1)

Lesson

Balance and going round

Part 2 — when do forces cancel so that a body stays at rest, where does the weight of a body act, and why does circling at constant speed still need a force?

In this lesson you will

State the conditions of equilibrium and the principle of moments
Use the principle of moments to find an unknown force or distance
Locate the centre of gravity of regular bodies and an irregular lamina
Explain uniform circular motion and distinguish centripetal from centrifugal force

Table of everyday situations and the idea each illustrates
Situation	Idea used
A balanced see-saw	Principle of moments
A book resting on a table	Equilibrium of forces
Balancing a ruler on one finger	Centre of gravity
A stone whirled on a string	Centripetal force

Concept

Conditions of equilibrium

A body is in equilibrium when it stays at rest (static) or moves with constant velocity without turning (dynamic). Two conditions: (1) the resultant force is zero — no translational acceleration; (2) the resultant moment about any point is zero — sum of anticlockwise moments = sum of clockwise moments, so no rotational acceleration. Static examples: a book on a table, a balanced see-saw; dynamic: a raindrop falling at constant (terminal) speed.

Table of the two conditions of equilibrium
Condition	Prevents	Equation
Resultant force = 0	Translation	Upward forces = downward forces
Resultant moment = 0	Rotation	Anticlockwise moments = clockwise moments

Diagram

The principle of moments

Principle of moments: when a body is in equilibrium under several forces, the sum of the anticlockwise moments about any point equals the sum of the clockwise moments about that point. Verification: balance a metre rule on a knife edge, hang weights on both sides, adjust until the rule is horizontal, and compare W₁ × d₁ with W₂ × d₂. In the figure, 40 gf × 20 cm = 20 gf × 40 cm = 800 gf cm.

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