9702 · 4.2
Equilibrium of Forces flashcards
Revision flashcards for Cambridge 9702 Equilibrium of Forces (syllabus 4.2). Flip, recall, then mark a real past-paper question.
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What two conditions define complete equilibrium?
Resultant force is zero and resultant moment (torque) about any point is zero.
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State the principle of moments.
For rotational equilibrium, sum of clockwise moments = sum of anticlockwise moments about the same pivot.
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How is moment calculated?
M = F × perpendicular distance from pivot to the line of action of F (unit: N m).
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When do three coplanar forces show equilibrium graphically?
Their vector diagram drawn tip-to-tail forms a closed triangle (resultant = 0).
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What is a couple?
A pair of equal, parallel, opposite forces on different lines — produces rotation only, no translation.
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Torque of a couple?
τ = F × perpendicular distance between the two lines of action.
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What is the centre of gravity?
The single point through which the entire weight of an object can be considered to act. For a uniform object, it's at its geometric centre.
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What is a strategic choice for the pivot point when solving equilibrium problems?
Choose a point through which one or more unknown forces act. This makes their moment about that pivot zero, simplifying the moment equation.