Further Maths · Further Mechanics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A rigid body has size, so a force can turn it as well as push it. The turning effect of a force about a point is its moment: the force multiplied by the perpendicular distance from the point to the line of action of the force.
The whole weight of a body can be treated as one force acting at its centre of mass. For a uniform body this point lies on every line of symmetry. For a composite body, replace each part by a particle at its own centre of mass and take moments.
A body is in equilibrium when the forces balance in every direction and the moments about any point balance. On a slope a body may slide or topple, so test which happens first.
Students multiply the force by the distance to the point where it acts, not by the perpendicular distance to its line of action. If the force is at an angle, first resolve it or use Fd sin θ.
Answer: (2, 3)
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