Edexcel IAL Physics revision · AS — Mechanics
Forces, moments and equilibrium
IGCSE moments with the safety rails removed: forces now act at angles, so every distance must be genuinely perpendicular, and equilibrium problems become geometry with physics attached.
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What the syllabus demands
- —Define and calculate the moment of a force and the torque of a couple
- —Apply the principle of moments with forces at angles
- —State the conditions for equilibrium: zero resultant force and zero resultant moment
- —Use the triangle of forces for three coplanar forces in equilibrium
- —Distinguish centre of gravity from centre of mass
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Moment of a force
- The product of the force and the perpendicular distance from the pivot to the line of action of the force.
- Couple
- A pair of equal, antiparallel forces whose lines of action do not coincide — producing rotation only, with zero resultant force. Torque of a couple = one force × perpendicular separation.
- Centre of gravity
- The single point at which the entire weight of a body may be considered to act.
The equations
More equations to practise: the Edexcel IAL formula sheet.
Where the marks die
Common mistakes to check
- 01
Using the straight-line distance instead of the perpendicular distance when the force is at an angle. Either resolve the force or find d cos θ — draw the line of action first.
- 02
Forgetting the second equilibrium condition. Zero resultant force alone is not equilibrium: a couple has zero resultant force and still rotates. Both conditions, both marks.
- 03
Taking moments about a random point when a cleverer pivot kills an unknown. Take moments about the point where the most unknown forces act — their moments vanish.
- 04
In ladder and hinge problems, forgetting the hinge or wall reaction entirely. List every force on the body before writing a single equation.
One worked example, done properly
Question
A uniform beam of weight 200 N and length 4.0 m is hinged at a wall and held horizontal by a vertical cable at its far end. Find the tension in the cable.
Method
- 1.Take moments about the hinge (eliminates the unknown hinge force).
- 2.The beam's weight acts at its centre, 2.0 m from the hinge: clockwise moment = 200 × 2.0 = 400 N m.
- 3.The cable acts at 4.0 m: anticlockwise moment = T × 4.0.
- 4.Equilibrium: 4.0T = 400.
T = 100 N