Weight is the gravitational force on a mass
Weight is the gravitational force on an object — a vector in N, (on Earth N/kg). Two elements: (1) gravitational force, (2) acting on an object/mass. Mass is the amount of matter in kg, the same everywhere; weight changes with location because does. Wrong forms that score zero: "how heavy it is", "gravitational force per unit mass" (that is ), and the vague "pull of gravity". Mini-example: a 3.0 kg rock weighs N on Earth.
Moment = force × perpendicular distance
The moment (turning effect) of a force about a pivot: , where is the perpendicular distance from the pivot to the line of action of the force (m). Unit: N m — never J. Two elements: force in N, perpendicular distance in m. Principle of moments: sum of clockwise moments = sum of anticlockwise moments about the pivot; use it to find an unknown force or distance. Mini-example: 25 N at 0.80 m gives N m.
Newton's First Law and resultant force
Newton's First Law: with zero resultant force an object stays at rest OR moves at constant velocity — never "stops". A non-zero resultant changes speed or direction. Resultant force is a vector: add forces along a line, subtract opposing ones, and state the direction. Free-body diagrams show every force on ONE object as a labelled arrow, length proportional to size. (E) Newton's Second Law: uses only the RESULTANT force ( in m/s²).
Drawn from real examiner reports.
Weight is not "force per unit mass"
"Weight is the gravitational force per unit mass" defines gravitational field strength (N/kg), not weight. The accepted form is "the gravitational force on an object" — both nouns, "gravitational force" and "object", are required. "Pull of gravity" is too vague (it names no object), and "how heavy it is" is everyday language.
June 2024 Paper 1 Q4: many chose the option defining weight as "gravitational force per unit mass" rather than "gravitational force on an object". June 2024 Paper 4 Extended Q1(a)(ii) accepted only the "gravitational force on a mass/object" form.
Moment: "perpendicular" and from the pivot
Two errors lose the moment mark: (1) writing "force × distance" without perpendicular — Cambridge needs the perpendicular distance from the pivot to the line of action; (2) measuring that distance to the centre of gravity instead of the pivot. Draw the perpendicular from the pivot, not from the beam's centre.
November 2023 Paper 3 Q2(c): the majority omitted "perpendicular" from moment = force × distance, and many drew it to the centre of gravity rather than to the line of action of the force.
No resultant force means constant velocity
A force is NOT needed to keep an object moving. With zero resultant force an object keeps moving at constant velocity — it does not stop. So "an object moves at constant speed; what is the resultant force?" has the answer 0 N, not "a forward force". At terminal velocity, air resistance = weight, resultant = 0, and the object falls at constant speed.
June 2024 Paper 3 Q2(b)(iii): many candidates believed "no force means no movement" and answered "the object stops" when the resultant force became zero; only stronger candidates gave "constant speed".
Moment unit is N m, never joules
The unit of a moment is the newton metre (N m), never the joule — even though 1 N m = 1 J dimensionally, Cambridge rejects J for moments. Do not write N/m either (that is force per metre). Work done also equals force × distance, but it uses the distance moved along the force and its unit is J; a moment uses the perpendicular distance from the pivot and its unit is N m.
State the direction of a resultant force
A resultant force is a vector, so Cambridge wants its magnitude AND its direction. For forces along a line, add those in one direction and subtract the opposing ones, then say which way the resultant points. A bare number, or a hand-wave at "wind/water/waves" with no direction, drops the direction mark.
June 2024 Paper 3 Q2(a): candidates gave vague reasons ("because of wind/water/waves") without specifying the direction of the resultant force, which the examiner required.
Moments: M = Fd, perpendicular, N m
For a moment, write first, identify the distance that is perpendicular to the line of action of the force (measured from the pivot, not the centre of gravity), substitute, and give the unit as N m — never J.
Explain motion: forces, then a law
For "describe/explain the motion", write two sentences: (1) which force is bigger, so the resultant is forward, zero or backward; (2) apply a law — Newton's First (zero resultant → constant velocity) or (E) Newton's Second (a resultant → acceleration in its direction).
(E) Equilibrium needs two conditions
(E) For full equilibrium, state BOTH conditions: (1) the resultant force = 0 (forces balance), and (2) the resultant moment about any pivot = 0 (clockwise moments = anticlockwise moments). Giving only one condition loses marks at Extended level.
| Quantity | Symbol | Formula | Unit |
|---|---|---|---|
| Weight | newton, N | ||
| Gravitational field strength (Earth) | — | 10 N/kg | |
| Moment of a force | newton metre, N m (not J) | ||
| Resultant force (collinear) | (opposite directions) | newton, N | |
| (E) Newton's Second Law | N (where in kg, in ) |
Where = mass (kg), = gravitational field strength (N/kg), = perpendicular distance from pivot to line of action of force (m), = force (N), = acceleration ().
Define weight. Give the formula and the unit.
Describe an experiment to determine the position of the centre of gravity of an irregularly shaped flat piece of card (a lamina). Your description should include:
