A force changes shape, speed or direction; forces are vectors
A force is a push or pull (newtons, N) and can change a body's SHAPE, SPEED or DIRECTION. Types to recognise: weight, friction, air resistance, tension, reaction and driving/thrust. Force is a VECTOR (magnitude AND direction); mass and speed are SCALARS. Forces along one line combine into a single RESULTANT — add same-direction, subtract opposing. A zero resultant means balanced forces (rest or constant velocity, Newton's First Law); a non-zero resultant causes acceleration.
F = m × a and W = m × g; mass (kg) vs weight (N)
Newton's Second Law: resultant force = mass × acceleration, F = m × a (F in N, m in kg, a in m/s^2); the acceleration is in the SAME direction as the resultant force. Weight: W = m × g, with g = 9.8 N/kg near the Earth's surface. Keep the quantities distinct: MASS is the amount of matter (a scalar, in kg, the same everywhere); WEIGHT is the force of gravity on that mass (a vector, in N, smaller where g is smaller). Never give a weight in kg or a mass in N.
Stopping distance = thinking distance + braking distance
Stopping distance = thinking distance + braking distance. THINKING distance = distance travelled during the driver's reaction time (before braking); it increases with higher SPEED and anything that lengthens reaction time — tiredness, alcohol or drugs, distraction. BRAKING distance = distance travelled while braking; it increases with higher SPEED, greater MASS, worn brakes or tyres, and wet or icy roads. Higher speed increases both, so stopping distance rises steeply.
Drawn from real examiner reports.
Mass is not weight (kg vs N)
Blurring mass and weight is heavily penalised. MASS is the amount of matter — a scalar, in kilograms (kg), the same everywhere. WEIGHT is the force of gravity on that mass — a vector, in newtons (N), and it changes if g changes (smaller on the Moon). "How heavy something is" is not a definition of weight, and a weight quoted in kg loses the mark.
Convert grams to kg for W = mg
A mass given in grams must be converted to kilograms before substituting into W = mg — leaving it in grams caps the score at 1 mark and produces a power-of-ten error. The same mass-unit conversion also trips up F = ma. Convert to SI base units (kg) first, then substitute; weight and force then come out in newtons.
W = mg: Jun 2024 1P Q10a, Jun 2023 1P Q9a (mass not converted, power-of-ten error, capped at 1 mark). F = ma: Jun 2024 1P Q5b(ii) (mass-unit conversion caused a power-of-ten error).
Terminal velocity is not "no force"
At terminal velocity it is wrong to write "no force acts" or "there is no gravity". Weight still acts down and air resistance still acts up — they are EQUAL, so the RESULTANT force is zero. A zero resultant does not mean zero speed: by Newton's First Law the object moves at a constant (terminal) velocity. Always say "balanced forces / zero resultant force", never "no force".
Jun 2024 1P Q12c; Jun 2024 1PR Q9c(ii): candidates recognised the steady speed but wrote "no force acts" instead of "resultant force is zero / forces balanced".
Balanced forces = equal-length arrows
On a free-body diagram, balanced forces (constant or terminal velocity) must be drawn as opposing arrows of EQUAL length; unequal arrows imply the candidate thinks the forces are unbalanced. Each arrow needs the correct direction, a roughly correct length and a named force. "Downthrust" is not an accepted force name.
Nov 2024 1P Q7a(ii) (equal-length arrows); Nov 2024 1P Q7a(i) ("downthrust" not accepted); Jun 2024 1PR Q3b (arrow needs direction, length and a named force).
Thinking vs braking distance
Do not mix up which factor changes which part. THINKING distance (travelled during the reaction time, before braking) is lengthened by tiredness, alcohol or drugs, and distraction. BRAKING distance (travelled while the brakes act) is lengthened by greater mass, worn brakes or tyres, and wet or icy roads. Higher speed increases BOTH parts.
Larger mass gives smaller acceleration
For a fixed resultant force, a larger mass gives a SMALLER acceleration, because a = F/m. Do not assume a heavier object always accelerates more — that is only true if the force also rises. This is the mass side of F = ma, distinct from weight: mass is what resists a change in motion (inertia).
Resultant force first, then F = ma
Before using F = m × a, combine the forces along the line to find the RESULTANT (add same-direction, subtract opposing) — it usually carries its own mark. Then substitute and rearrange for the unknown. Do not put a single driving force into F = ma when other forces also act.
Explain as a chain, not a jump
"Explain" marks reward a linked chain, not a leap to the conclusion. Show each step in terms of forces: as speed rises, air resistance rises, so the resultant force falls, so the acceleration falls. Restating the question, or jumping straight to the answer, scores nothing.
Check if terminal velocity is reached
Read the stem carefully: if it states that terminal velocity is NOT reached, any "air resistance = weight" or "balanced forces" answer is irrelevant and earns nothing. Only invoke terminal velocity when the object is actually at its constant maximum speed.
Let the mark tariff guide the answer
Use the number of marks as a guide to how many distinct points to give, and match the command word: "describe" states what happens, "explain" needs reasons. A 4-mark question needs four separate creditworthy points; repeating the wording of the question is never one of them.
where = resultant force (N), = mass (kg), = acceleration (m/s²). The acceleration is in the same direction as the resultant force.
where = weight (N), = mass (kg), = gravitational field strength N/kg near the Earth's surface.
Two-element definitions to learn exactly (vague wording scores zero):
| Term | Precise (mark-scheme) definition |
|---|---|
| Force | A push or a pull; a vector; measured in newtons (N) |
| Mass | A measure of the amount of matter in a body; a scalar; measured in kg; same everywhere |
| Weight | The force acting on a body due to gravity; a vector; ; measured in N |
| Resultant force | The single force that has the same effect as all the forces acting on a body combined |
| Scalar | A quantity with magnitude only (e.g. mass, speed, distance) |
| Vector | A quantity with magnitude and direction (e.g. force, weight, velocity) |
| Friction | A force that opposes the relative motion of two surfaces in contact |
| Terminal velocity | The constant maximum velocity of a falling object, reached when air resistance equals weight (resultant force = 0) |
Define weight and state its unit.
A student has a mass of 50 kg. The gravitational field strength is N/kg.
Calculate the student's weight. (2 marks)
