What this covers
Weight is part of the Force and Motion I chapter. We teach it the way it is tested: the concept in plain English, then a worked example, then a question the student tries while the teacher checks the method.
Formulas you may need
How it is examined
It can appear in Paper 1 (objective) and Paper 2 (structured), and where an experiment applies, in Paper 3. We do not predict which questions appear; we prepare the technique for all of them.
A common mistake
How to study it
Learn the definition precisely, practise one or two SPM-style questions with full working, and link it to the rest of Force and Motion I. If it keeps costing marks, a one-to-one lesson fixes exactly that.
Force and Motion I · Formulas · Exam Papers
What you need to know
Weight is the gravitational force acting on an object, calculated as W = mg, where W is weight in newtons (N), m is mass in kilograms (kg), and g is the gravitational field strength, or acceleration due to gravity, 9.81 m s⁻² near Earth's surface. Weight is a vector quantity, always directed towards the centre of the Earth, and it is measured using a spring balance or a newton meter, not a beam balance, since a beam balance actually compares mass.
Mass and weight are frequently confused but are fundamentally different quantities. Mass is the amount of matter in an object, measured in kilograms, and it remains constant wherever the object is located, since it does not depend on gravity.
Weight, however, depends on the local gravitational field strength and therefore changes from place to place; for example, an astronaut's mass stays the same on the Moon as on Earth, but their weight is smaller on the Moon because the Moon's gravitational field strength is weaker than Earth's. For a mass of 5 kg on Earth, the weight is W = mg = 5 kg × 9.81 m s⁻² = 49.05 N (often rounded to 49.1 N).
Worked example
Question: An astronaut has a mass of 70 kg. Calculate the astronaut's weight on Earth, where g = 9.81 m s⁻², and on the Moon, where the gravitational field strength is 1.62 m s⁻².
On Earth: W = mg = 70 kg × 9.81 m s⁻² = 686.7 N. On the Moon: W = mg = 70 kg × 1.62 m s⁻² = 113.4 N. The astronaut's mass remains 70 kg in both locations because mass does not depend on gravity, but the weight on the Moon is much smaller than on Earth because the Moon's gravitational field strength is weaker. This shows clearly that mass is a fixed property of the astronaut's body, while weight depends entirely on the surrounding gravitational field.
How it is examined
Paper 1 objective items commonly ask candidates to calculate weight using W = mg, distinguish mass from weight, or identify the correct instrument for measuring weight, a spring balance or newton meter, as opposed to mass, measured with a beam balance. Paper 2 structured and essay questions frequently require candidates to calculate weight given mass and gravitational field strength, explain why weight changes on different planets or moons while mass does not, or interpret a scenario involving an object in free fall or on an incline in terms of its weight component.
Paper 3 practical work may involve using a spring balance to measure the weight of objects and relating the readings to their known masses.
Common mistakes include treating mass and weight as interchangeable, giving weight in kilograms instead of newtons, and using the wrong value of g when a question specifies a location other than Earth. Candidates should always state units clearly, mass in kg and weight in N, and should remember that although g = 9.81 m s⁻² is standard for Earth, some questions give a different local value that must be used instead of the memorised Earth value.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)