Not given in the exam, recall it.
What it is for
Weight is used in Force and Motion I (SPM Physics Form 4). Keep the SI units in every line of working. Force and Motion I
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| W | weight | N |
| m | mass | kg |
| g | gravitational acceleration | m s⁻² |
Rearrangements
m = W / gg = W / m
Worked example
Common trap
Understanding Weight
Weight is the gravitational force pulling an object downward, given by W = mg.
In this equation W is the weight in newtons (N), m is the mass in kilograms (kg), and g is the gravitational acceleration (gravitational field strength) in metres per second squared (m s⁻²). Mass measures how much matter an object contains and never changes; weight depends on where the object is, because g varies.
The two quantities relate because gravity gives every kilogram of mass the same acceleration g, so the total pull is mass × g. To find mass from a measured weight, rearrange to m = W / g; to find the field strength, use g = W / m.
A worked example, step by step
An astronaut with mass m = 50 kg stands on the Moon, where g = 1.6 m s⁻². Find her weight there.
Write the formula and substitute with units:
W = mg = 50 kg × 1.6 m s⁻²
W = 80 N.
Her mass is still 50 kg on the Moon, but her weight is only 80 N, compared with about 490 N on Earth (where g ≈ 9.8 m s⁻²). This is the clearest way to see that weight, unlike mass, changes with location because it depends on the local value of g.
The answer is given to two significant figures with the correct unit, the newton.
Common mistakes and how this is tested
The headline mistake is confusing weight with mass: weight is a force in newtons, mass is in kilograms, and they are not interchangeable.
Other slips: leaving mass in grams instead of kilograms; using g = 9.8 m s⁻² on the Moon or another planet when a different value is given; and writing the unit as kg. Remember weight also changes slightly between the equator and the poles on Earth because g is not perfectly uniform.
This appears under define (state that weight is the gravitational force on a mass), calculate (find W, m or g), and explain (why an object weighs less on the Moon yet keeps the same mass).
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)