The symptom
You can recite both definitions. Then a question asks for a weight, and your answer comes out in kilograms. Or a mass appears in a pressure question, and it goes straight into the equation as a force.
Each slip costs a mark. On a multi-step question it can cost more, because every later line inherits the error. The Glide Physics notes list it as a common way to lose marks in this topic.
The 0625 syllabus keeps the two ideas apart. Mass is a measure of the quantity of matter in an object. Weight is a gravitational force on an object that has mass. One is an amount of stuff; the other is a pull.
Six reasons mass and weight get mixed up
Most mix-ups come from one of these habits. Check your last marked test against the list.
The fix: six habits that keep them apart
Use these on every question that mentions mass, weight or gravity. They take seconds once they are routine.
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Ask what the quantity is
Is it an amount of matter, or a pull of gravity? An amount is a mass, in kg. A pull is a weight, in N.
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Label every value with its unit
Write “m = 75 kg” or “W = 735 N”, never a bare number. The unit tells you which quantity you are holding.
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Convert with one equation
Use W = m × g to go from mass to weight. Rearrange it to m = W ÷ g to go back. Change grams to kilograms first.
Tip. The syllabus defines g as force per unit mass, g = W ÷ m. It is the same equation, rearranged. -
Use the g the question gives
Near the Earth’s surface g is about 9.8 N/kg. If a question gives another value, such as 10 N/kg or a value for the Moon, use that one.
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Turn mass into weight before using it as a force
When an equation needs F, and the question gives a mass, write the line W = m × g first. That line can earn a method mark even if a later step goes wrong.
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Check the final unit
A weight must end in N and a mass in kg. If the unit does not match the quantity, go back to step 1.
For Extended (Supplement) candidates there is one more check. Mass is a scalar: it has size only. Weight is a vector: it has size and a direction, towards the centre of the Earth or other body.
Before and after: the same answer, fixed
Here is an original question on mass and weight, first with a typical weak answer, then with a full-mark one.
Both parts go wrong. Her mass has not changed, because she has not lost any matter. And the weight is given in kilograms, so it is not a force at all. Against the three mark points below, it scores nothing.
An astronaut has a mass of 75 kg on Earth. The gravitational field strength on the Moon is 1.6 N/kg. State her mass on the Moon and calculate her weight there.
Look at what changed. The mass stays in kilograms and keeps its value. The weight is worked out with W = m × g and ends in newtons. For comparison, on Earth she weighs 75 × 9.8 = 735 N.
Check yourself
Use the six habits on this question. Label each value with its unit before you calculate.
A rock sample has a weight of 4.9 N on Earth, where g = 9.8 N/kg. Calculate its mass. Then calculate its weight on Mars, where g = 3.7 N/kg.
Show the answer
The mass is the same on Mars: 0.50 kg
W on Mars = m × g = 0.50 × 3.7 = 1.85 N (1.9 N to 2 significant figures)
The mass ends in kg and both weights end in N. The weight changed; the mass did not.