Mass vs Weight
Distinguish between mass (amount of matter, measured in kg) and weight (gravitational force, measured in N), use the equation weight = mass × gravitational field strength, and explain why g differs on other planets and stars
What a learner can do afterwards
- Explains the difference between mass and weight with correct units for each
- Calculates weight using W = mg with g = 10 N/kg on Earth
- Predicts what would happen to an object's weight on the Moon or Jupiter
- Explains why mass stays constant but weight changes in different gravitational fields
The lesson
Mass and weight sound alike, but they are not the same thing. Mass is the amount of matter (stuff) packed into something, measured in kilograms (kg). Weight is the pull of gravity on that mass, measured in newtons (N). Your mass never changes, no matter where you go. Your weight does change, because it depends on how strong gravity is around you.
Sam has a mass of 5 kg. On Earth, gravitational field strength is g = 10 N/kg, so his weight is weight = mass x g = 5 x 10 = 50 N. If Sam's mass stayed exactly the same but he stood on the Moon, where g is about 1.6 N/kg, his weight would drop to 5 x 1.6 = 8 N.
Use the units to check yourself. Kilograms (kg) always mean mass. Newtons (N) always mean weight. If a question gives you kg and asks for N, multiply by g.
Mass (kg) is fixed and measures how much matter you have. Weight (N) is a force that changes with gravity, found using weight = mass x g.
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.