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Gravity pulls everything down. How hard it pulls is one number. Another number stays the same wherever you go. Sam packs a bag with exactly 5 kg of stuff.
What does that 5 kg measure?
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
1 · Read
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.
Try it together
Sam packs a bag with a mass of 5 kg. On Earth, gravitational field strength is g = 10 N/kg, so the bag's weight is weight = mass x g = 5 x 10 = 50 N. If the bag's mass stayed exactly the same but it travelled to the Moon, where g is about 1.6 N/kg, its weight would drop to 5 x 1.6 = 8 N.
Read g as newtons per kilogram. On Earth, g = 10 N/kg means each kilogram is pulled with 10 newtons. To work backwards, divide: mass = weight / g. A crate weighing 120 N on Earth has a mass of 120 / 10 = 12 kg.
Good to know
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.
Reading summaries adapted from OpenStax University Physics Volume 1 (CC BY-NC-SA 4.0), openstax.org. Practice items x1-x4 are original to this dataset.
2 · Watch
3 · Play
Try it yourself
Weigh the same rock on Earth, the Moon, and Jupiter, and watch which number never moves.
Take it off screen
Print a worksheetA4 with an answer key page for grown-ups. No screen, no internet.
Where it sits
Learn first
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
Then practise
28 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.