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Gravity pulls Sam's bag toward the world he stands on. That pull is its weight. Sam packs exactly 5 kg of stuff. Nothing is added or removed during the trip.
What does that 5 kg measure?
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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 is the amount of matter in an object, measured in grams (g) or kilograms (kg). Weight is gravity's pull on that matter, measured in newtons (N). A change of world or gravity alone does not change mass. Adding matter increases mass: adding 1 kg to a 6 kg sample gives 7 kg. Removing matter reduces mass. If no matter is added or removed, the same object keeps its mass while its weight can change with gravity.
Try it together
Sam packs a bag with a mass of 5 kg. On Earth, gravitational field strength is g = 10 N/kg. Use weight = mass x g = 5 x 10 = 50 N to find the bag's weight. 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
Check what each unit measures. Grams (g) and kilograms (kg) measure mass. Newtons (N) measure forces, including weight. N/kg measures gravitational field strength. To calculate weight with weight = mass x g, use mass in kg and g in N/kg.
For a decimal mass on Earth, multiplying by 10 makes each digit worth ten times as much: 2.4 kg weighs 24 N. On the Moon, g is about 1.6 N/kg. Since 1.6 is 16 tenths, multiply mass by 16, then divide by 10. Sam's 5 kg bag weighs (5 x 16) / 10 = 80 / 10 = 8 N. Always use g for the place in the question.
To find mass from weight, divide by g. When g is 1.6 N/kg, multiply both numbers in the division by 10 to make the divisor a whole number. A box weighing 19.2 N on the Moon has mass 19.2 / 1.6 = 192 / 16. Since 16 x 12 = 192, its mass is 12 kg. Check by multiplying: 12 x 1.6 = 19.2 N.
Mass (kg) measures matter and stays the same when none is added or removed. Weight (N) is gravity's pull: weight = mass x g.
The mass-and-weight teaching, worked calculations and scenes were written for this dataset. The explanations about adding and removing matter and the revised assessment questions were also written for this dataset.
Includes readings adapted from Khan Academy records. The linked videos are Gravitational force by Khan Academy and Intro to Mass Vs. Weight for Middle School and Older Students. by Test Prep LLC. The video links were selected for this dataset. Calculating weight on Earth and the Moon, Why your weight changes between worlds, and the four extra questions were written for this dataset. Clarifications about unchanged matter were written for this dataset.
More to read
Mass & weight review
Mass and weight are not the same thing. Mass is the amount of matter in an object. It is measured in kilograms (kg). Moving to a different planet alone does not change mass. If no matter is added or removed, a 60 kg person has 60 kg of mass on Earth, on the Moon, and on Jupiter.
Weight is the force of gravity on an object. It is measured in newtons (N). Weight changes with gravity. On Earth, gravity is about 9.8 N/kg (often rounded to 10 N/kg). Weight = mass x gravity. On the Moon, gravity is about 1.6 N/kg. On Jupiter, gravity is about 25 N/kg. With the amount of matter unchanged, mass stays the same while weight changes with gravity.
Calculating weight on Earth and the Moon
Calculate weight by multiplying mass by gravitational field strength: W = m x g. Here, W names the weight (newtons), m the mass (kilograms), and g the gravitational field strength (N/kg).
Using Earth's rounded value g = 10 N/kg, multiply each mass by 10. This gives 10 N for 1 kg and 700 N for a person of 70 kg.
Use about 1.6 N/kg for the Moon instead: 70 x 1.6 = 112 N. If no matter is added or removed, the person's mass remains 70 kg, but their weight is smaller.
Why your weight changes between worlds
Gravity is a pull between objects with mass. The strength of that pull depends on their masses and how far apart they are. Worlds have different masses and sizes, so their surface gravity can differ.
Gravitational field strength, called g, tells you the pull on each kilogram. Its unit is newtons per kilogram, written N/kg. Near Earth's surface, we often round g to 10 N/kg. Near the Moon's surface, it is about 1.6 N/kg.
To calculate weight, multiply mass in kilograms by g in N/kg. The answer is a force in newtons. The same object can therefore weigh less on the Moon while keeping the same amount of matter. Moving it does not remove matter from it.
Gravitational force
Astronauts in orbit feel weightless, but they still have mass. They are in free fall, constantly falling toward Earth but moving forward fast enough to miss it. The scale reads zero because both the astronaut and the scale are falling together. But if they hit something, they would feel the impact. If no matter is added or removed, their mass stays the same as on Earth. Their weight is not zero: gravity is still pulling them, even though they do not feel it.
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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.