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⚖️ Mass and Weight

They sound like the same thing — but they're not. Learn why mass stays fixed while weight changes, and how the formula W = m × g links them.

3
lessons
~15 min
to learn
🔬 Science
subject
Ages 6–12
level
Start the course →

What you’ll learn

  1. Mass: How Much StuffDefine mass as the amount of matter, measured in kilograms, and explain that it never changes.Mass is the amount of matter in an object, measured in kilograms. The more material an object contains, the greater its mass. Mass does not change when you move an object — a 5 kg bag is 5 kg on a mountain, underwater, or on the Moon, because none of its matter is lost.
  2. Weight: Gravity's PullDefine weight as the force of gravity on a mass, measured in newtons, and contrast it with mass.Weight is the force of gravity pulling on an object's mass, so it is measured in newtons, not kilograms. Unlike mass, weight depends on gravity and can change from place to place. Scales really measure this downward force, even when they display a kilogram number.
  3. Calculating WeightUse W = m × g to calculate weight and explain why weight differs on the Moon.Weight is calculated with W = m × g, where g is the gravitational field strength — about 9.8 N/kg on Earth. So a 5 kg bag weighs about 49 N on Earth. On the Moon, g is only about 1.6 N/kg, so the same 5 kg bag weighs about 8 N: the mass is unchanged, but the weaker gravity means less weight.

Questions this course answers

What does mass measure?

Mass is the amount of matter (stuff) in an object, measured in kilograms.

A 5 kg bag is taken to the Moon. What happens to its mass?

Mass is the amount of matter and none is lost on the trip, so it stays 5 kg everywhere.

Weight is measured in which units?

Weight is a force — the pull of gravity — and forces are measured in newtons.

Which statement is true?

Mass is fixed matter, while weight depends on gravity, so weight can change from place to place.

Using W = m × g with g = 9.8 N/kg, what is the weight of a 10 kg object on Earth?

Multiply mass by g: 10 kg × 9.8 N/kg = 98 N.

The same object weighs less on the Moon than on Earth because...

Weight is mass × g, and g is smaller on the Moon, so the same mass has less weight there.

Grounded in trusted sources

  • BBC Bitesize
  • Britannica
  • The Physics Classroom
  • NASA

Every Wunder lesson is built from real, reputable sources — never invented.

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