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Mass is the amount of matter in an object and is measured in kilograms. Weight is the gravitational force acting on that mass, calculated by W = mg, where g is the acceleration due to gravity (about 9.8 m/s² on Earth). Mass stays the same no matter where you are, but weight changes depending on the strength of gravity at your location.
Compare the mass of an object on a balance scale (which compares masses) with its weight on a spring scale (which measures gravitational force). Discuss how an astronaut's mass stays the same on the Moon but their weight drops to about one-sixth.
In everyday life, people use "mass" and "weight" interchangeably. You might say you weigh 70 kilograms, but in physics, those two words mean very different things. Mass is the amount of matter in an object, measured in kilograms (kg). Weight is the gravitational force pulling on that mass, measured in newtons (N).
The connection between them comes from Newton's Second Law. Since weight is just the force of gravity on an object, and F = ma, we can write W = mg, where W is weight, m is mass, and g is the gravitational acceleration at that location. On Earth, g is approximately 9.8 m/s², so a 10 kg object weighs about 98 N.
Here is why the distinction matters: if you traveled to the Moon, your mass would stay exactly the same — you are still made of the same atoms. But the Moon's gravitational acceleration is only about 1.6 m/s², roughly one-sixth of Earth's. So your weight would drop to about one-sixth of what it is on Earth. You would feel incredibly light and could jump much higher, even though nothing about your body changed.
This also explains why astronauts aboard the International Space Station appear weightless. The ISS orbits only about 400 km above Earth, where gravity is still about 90% as strong as on the surface. The astronauts float not because gravity is absent, but because both they and the station are in free fall — constantly falling toward Earth but moving sideways fast enough to keep missing it. In free fall, there is no normal force pushing up on you, so you experience apparent weightlessness even though gravitational force still acts on your mass.
Understanding the mass-weight distinction is essential for solving physics problems correctly. Whenever a problem gives you mass in kilograms and asks for a force, you need to multiply by g to get weight. And whenever you see a force measured in newtons, remember that it is not the same as mass — it is mass multiplied by gravitational acceleration.