Apollo astronauts brought Moon rocks back to Earth. On the Moon, a case of rock samples swung easily off the ground. Back on Earth, the same case dragged down six times harder. Nobody added any rock.
Two quantities rode home with those rocks. Only one of them changed.
Start with the one that changed. On the Moon, a rock weighs about a sixth of its Earth weight.
The reason is where the rock is. The Moon's gravity pulls more weakly than Earth's. A weaker pull on the same rock means a smaller weight.
The rock itself is unchanged: the same matter, so the same mass, in the same kilograms.
The pattern: weight changes with location. Mass does not.
A geology sample is carried from Earth to the Moon. Its mass:
The same geology sample again, carried from Earth to the Moon. Its weight:
Why do the two split like that? Weight is a pull on the object, so it depends on where the object is. Mass is the quantity of matter in the object, so it goes with the object, unchanged, everywhere.
Take the idea to the extreme: far out in deep space, far from every planet and moon, the rock would weigh almost nothing. Its mass would still be exactly the same.
Lena says: 'A barbell would be easier to hold up on the Moon, because its weight is smaller there.' Is Lena correct?
Mars's gravity is weaker than Earth's. A rover built on Earth lands on Mars. Compare its weight and its mass on Mars with their Earth values.
Weight changes with location: a weaker pull means a smaller weight.
Mass is the quantity of matter, unchanged by the trip.
On Mars the rover's weight is smaller than on Earth, and its mass is the same.
A 2 kg Moon rock is flown from the Moon to a lab on Earth. What happens to its mass, and what happens to its weight?
Mass follows the matter: still 2 kg.
Earth's gravity pulls harder than the Moon's, so the pull on the rock grows.
Its mass stays 2 kg. Its weight is larger on Earth, about six times its Moon weight.
A lunar lander carries a camera from Earth to the Moon. Which row is correct?
A bolt drifts in deep space, far from every planet and moon. Which statement is correct?
Why does a rock weigh less on the Moon than on Earth?