Physics
Weight vs. Mass
Quick fact
An astronaut orbiting Earth is weightless (apparent weight near zero) but still has the same mass as on the ground. If they pushed off a wall, they would float—demonstrating their inertia depends on mass, not weight.
Why this is interesting
You step on a bathroom scale and read 70 kg—but if you take the same scale to the Moon, you'd weigh only about 12 kg. Yet you haven't lost any 'stuff'. How can the same object give a different number?
Read the full explanation
Understanding Weight vs. Mass
Think of mass as the 'amount of stuff' inside an object—atoms, molecules, total material. It doesn't change whether you're on Earth, the Moon, or in deep space. Weight, by contrast, is the pull of gravity on that mass. When you stand on a scale, the scale measures the force your body exerts downward due to gravity, then converts it to a mass reading (assuming Earth's gravity). On the Moon, gravity is about 1/6 as strong, so the same mass experiences only 1/6 the weight. A balance scale—which compares masses by using a lever—would still give the same reading because it compares your mass to known masses, not to the gravitational pull. So mass is an intrinsic property of matter, while weight is a force that depends on the local gravitational field.
A deeper explanation
The fundamental relationship is Newton's second law: F = ma. Weight (W) is the force of gravity acting on an object, given by W = mg, where m is mass and g is the acceleration due to gravity. On Earth's surface, g ≈ 9.8 m/s²; on the Moon, g ≈ 1.6 m/s². Thus an object's weight changes, but its mass m remains the same. This distinction is essential because inertia (resistance to change in motion) depends on mass, not weight. In free fall, objects experience apparent weightlessness because there is no contact force supporting them, but their mass is unchanged. Understanding this difference prevents common errors, such as confusing 'lightness' with low mass (e.g., a feather vs. a lead sphere—feather has less mass, but both fall at the same rate in vacuum due to same gravitational acceleration). Applications range from calibrating scales in different gravitational environments to designing spacecraft trajectories and understanding planetary geology.