Mass vs Weight

Lesson 4 of 11 · 7 min · Free, no account needed

What you will learn

Mass (kg) is how much stuff, and how hard it is to speed up. It is the same on every world.
Weight is the pull of gravity on that mass, a force in newtons: W = m × g.
Spring scales measure weight; a balance compares masses. On the Moon you weigh about a sixth as much, but you are just as hard to stop.

On the Moon you can lift a 60 kg astronaut. Could you stop one sliding into you just as easily?
Hang the same 10 kg backpack on a spring scale on each world.
Now push it sideways on smooth ice, same push on Earth and on the Moon.
Compare how fast it speeds up.
Two different things: mass and weight.
Mass (kg): how much stuff, and how hard it is to speed up or stop. The same on every world.
Weight (N): the force of gravity on that mass. W = m × g, with g = 9.8 m/s² on Earth and 1.6 m/s² on the Moon.
In everyday talk people say weight and mean kilograms. In physics, weight is a force. Simplified model: g is the same everywhere near each surface.
Game: which scale lies? Four crates, four answers.
A spring scale marked in kg (set up on Earth) and a balance with kilogram masses weigh the same crate.
Somewhere new, the 10 kg backpack weighs 37 N. Turn g to match.
Code: build a weight function and call it for Earth, the Moon and Mars, in that order.
The backpack is 10 kg. Your function reads the scales in the last step.
A 60 kg astronaut stands on a scale on the Moon, where g = 1.62 m/s². What does it read, in newtons?
Your weight function on tour. Press Play.

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