Mass and Weight
Simple Explanation
Mass is the amount of matter in an object, measured in kilograms — it never changes, no matter where the object is. Weight is the force of gravity pulling on that mass, measured in newtons — it depends on the local gravitational field strength, so the same object weighs less on the Moon than on Earth.
Why Do We Need It?
Confusing mass and weight is one of the most common errors in physics — they have different units, different values in different places, and are genuinely different physical quantities.
Formula
Weight
W = mg
Weight is the force of gravity pulling on an object's mass. Unlike mass, weight depends on the local gravitational field strength, so the same object weighs less on the Moon than on Earth.
- W
- — weight, in newtons (N)
- m
- — mass of the object, in kilograms (kg)
- g
- — the local gravitational field strength (acceleration due to gravity), in N/kg or m/s² — approximately 9.8 on Earth's surface
When to use it: Whenever an object's weight needs to be found from its mass, at a known location.
Worked Example
Find an astronaut's weight on the Moon
An astronaut has a mass of 80 kg. Earth's g ≈ 9.8 m/s², the Moon's g ≈ 1.6 m/s². Find the astronaut's weight on the Moon.
Why Does This Work?
Since weight is defined as W = mg, and the astronaut's mass m doesn't change by travelling to the Moon, the entire difference in weight comes purely from the Moon's much weaker gravitational field strength, g.
Real-Life Example
Why astronauts can jump so high on the Moon
Footage of Apollo astronauts shows them making huge leaps despite bulky spacesuits.
Their mass (and muscular strength) is unchanged, but their weight is only about a sixth of what it is on Earth, so the same muscular push produces a much bigger leap.
Practice
Find the weight of a 5 kg object on Earth (g = 9.8 m/s²).
EasyAn object is taken from Earth to the Moon. Which statement is correct?
MediumCommon mistake
Using the words "mass" and "weight" interchangeably, or measuring weight in kilograms — mass is measured in kilograms (kg), weight is a force, measured in newtons (N).
Quick Review
- W = mg
- Mass (kg): amount of matter, the same everywhere.
- Weight (N): the force of gravity on that mass, different in different gravitational fields.