Mass and Weight Are Not the Same
This is the distinction the whole page rests on.
| Mass | Weight | |
|---|---|---|
| What it is | The quantity of matter | The force of gravity on that matter |
| SI unit | Kilogram (kg) | Newton (N) |
| Changes with location? | No | Yes |
| In free fall | Unchanged | Effectively zero |
| Measured with | A balance | A spring scale |
W = m × g
A bathroom scale measures force and then divides by Earth's gravity to display a mass. Take the same scale to the Moon and it would read a sixth of the figure — not because you lost matter, but because it is calibrated for Earth.
Gravity Across the Solar System
| Place | g (m/s²) | Relative to Earth | A 70 kg person reads |
|---|---|---|---|
| Pluto | 0.62 | 0.06× | 4.4 kg |
| The Moon | 1.62 | 0.17× | 11.6 kg |
| Mars | 3.72 | 0.38× | 26.6 kg |
| Mercury | 3.70 | 0.38× | 26.4 kg |
| Uranus | 8.87 | 0.90× | 63.3 kg |
| Venus | 8.87 | 0.90× | 63.3 kg |
| Earth | 9.807 | 1.00× | 70.0 kg |
| Saturn | 10.44 | 1.06× | 74.5 kg |
| Neptune | 11.15 | 1.14× | 79.6 kg |
| Jupiter | 24.79 | 2.53× | 177.0 kg |
| The Sun | 274 | 27.9× | 1,956 kg |
Two surprises sit in that table. Mercury and Mars have almost identical surface gravity despite Mercury being much smaller — it is far denser, being mostly iron. And Saturn's gravity is barely above Earth's, even though it is 95 times more massive, because it is so large and so diffuse that its surface is far from its centre. Saturn's average density is lower than water's.
Why Astronauts Float
The common explanation — that there is no gravity in space — is wrong. At the International Space Station's altitude of about 400 km, Earth's gravity is still roughly 89% of its surface value.
Astronauts float because the station is in continuous free fall. It moves sideways fast enough — about 7.7 km/s — that as it falls, the Earth's surface curves away beneath it at the same rate. Everything inside falls together, so nothing presses against anything else. The correct term is microgravity, not zero gravity.
Gravity Varies on Earth Too
The standard value of 9.80665 m/s² is a defined average. Real local gravity varies by about 0.5%:
| Place | g (m/s²) | Why |
|---|---|---|
| North Pole | 9.832 | Closer to the centre; no centrifugal effect |
| London | 9.816 | — |
| New York | 9.802 | — |
| Equator, sea level | 9.780 | Earth bulges, and rotation reduces effective gravity |
| Top of Everest | 9.765 | 8.8 km further from the centre |
A person weighing exactly 70.00 kg at the pole would read about 69.63 kg at the equator on the same spring scale. This is why laboratory balances are calibrated on site.
Units of Force
| Unit | Equivalent | Notes |
|---|---|---|
| Newton (N) | 1 kg·m/s² | The SI unit; roughly the weight of an apple |
| Pound-force (lbf) | 4.448 N | The weight of one pound on Earth |
| Kilogram-force (kgf) | 9.807 N | Deprecated but still seen in engineering |
| Dyne | 10−5 N | CGS system, largely obsolete |
Where the Distinction Actually Matters
- Spacecraft design. Thrust is a force; propellant is a mass. Confusing them ended the Mars Climate Orbiter.
- Structural engineering. Loads are forces, and must be in newtons or pounds-force.
- Physics problems. F = ma requires mass; W = mg produces a force.
- Trade. Goods are sold by mass, which is why balances rather than spring scales are legal for trade — a balance compares masses and is unaffected by local gravity.
Frequently Asked Questions
What would I weigh on the Moon?
About 16.5% of your Earth reading. A 70 kg person's scale would show 11.6 kg — while their mass remains exactly 70 kg.
Is my bathroom scale measuring mass or weight?
It measures weight, a force, then divides by Earth's gravity to display mass. That is why it would be wrong on another planet.
How many newtons do I weigh?
Multiply your mass in kilograms by 9.807. A 70 kg person weighs about 687 newtons on Earth.
Could you stand on Jupiter?
There is no surface to stand on — it is a gas giant. At the level where pressure equals Earth's sea level, gravity is about 2.5 times Earth's, so a 70 kg person would feel like 177 kg. Standing would be extremely difficult and breathing impossible.