What is mass?
Everything is made of matter. Mass tells us how much matter is in an object. A big iron ball has more mass than a small iron ball. A bag of sand has more mass than a bag of cotton of the same size.
Mass does not change when you move the object. It does not change if you heat it, squash it, or carry it to another place. Mass is a property of the object.
The SI unit of mass is the kilogram (kg). Smaller units: gram (g), 1 kg = 1000 g. Bigger units: tonne, 1 tonne = 1000 kg. Tiny things use milligram (mg), 1 g = 1000 mg.
Measuring mass with a balance
A pan balance compares two masses. Put the object on the left pan. Put standard masses (known weights) on the right pan. Add or remove them until the beam is level and the pointer is in the middle. Then the two masses are equal.
- Check the pointer is at zero with empty pans.
- Put the object on the left pan. Use clean, dry hands or tweezers for the masses.
- Add the biggest standard mass that does not tip the beam. Then add smaller ones.
- When the beam is level, add up the standard masses. That is the mass of the object.
A balance works on the Moon too, because both pans are pulled by the same gravity. That is why a balance measures mass, not weight.
What is weight?
Gravity pulls every object towards the centre of its planet. This pull is the weight of the object. Weight is a force, so its unit is the newton (N).
W = m × g. Here m is mass in kg and g is the strength of gravity. On Earth g is about 9.8 N per kg (often rounded to 10). On the Moon it is about 1.6 N per kg.
A spring scale measures weight. The heavier the pull, the more the spring stretches.
Mass and weight side by side
| Mass | Weight | |
|---|---|---|
| What is it? | Amount of matter | Pull of gravity on it |
| Unit | kg | newton (N) |
| Tool | Pan balance | Spring scale |
| On the Moon | Same | About 1/6 of Earth |
| Can be zero? | No | Yes, far from all planets |
Try it
Make a simple balance at home: hang a ruler from a string at its middle, and hang two small bags from its ends. Put 10 identical coins in one bag. Add coins to the other bag until the ruler is level. Now you know that bag has the same mass as 10 coins. In the 3D above, drag the sliders and guess before you check: when will the beam be level?
Key formulas and definitions
- Weight W = m × g
- m in kilograms (kg), W in newtons (N)
- g on Earth ≈ 9.8 N/kg (use 10 for quick sums)
- g on the Moon ≈ 1.6 N/kg
- 1 kg = 1000 g, 1 tonne = 1000 kg, 1 g = 1000 mg
Worked examples
1. A bag of rice has a mass of 2 kg. What is its weight on Earth? (g = 10 N/kg)
W = m × g = 2 × 10 = 20 N.
2. A brick has a mass of 500 g. Write this in kg and find its weight on Earth (g = 9.8 N/kg).
500 g = 0.5 kg. W = 0.5 × 9.8 = 4.9 N.
3. The same 500 g brick goes to the Moon (g = 1.6 N/kg). Find its mass and weight there.
Mass stays 0.5 kg. Weight = 0.5 × 1.6 = 0.8 N.
Common mistakes
- Saying "my weight is 50 kg". Kilogram is the unit of mass. Weight is in newtons.
- Thinking mass becomes less on the Moon. Only weight becomes less.
- Forgetting to change grams to kilograms before using W = m × g.
- Thinking a balance gives a different reading on the Moon. It does not, because both pans are pulled equally.