Mass, Weight & Density
Mass is the amount of matter in an object (always the same anywhere), weight is how hard gravity pulls on it (changes with location), and density is how tightly that matter is packed together.
Mass
The quantity of matter in an object. Measured in kg. Stays the same everywhere — on Earth, the Moon, or in space.
Weight
The gravitational force pulling on your mass. Measured in newtons (N). Changes depending on gravity at your location.
Density
How much mass is packed into a volume. Measured in kg/m³. Tells you if something will float or sink.
Mass & Weight
What is Mass?
Mass is the amount of matter in an object. Think of it as "how many atoms are packed into this thing." A bowling ball has more mass than a tennis ball because it has more stuff inside it.
Key point:
Mass is a scalar quantity — it has a size (magnitude) but no direction. You don't say "5 kg downwards" — you just say "5 kg."
Mass is always measured in kilograms (kg) in physics. Sometimes you'll see grams (g), but you need to convert to kg for calculations.
Converting between grams and kilograms
Remember this simple rule:
- 1 kg = 1000 g
- To convert kg to g: multiply by 1000
Example: 5 kg = 5 × 1000 = 5000 g
- To convert g to kg: divide by 1000
Example: 2500 g = 2500 ÷ 1000 = 2.5 kg
Practice: Unit conversion
Convert 4.5 kg into grams.
What is Weight?
Weight is the gravitational force pulling down on an object. Gravity is constantly trying to pull you towards the Earth (or the Moon, or the Sun, or any massive object).
Analogy:
Think of mass as your actual body — the stuff you're made of. Weight is how hard a scale pushes back when you stand on it. On the Moon, you're made of the same stuff (same mass) but the scale pushes back with less force because Moon gravity is weaker.
Weight is measured in newtons (N) — the same unit we use for all forces.
Key point:
Weight is a vector quantity — it has both magnitude (size) and direction. It always points downwards (towards the centre of the Earth).
The relationship between weight, mass, and gravity
Your weight depends on how much mass you have and how strong the gravity is where you are. This is expressed by one of the most important formulas in physics:
What does "9.8 N/kg" mean? For every kilogram of mass you have, Earth's gravity pulls down with 9.8 newtons of force. If you have 10 kg of mass, gravity pulls with 98 N.
Worked Example: Comparing Weight on Earth and the Moon
The scenario: Astronaut Isabelle has a mass of 42 kg. What is her weight on Earth? What is her weight on the Moon?
Given information:
- Mass = 42 kg (this never changes)
- Earth's gravitational field strength: g = 9.8 N/kg
- Moon's gravitational field strength: g = 1.6 N/kg
Step 1: Calculate weight on Earth
WEarth = m × g
WEarth = 42 × 9.8
WEarth = 411.6 N (or 410 N to 2 s.f.)
Step 2: Calculate weight on the Moon
Moon
Moon
Moon
67.2 N
Density