Gravity, weight and mass (AQA GCSE Physics): Revision Notes
Gravity, weight and mass
Understanding the difference between weight and mass is crucial for GCSE Physics. These terms are often confused, but they mean completely different things.
Many students confuse weight and mass because we often use these words interchangeably in everyday language. However, in physics, they have very specific and different meanings. Understanding this distinction is fundamental to success in mechanics topics.
What is weight?
Weight is the force that pulls an object towards the centre of a planet or moon. Think of it as the pulling force you feel when gravity acts on your body.
Key facts about weight:
- Weight is a force, so it's measured in newtons (N)
- Weight is a vector quantity (it has direction - always pointing towards the centre of the planet)
- Weight changes depending on which planet or moon you're on
- On Earth, weight always points downward towards the Earth's centre
What is mass?
Mass is the amount of matter (or "stuff") that makes up an object. It tells you how much material an object contains.
Key facts about mass:
- Mass is measured in kilogrammes (kg)
- Mass is a scalar quantity (it has no direction)
- Mass never changes - it stays the same wherever you go in the universe
- Your mass on Earth is exactly the same as your mass on the Moon
The connection between weight and mass
Weight and mass are linked by this important equation:
The Weight Equation:
Where:
- W = weight (in newtons, N)
- m = mass (in kilogrammes, kg)
- g = gravitational field strength (in N/kg)
This means weight is directly proportional to mass. If you double the mass, you double the weight.
Gravitational field strength
Different planets and moons have different gravitational field strengths. This is why your weight changes depending on where you are in the solar system, even though your mass stays the same.
Gravitational field strength values:
- Earth: 10 N/kg
- Moon: 1.6 N/kg
- Jupiter: 26 N/kg
- Mars: 3.75 N/kg
This explains why astronauts can jump much higher on the Moon - the gravitational field strength is much weaker there, so their weight is much less even though their mass is unchanged.
Measuring weight
Weight is measured using a spring balance (also called a newtonmeter). This is different from a normal bathroom scale, which actually measures weight but displays the reading in kilogrammes (which can be confusing!).
How a spring balance works:
- The object stretches the spring inside the device
- The heavier the object, the more the spring stretches
- The scale reading shows the weight in newtons
Worked example
Worked Example: Astronaut on Different Celestial Bodies
An astronaut has a mass of 58 kg on Earth. Let's calculate their weight on different celestial bodies.
Step 1: On the Moon
- Mass stays the same: 58 kg
- Using : Weight =
Step 2: On Jupiter
- Mass stays the same: 58 kg
- Using : Weight =
Step 3: Comparison Notice how the mass never changes, but the weight is very different on each celestial body. On Jupiter, the astronaut weighs over 15 times more than on the Moon!
Common mistakes to avoid
Watch out for these common errors:
- Don't say "my mass is 70 kilogrammes" when you mean "my weight is 70 kilogrammes worth" (your actual weight on Earth would be about 700 N)
- Don't confuse the units: mass is in kg, weight is in N
- Remember that bathroom scales show mass in kg, but they're actually measuring your weight and converting it
Key Points to Remember:
- Weight is a force measured in newtons (N) - it changes depending on location
- Mass is the amount of matter measured in kilogrammes (kg) - it never changes
- Use to calculate weight from mass and gravitational field strength
- Weight is measured using a spring balance or newtonmeter
- Earth's gravitational field strength is 10 N/kg
- The Moon's gravitational field strength is 1.6 N/kg