Gravity, weight and mass Revision Notes for AQA GCSE Physics Combined Science

Gravity, weight and mass

Understanding weight and mass

Many students mix up weight and mass, but they are completely different things. Let's clear this up once and for all.

Weight is a force that pulls objects towards the centre of a planet or moon. It's what you feel when gravity acts on your body. Weight is measured in newtons (N) and it can change depending on where you are in the universe.

Mass is simply the amount of matter (stuff) that makes up an object. Mass is measured in kilograms (kg) and it never changes - whether you're on Earth, the Moon, or floating in space, your mass stays exactly the same.

Think of it this way: your mass is how much "stuff" you're made of, but your weight is how hard gravity pulls on that "stuff".

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The key distinction: Mass tells us how much matter an object contains, while weight tells us how strongly gravity pulls on that matter. This is why your weight changes on different planets, but your mass remains constant!

The connection between mass and weight

There's a simple equation that links mass and weight together:

$\text{Weight} = \text{mass} \times \text{gravitational field strength}$

Or in symbols: $W = mg$

Where:

$W$ = weight (in newtons, N)
$m$ = mass (in kilogrammes, kg)
$g$ = gravitational field strength (in N/kg)

This means weight and mass are directly proportional. If you double the mass, you double the weight. If you halve the mass, you halve the weight.

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Understanding this relationship is crucial: Weight depends on both mass AND gravity. The same object will have different weights on different planets, but its mass remains unchanged.

Gravitational field strength

Gravitational field strength ( $g$ ) tells us how strong gravity is in different places. On Earth, $g = 10$ N/kg, which means every kilogramme of mass experiences 10 newtons of gravitational force.

Here are the $g$ values for different places in our solar system:

Location	Gravitational field strength (N/kg)
Earth	10
Moon	1.6
Jupiter	26
Neptune	13.3
Mercury	3.6
Mars	3.75
Neutron star	10¹²

This is why astronauts feel much lighter on the Moon - gravity there is only 1.6 N/kg compared to Earth's 10 N/kg!

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Notice how dramatically different gravitational field strengths can be! Jupiter's gravity is more than twice as strong as Earth's, while the Moon's is less than one-sixth. On a neutron star, gravity is so intense that it's a trillion times stronger than Earth's!

Measuring weight

We measure weight using a spring balance (also called a newtonmeter). When you hang an object from the spring, gravity pulls it down and stretches the spring. The heavier the object, the more the spring stretches.

The scale on a spring balance shows the weight in newtons, not the mass in kilogrammes.

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Remember: Spring balances measure weight (in newtons), not mass (in kilogrammes). This is a common source of confusion in physics problems!

Worked example

Let's see how this works in practice with a complete calculation:

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Worked Example: Astronaut on Different Planets

An astronaut has a mass of 58 kg on Earth.

(a) What is the astronaut's mass and weight on the Moon?

Step 1: Determine the mass Mass stays the same: 58 kg

Step 2: Calculate weight using $W = mg$ Weight = mass × $g$ Weight = $58 \text{ kg} \times 1.6 \text{ N/kg} = 92.8 \text{ N}$

(b) What is the astronaut's mass and weight on Jupiter?

Step 1: Determine the mass Mass stays the same: 58 kg

Step 2: Calculate weight using $W = mg$
Weight = mass × $g$ Weight = $58 \text{ kg} \times 26 \text{ N/kg} = 1508 \text{ N}$

Notice how the mass never changes, but the weight is very different on each planet!

Key concepts summary

Understanding the relationship between gravity, weight, and mass is fundamental to physics. Remember that while your mass represents the amount of matter you contain, your weight represents how strongly gravity pulls on that matter.

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Key Points to Remember:

Weight is a force measured in newtons (N) - it changes depending on gravity
Mass is the amount of matter measured in kilogrammes (kg) - it never changes
Use $W = mg$ to calculate weight from mass and gravitational field strength
On Earth, $g = 10$ N/kg - this is the value you'll use in most calculations
Spring balances measure weight, not mass

Gravity, weight and mass (AQA GCSE Physics Combined Science): Revision Notes