# Gravity

> Why things fall, the difference between mass and weight, and what keeps the Moon in orbit.

From HSC Physics on WhizzNotes, free study notes for NSW students: https://whizznotes.com/physics/gravity/. Updated 2 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The astronaut's scales
Before her mission, astronaut Kate steps onto a set of bathroom scales. They read 60 kg. On the Moon base, she tries the same scales. They read just 10 kg.

"Did I lose 50 kilograms?" she laughs.

She did not. Her body has exactly the same amount of stuff in it. What changed is how hard the Moon pulls on her. The scales were really measuring a force, not her mass. Telling these two apart is the key to this chapter.

**In short:** Gravity is a non-contact force that pulls any two masses towards each other. Your weight is the force of gravity on your mass, $W = mg$, with $g \approx 9.8$ N/kg on Earth: your mass stays the same everywhere, but your weight changes on other planets.

## What is gravity?

**Gravity** is a force of attraction between **any two objects that have mass**. It is a **non-contact** force.

- Every object pulls on every other object.
- The pull is only noticeable when at least one object is **very massive**, like a planet or a star.
- The pull gets **weaker** as objects get **further apart**.

Gravity holds you on the ground, makes rain fall, keeps the Moon orbiting Earth and keeps Earth orbiting the Sun.

## Mass and weight

| | Mass | Weight |
|---|---|---|
| What it is | The amount of matter in an object | The force of gravity on an object |
| Unit | kilogram (kg) | newton (N) |
| Scalar or vector? | Scalar | Vector (points towards the centre of the planet) |
| Changes on the Moon? | **No** | **Yes** |
| Measured with | Beam balance | Spring balance (newton meter) |

$$W = m \times g$$

- $W$ = weight (N)
- $m$ = mass (kg)
- $g$ = gravitational field strength (N/kg)

On Earth, $g \approx 9.8$ N/kg. This is the same number as the acceleration due to gravity, 9.8 m/s². The units N/kg and m/s² are actually equal.

### Worked example: Kate on Earth and the Moon
Kate's mass is 60 kg everywhere.

- On Earth: $W = 60 \times 9.8 = 588$ N
- On the Moon ($g = 1.6$ N/kg): $W = 60 \times 1.6 = 96$ N

Her weight on the Moon is about one sixth of her weight on Earth.

| Place | g (N/kg) |
|---|---|
| Moon | 1.6 |
| Mars | 3.7 |
| Earth | 9.8 |
| Jupiter | 24.8 |

### Watch out: "I weigh 50 kg"
In everyday speech people say this, but in physics it is wrong. 50 kg is your **mass**. Your **weight** is about 490 N on Earth.

## Falling objects

Without air, everything falls with the same acceleration, $g$. With air, there is also **air resistance**.

1. **Just after jumping**, a skydiver's only big force is weight. She accelerates at about 9.8 m/s².
2. **As she speeds up**, air resistance grows. Net force gets smaller, so acceleration gets smaller.
3. **Eventually** air resistance equals weight. Net force is zero. She falls at a steady top speed: **terminal velocity**.
4. **She opens the parachute.** Air resistance jumps up, now bigger than weight. She slows down.
5. **Slower speed** means less air resistance, until it balances weight again at a new, much lower terminal velocity. Safe landing.

## Gravity in space

The Moon is constantly falling towards Earth. But it is also moving sideways so fast that it keeps missing. That curved path is an **orbit**. Satellites and the International Space Station orbit Earth the same way.

Astronauts in the space station feel "weightless". Gravity there is still about 90% as strong as on the ground. They float because they and their station are **falling together** around Earth.

**Mind map**

```text
Gravity
  The force
    Attraction between masses
    Non-contact
    Weaker with distance
  Mass vs weight
    Mass: kg, never changes
    Weight: N, depends on g
    W = mg
  Falling
    g = 9.8 m/s²
    Air resistance
    Terminal velocity
  Space
    Orbits
    Moon falls and misses
    Weightlessness = free fall
```

### Year 11: Gravitational field strength
A **gravitational field** is the region around a mass where another mass feels a force. Field strength is the force per kilogram:

$$g = \frac{F}{m}$$

Near Earth's surface the field is roughly uniform at 9.8 N/kg. On a field diagram, arrows point towards the centre of the planet and get closer together where the field is stronger.

**Apparent weight** is what a scale reads. In a lift accelerating upwards at $a$, the scale reads $m(g + a)$, so you feel heavier. Accelerating downwards, it reads $m(g - a)$. In free fall, it reads zero.

### Year 12: Newton's law of universal gravitation
The force between two masses $M$ and $m$ whose centres are distance $r$ apart is:

$$F = \frac{GMm}{r^2}$$

where $G = 6.67 \times 10^{-11} \text{ N m}^2 \text{ kg}^{-2}$.

This is an **inverse square law**. Double the distance and the force drops to one quarter. Triple it and the force is one ninth.

Field strength at distance $r$ from a planet: $g = \dfrac{GM}{r^2}$. A planet with twice Earth's mass and twice its radius has $g = \dfrac{2}{2^2} = \dfrac{1}{2}$ of Earth's value.

Kepler's third law for orbits follows from this: $\dfrac{r^3}{T^2} = \dfrac{GM}{4\pi^2}$.

## Quick summary

- Gravity is a non-contact force of attraction between masses.
- **Mass** (kg) is the amount of matter and never changes. **Weight** (N) is the force of gravity.
- $W = mg$. On Earth $g \approx 9.8$ N/kg.
- Your weight changes on other planets. Your mass does not.
- Falling objects reach **terminal velocity** when air resistance equals weight.
- Orbits happen when an object falls towards a planet while moving sideways fast enough to keep missing it.

## Quiz

Try these questions from the chapter quiz.

1. The unit of weight is the:
   - A. Kilogram
   - B. Newton
   - C. Joule
   - D. Metre
   - Answer: B. Newton. Weight is a force, so it is measured in newtons.

2. A 5 kg bag on Earth (g = 9.8 N/kg) has a weight of:
   - A. 5 N
   - B. 0.51 N
   - C. 49 N
   - D. 14.8 N
   - Answer: C. 49 N. W = mg = 5 × 9.8 = 49 N.

3. An astronaut with a mass of 70 kg goes to the Moon. Her mass on the Moon is:
   - A. 11.7 kg
   - B. 70 kg
   - C. 112 kg
   - D. 0 kg
   - Answer: B. 70 kg. Mass is the amount of matter. It does not change from place to place.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/physics/gravity/#quiz