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Gravity Affects
Motion

Discover how gravity shapes the movement of everything: from a falling apple to planets orbiting the Sun.

Earth & Space Science

What Is Gravity?

Gravity is an invisible force of attraction between any two objects that have mass. The more mass an object has, the stronger its gravitational pull. You feel it every second of every day: it is the force that keeps your feet on the ground and pulls a basketball back down after you shoot it.

Mass vs. Weight

Mass
The amount of matter in an object. Measured in kilograms (kg). Stays the same everywhere in the universe.
Weight
The force of gravity on an object. Measured in newtons (N). Changes depending on where you are.

Why Gravity Matters

1
Keeps us grounded: Without gravity, you would float away from Earth's surface.
2
Shapes the solar system: Gravity holds planets in orbit around the Sun and moons in orbit around planets.
3
Drives tides: The Moon's gravitational pull creates ocean tides on Earth.
4
Governs falling objects: Every object dropped near Earth accelerates downward at the same rate (ignoring air resistance).

The Law of Universal Gravitation

Newton discovered that every pair of masses attracts each other with a force given by:

\( F \)
Gravitational force (N)
\( G \)
Gravitational constant
\( m_1, m_2 \)
Masses of the two objects
\( r \)
Distance between centres

Visualising Gravitational Force

\( m_1 \)
\( m_2 \)
\( r \)
\( F \)
\( F \)

Free Fall and \( g \)

Near Earth's surface, gravity accelerates all falling objects at a constant rate (ignoring air resistance). We call this gravitational acceleration:

This means that every second an object falls, its speed increases by about \(9.8 \;\text{m/s}\). A feather and a bowling ball, dropped in a vacuum, hit the ground at the same time.

Gravity at Different Scales

Everyday
A ball thrown in the air follows a curved path back to the ground.
Planetary
Earth orbits the Sun because the Sun's gravity continuously curves Earth's path.
Galactic
Gravity holds billions of stars together in spiral galaxies like the Milky Way.

Gravity vs. Magnetism

Gravity Magnetism
Acts on All objects with mass Only magnetic materials or charges
Direction Always attractive Can attract or repel
Range Infinite (weakens with distance) Infinite (weakens faster)
Strength Very weak per unit mass Much stronger at small scales

How to Solve Gravity Problems

1
Identify the objects: What two masses are interacting?
2
List known values: Masses (\( m_1, m_2 \)), distance (\( r \)), or acceleration (\( g \)).
3
Choose the right equation: Use \( F = G\frac{m_1 m_2}{r^2} \) for force, or \( W = m \cdot g \) for weight near a surface.
4
Substitute and solve: Plug in values, watch your units, and calculate.
5
Check reasonableness: Does the answer make physical sense?

Example 1: Weight on Earth

A student has a mass of 50 kg. What is their weight on Earth?

The student's weight is 490 newtons directed downward toward Earth's centre.

Example 2: Weight on the Moon

The same 50 kg student travels to the Moon, where \( g_{\text{Moon}} \approx 1.6 \;\text{m/s}^2 \). What is their weight there?

Their mass stays 50 kg, but their weight drops from 490 N on Earth to just 80 N on the Moon.

Example 3: Free Fall Speed

A rock is dropped from rest off a cliff. How fast is it falling after 3 seconds? (Ignore air resistance.)

Cliff
\( t = 0 \;\text{s}, v = 0 \)
\( t = 1 \;\text{s}, v = 9.8 \;\text{m/s} \)
\( t = 2 \;\text{s}, v = 19.6 \;\text{m/s} \)
\( t = 3 \;\text{s}, v = 29.4 \;\text{m/s} \)
\( v = g \cdot t \)
\( v = 9.8 \times 3 \)
\( v = 29.4 \;\text{m/s} \)

Orbits: Gravity in Action

Sun
Earth
velocity
gravity

An orbit happens when an object moves fast enough sideways that it keeps falling around the larger body instead of crashing into it. Gravity constantly pulls Earth inward, while Earth's velocity carries it forward, creating a curved path.

Gravity and Tides

Earth
Moon
High tide
High tide
Low tide at sides

Common Mistakes

✕ Heavier falls faster
Without air resistance, all objects fall at the same rate regardless of mass. A feather and hammer dropped on the Moon land together.
✕ No gravity in space
Astronauts feel "weightless" because they are in free fall, not because gravity is absent. The ISS still experiences about 90% of surface gravity.
✕ Mass equals weight
Mass (kg) is how much matter you have; weight (N) is the gravitational force on that matter. They are related but not the same thing.

Summary Table

Concept Formula / Value Key Point
Gravitational force \( F = G\frac{m_1 m_2}{r^2} \) Acts between all masses
Weight \( W = m \cdot g \) Changes with location
Earth's \( g \) \( 9.8 \;\text{m/s}^2 \) Same for all objects
Free-fall speed \( v = g \cdot t \) Increases every second
Orbits Continuous free fall Sideways speed + gravity = curve

Question 1 of 5

A bowling ball and a marble are dropped from the same height in a vacuum. Which hits the ground first?

Question 2 of 5

An astronaut has a mass of 80 kg. What is their weight on Earth? (Use \( g = 9.8 \;\text{m/s}^2 \))

Question 3 of 5

According to Newton's law, what happens to gravitational force if the distance between two objects is doubled?

Question 4 of 5

Why do astronauts on the International Space Station appear to be weightless?

Question 5 of 5

A rock is dropped from rest. What is its speed after 4 seconds of free fall? (Ignore air resistance; \( g = 9.8 \;\text{m/s}^2 \))