1. What Is Motion?

Learning outcomes

I can define motion as a change in position over time.
I can explain the importance of a reference point when describing motion.
I can distinguish between an object at rest and an object in motion.
I can describe motion using appropriate scientific terminology.
I can identify examples of motion in everyday life.

Introduction

Motion is one of the most fundamental ideas in physics. Everything around us is either moving or has the potential to move—from cars travelling along roads and planets orbiting the Sun to leaves blowing in the wind and people walking across a room. Understanding motion allows scientists and engineers to explain how objects move, predict where they will be in the future, and design everything from bicycles and roller coasters to satellites and spacecraft.

Before we can measure speed, calculate acceleration, or study forces, we first need to understand what motion actually is. In this lesson, you will learn how physicists define motion, why a reference point is essential, and how to describe movement using scientific language.

What is Motion?

Motion is the change in the position of an object over time.

If an object's position changes as time passes, it is said to be in motion.

If its position does not change, it is said to be at rest.

Both position and time are needed to describe motion.

For example:

A cyclist riding along a road is in motion.
A bird flying through the sky is in motion.
The Moon orbiting Earth is in motion.
A parked bicycle is at rest (relative to the ground).

Position and Time

To describe motion, we need two pieces of information:

Position

Position is the location of an object relative to a chosen reference point.

For example:

A student is sitting 2 metres from the classroom door.
A bus is 500 metres from the bus station.
A satellite is 400 km above Earth's surface.

Time

Motion always occurs over a period of time.

Examples include:

A runner completes a race in 12 seconds.
A train travels between stations in 8 minutes.
Earth takes about 365¼ days to orbit the Sun.

Without time, motion cannot be measured or described.

The Importance of a Reference Point

A reference point is a fixed object or location used for comparison when describing motion.

Without a reference point, it is impossible to say whether an object is moving.

For example:

Imagine you are sitting on a train.

Relative to:

Your seat, you are at rest.
A person standing beside the railway tracks, you are moving.

The same object can appear to be either moving or stationary depending on the chosen reference point.

Examples of Reference Points

Common reference points include:

The ground.
A building.
A tree.
A road sign.
A starting line.
Another moving object.

Scientists always state the reference point when describing motion.

Objects at Rest

An object is at rest if its position does not change relative to the chosen reference point.

Examples:

A parked car in a car park.
A book on a desk.
A tree growing in a field.
A lamp hanging from the ceiling.

Although these objects appear stationary relative to Earth, they are actually moving through space as Earth rotates and orbits the Sun.

This shows that rest is relative, not absolute.

Objects in Motion

An object is in motion if its position changes relative to the chosen reference point.

Examples:

A football rolling across a field.
A bird flying overhead.
A lift moving between floors.
A boat travelling across a lake.
A cyclist riding along a road.

The amount of motion depends on how quickly the position changes over time.

Motion is Relative

There is no such thing as absolute motion.

Motion is always described relative to something else.

For example:

A passenger sitting on a moving bus is:

At rest relative to the bus.
Moving relative to the road.
Moving even faster relative to the Sun because Earth is orbiting the Sun.
Moving faster still relative to distant stars as our Solar System travels through the Milky Way.

This idea is known as relative motion and is one of the most important concepts in physics.

Describing Motion Using Scientific Terms

Physicists use precise vocabulary to describe motion.

Some important terms include:

Term	Meaning
Motion	A change in position over time
Position	The location of an object relative to a reference point
Reference Point	A fixed object or location used for comparison
At Rest	Position does not change relative to the reference point
In Motion	Position changes relative to the reference point
Relative Motion	Motion described with respect to another object

Learning these terms helps scientists communicate clearly.

Everyday Examples of Motion

Motion can be observed almost everywhere.

Examples include:

A child riding a bicycle.
Cars travelling on a motorway.
Rain falling from clouds.
Fish swimming in a river.
Birds flying.
Elevators moving between floors.
A spinning ceiling fan.
Earth rotating on its axis.
The Moon orbiting Earth.

Some motions are easy to observe, while others, such as Earth's movement through space, are not immediately obvious.

Why Understanding Motion is Important

Motion is the foundation of many areas of physics.

It allows scientists and engineers to:

Predict where objects will be.
Design safer vehicles.
Launch satellites into orbit.
Build roller coasters.
Study sports performance.
Understand planetary motion.

Every topic in mechanics begins with understanding motion.

Real-World Connections

Understanding motion is essential in many careers and technologies.

Examples include:

Engineers design vehicles that move efficiently and safely.
Pilots monitor the motion of aircraft during flight.
Astronomers study the motion of planets, stars, and galaxies.
Sports scientists analyse athletes' movements to improve performance.
Robotics engineers program robots to move accurately and safely.

The study of motion affects almost every aspect of modern technology.

Worked Example

A passenger is sitting on a train that is travelling at 90 km/h.

Question

Is the passenger at rest or in motion?

Solution

The answer depends on the reference point.

Relative to the train seat, the passenger is at rest.
Relative to the ground, the passenger is in motion.
Relative to the Sun, the passenger is moving even faster because Earth is orbiting the Sun.

This example shows that motion is always relative to a reference point.

Did You Know?

Even when you are standing still, you are moving at about 1,670 km/h due to Earth's rotation at the equator.
Earth travels around the Sun at an average speed of about 107,000 km/h, carrying everything on its surface with it.
The International Space Station travels around Earth at about 28,000 km/h, completing one orbit approximately every 90 minutes.

Key Terms

Motion — A change in an object's position over time.

Position — The location of an object relative to a reference point.

Reference Point — A fixed object or location used to determine whether an object is moving.

At Rest — When an object's position does not change relative to a chosen reference point.

Relative Motion — Motion described with respect to another object or reference point.

Time — The interval over which motion occurs.

Key Takeaways

Motion is defined as a change in position over time.
A reference point is essential because motion can only be described relative to another object or location.
An object is at rest if its position does not change relative to the chosen reference point and in motion if it does.
Motion is relative, meaning the same object may be at rest with respect to one reference point but moving with respect to another.
Scientists use terms such as position, reference point, motion, and relative motion to describe movement accurately.
Understanding motion provides the foundation for studying speed, velocity, acceleration, forces, and all of mechanics.