Describing Motion

Site:	Young Education
Cours:	Kinematics
Livre:	Describing Motion

Imprimé par:	Guest user
Date:	vendredi 25 septembre 2026, 01:01

Table des matières

1. What Is Motion?
2. Distance and Displacement
3. Speed
4. Velocity
5. Motion in Everyday Life

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.

2. Distance and Displacement

Learning outcomes

I can define distance as the total path traveled by an object.
I can define displacement as the change in position of an object.
I can distinguish between distance and displacement.
I can calculate distance traveled in simple situations.
I can determine the displacement of an object and describe its direction.

Introduction

When describing motion, scientists need more than just the words "moving" or "not moving." They need precise ways to describe how far an object has travelled and where it ends up. These ideas are represented by two related but different quantities: distance and displacement.

Although these terms are often used interchangeably in everyday conversation, they have very different meanings in physics. Understanding the difference between them is essential before studying speed, velocity, and acceleration. In this lesson, you will learn how to measure both distance and displacement and why direction is important when describing motion.

What is Distance?

Distance is the total length of the path travelled by an object.

Distance tells us how much ground an object has covered, regardless of the direction of travel.

Distance is a scalar quantity, meaning it has:

Magnitude only.
No direction.

Distance is always:

Positive.
Measured in units such as metres (m) or kilometres (km).

Examples:

A person walks 200 m to school.
A car travels 15 km to work.
A runner completes a 400 m race.

What is Displacement?

Displacement is the change in an object's position from its starting point to its ending point.

Displacement describes the shortest straight-line distance between the initial and final positions.

Unlike distance, displacement is a vector quantity, meaning it has:

Magnitude.
Direction.

Examples:

50 m east
12 km north
5 m downward

Without a direction, displacement is incomplete.

Distance vs Displacement

The key difference is:

Distance measures the entire path travelled.
Displacement measures the straight-line change in position.

A journey may have:

A large distance.
A much smaller displacement.

Or even:

A non-zero distance.
A zero displacement.

Comparing Distance and Displacement

Distance	Displacement
Total path travelled	Straight-line change in position
Scalar quantity	Vector quantity
Magnitude only	Magnitude and direction
Always positive	Can be positive, negative, or zero (depending on the chosen coordinate system)
Depends on the path taken	Depends only on the starting and ending positions

Example 1: Walking in a Straight Line

A student walks 30 m east.

Distance

30 m

Displacement

30 m east

Since the student travelled in a straight line without changing direction:

Distance = Displacement

Example 2: Taking a Longer Route

A student walks:

40 m east.
Then 30 m north.

Distance

40 + 30 = 70 m

Displacement

The displacement is the straight-line distance from the starting point to the ending point.

Using Pythagoras' Theorem:

$ \sqrt[]{40^2 + 30^2} = 50 \ m $

Direction:

Approximately 37° north of east.

Here:

Distance = 70 m
Displacement = 50 m northeast

Example 3: Returning to the Start

A runner jogs:

100 m east.
Then 100 m west.

Distance

100 + 100 = 200 m

Displacement

The runner finishes exactly where they started.

Displacement:

0 m

Even though the runner travelled 200 m, there was no overall change in position.

Why Direction Matters

Direction is essential when describing displacement.

Consider two students.

Student A:

Walks 20 m east.

Student B:

Walks 20 m west.

Both travel the same distance.

However, their displacements are different because the directions are opposite.

Direction allows scientists to describe motion more accurately.

Calculating Distance

When an object travels along several sections of a journey:

Distance = Total length of every section added together

Example:

A cyclist rides:

3 km north.
2 km east.
4 km south.

Distance:

3 + 2 + 4 = 9 km

The total path travelled is 9 km.

Determining Displacement

To find displacement:

Identify the starting position.
Identify the ending position.
Draw the shortest straight line between them.
Determine its length.
State its direction.

Unlike distance, displacement ignores the route taken.

Only the initial and final positions matter.

When are Distance and Displacement Equal?

Distance and displacement are equal only when:

The object moves in a straight line.
The object never changes direction.

Examples:

Walking directly across a field.
Driving straight along a road.
A lift moving directly upward.

If the path changes direction, the distance becomes greater than the displacement.

Everyday Examples

Distance and displacement appear in many everyday situations.

Examples include:

A runner completing laps around a track.
A delivery driver following city streets.
A hiker walking along winding trails.
An aircraft flying between airports.
A ship crossing the ocean.

Engineers, pilots, and GPS navigation systems often use displacement to determine the shortest route between two locations.

Why Understanding Distance and Displacement is Important

Distance and displacement are fundamental ideas in mechanics.

They are used to calculate:

Speed.
Velocity.
Acceleration.
Average speed.
Average velocity.

Understanding the difference prevents common mistakes when solving motion problems.

Real-World Connections

Many technologies rely on displacement rather than distance.

Examples include:

GPS navigation calculates your displacement to determine the quickest route.
Pilots plan direct flight paths between airports.
Engineers measure structural movement relative to an original position.
Scientists track the motion of satellites and planets using displacement vectors.

Recognising the difference between distance and displacement allows these systems to make accurate predictions.

Worked Example

A student walks:

60 m north
40 m south

Question

Determine:

The total distance travelled.
The displacement.

Solution

Step 1: Calculate Distance

60 + 40 = 100 m

Distance = 100 m

Step 2: Calculate Displacement

The student finishes:

60 - 40 = 20 m north

Displacement = 20 m north

Did You Know?

During a 400 m race, athletes travel a distance of 400 m, but because they finish where they started, their displacement is 0 m.
GPS systems often calculate the shortest displacement between two locations but must account for roads and obstacles when determining the actual distance to travel.
Astronauts aboard the International Space Station travel over 28,000 km every hour, but after each orbit they return close to their original position relative to Earth, illustrating the difference between distance travelled and displacement.

Key Terms

Distance — The total length of the path travelled by an object.

Displacement — The straight-line change in position from an object's starting point to its ending point, including direction.

Scalar Quantity — A quantity that has magnitude only.

Vector Quantity — A quantity that has both magnitude and direction.

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

Direction — The orientation of movement, such as north, south, east, or west.

Key Takeaways

Distance is the total path travelled, while displacement is the straight-line change in position.
Distance is a scalar quantity with magnitude only, whereas displacement is a vector quantity that includes both magnitude and direction.
Distance is always positive, while displacement depends on both the final position and the chosen direction.
Distance and displacement are equal only when an object travels in a straight line without changing direction.
Calculating distance involves adding the lengths of all parts of a journey, while determining displacement requires comparing the starting and ending positions.
Understanding the difference between distance and displacement provides the foundation for learning speed, velocity, and acceleration.

3. Speed

Learning outcomes

I can define speed as the rate at which distance is traveled.
I can calculate speed using distance and time measurements.
I can identify the SI units used for speed.
I can distinguish between average speed and instantaneous speed.
I can solve problems involving speed, distance, and time.

Introduction

Imagine watching a race between a cyclist, a sprinter, and a racing car. Although they are all moving, they are clearly not moving at the same rate. To compare how quickly objects move, physicists use a quantity called speed.

Speed is one of the most common measurements used in everyday life. It tells us how fast a car is travelling, how quickly an athlete completes a race, or how long it will take to reach a destination. In physics, speed is closely linked to distance and time, making it one of the first mathematical concepts students encounter in the study of motion.

What is Speed?

Speed is the rate at which distance is travelled.

In other words, speed tells us how much distance an object travels in a given amount of time.

An object moving at a higher speed covers more distance in the same amount of time than an object moving at a lower speed.

Examples:

A walking person travels about 1.4 m/s.
A cyclist may travel at 8 m/s.
A car on a highway may travel at 100 km/h.
A commercial aircraft cruises at about 900 km/h.

Calculating Speed

Speed can be calculated if the distance travelled and the time taken are known.

$ v = \frac{d}{t} $

$ v_{avg} = \frac{d}{t} = \frac{60}{5} = 12 \ m/s $

The relationship between these quantities is:

$ Speed = \frac{distance}{time} $

This formula can also be rearranged:

Distance = Speed \times Time

$ Time = \frac{Distance}{Speed} $

These three equations are among the most frequently used in mechanics.

SI Units of Speed

The SI unit (International System of Units) for speed is:

metres per second (m/s)

This means the object travels a certain number of metres every second.

Examples:

2 m/s means 2 metres every second.
15 m/s means 15 metres every second.

Other common units include:

kilometres per hour (km/h)
centimetres per second (cm/s)
miles per hour (mph)

Scientists usually use m/s because it is the SI unit.

Example 1: Calculating Speed

A runner travels 100 m in 12.5 s.

Step 1

Write the known values.

Distance = 100 m

Time = 12.5 s

Step 2

Use the speed equation.

$ Speed = \frac{100}{12.5} $

Step 3

Calculate.

Speed = 8.0 m/s

Answer: The runner's speed is 8.0 m/s.

Average Speed

Average speed is the total distance travelled divided by the total time taken.

It describes the overall rate of motion during an entire journey.

Example:

A car travels:

60 km in the first hour.
40 km in the second hour.

Total distance:

60 + 40 = 100 km

Total time:

2 hours

Average speed:

$ \frac{100}{2} = 50 \ km/h $

Although the car travelled at different speeds during the journey, its average speed was 50 km/h.

Instantaneous Speed

Instantaneous speed is the speed of an object at a particular moment in time.

It may change continuously during a journey.

Examples:

The speed shown on a car's speedometer.
A cyclist accelerating downhill.
A runner sprinting across the finish line.

Instantaneous speed tells us how fast an object is moving right now.

Average Speed vs Instantaneous Speed

Average Speed	Instantaneous Speed
Calculated over an entire journey	Measured at one specific moment
Uses total distance and total time	Changes from moment to moment
Gives an overall rate of motion	Gives the speed at a particular instant

For journeys with changing speeds, the two values are often different.

Solving Speed, Distance, and Time Problems

Many motion problems involve finding one unknown quantity.

A simple strategy is:

Step 1

Identify the known quantities.

Step 2

Choose the correct equation.

Step 3

Substitute the values.

Step 4

Calculate the answer.

Step 5

Include the correct units.

Always check that:

Distance is measured in metres (or kilometres).
Time is measured in seconds (or hours).
Units are consistent before calculating.

Common Mistakes

Students often make these mistakes:

Mixing metres with hours.
Mixing kilometres with seconds.
Forgetting to include units.
Confusing distance with displacement.
Using the wrong equation.

Always convert units before solving problems.

Everyday Examples of Speed

Speed is measured in many situations.

Examples include:

Speed limits on roads.
Athletes running races.
Aircraft travelling between cities.
Ships crossing oceans.
Elevators moving between floors.
Internet data transfer (using a different definition of "speed").

Understanding speed helps us compare how quickly different objects move.

Why Understanding Speed is Important

Speed is one of the most important quantities in physics.

It allows scientists and engineers to:

Predict travel times.
Design safer roads.
Analyse sports performance.
Control robots and machines.
Plan aircraft and spacecraft trajectories.

Speed is also the foundation for understanding velocity and acceleration.

Real-World Connections

Speed measurements are used every day in science, engineering, and technology.

Examples include:

Police use radar guns to measure vehicle speeds.
Engineers test vehicle performance and safety.
Pilots monitor aircraft speed throughout a flight.
Sports scientists analyse athletes' running and cycling speeds.
Space agencies calculate spacecraft speeds during launches and planetary missions.

Accurate speed measurements help improve safety, efficiency, and performance.

Worked Example

A cyclist rides 18 km in 45 minutes.

Question

Calculate the cyclist's average speed in km/h.

Solution

Step 1

Convert the time.

45 minutes = 0.75 hours

Step 2

Use the equation.

$ Speed = \frac{18}{0.75} $

Step 3

Calculate.

Speed = 24 km/h

Answer: The cyclist's average speed is 24 km/h.

Did You Know?

The fastest land animal, the cheetah, can reach speeds of about 110 km/h, but only for short distances.
The speed of sound in air at room temperature is approximately 343 m/s, while the speed of light in a vacuum is about 300 million m/s.
The International Space Station travels at about 28,000 km/h, allowing it to orbit Earth approximately every 90 minutes.

Key Terms

Speed — The rate at which distance is travelled.

Average Speed — The total distance travelled divided by the total time taken.

Instantaneous Speed — The speed of an object at a particular moment in time.

Distance — The total path travelled by an object.

Time — The duration over which motion occurs.

SI Unit — The standard international unit used for measurement. For speed, this is metres per second (m/s).

Key Takeaways

Speed measures how quickly distance is travelled.
Speed is calculated by dividing distance by time.
The SI unit of speed is metres per second (m/s), although km/h is also commonly used.
Average speed describes the overall rate of motion during a journey, while instantaneous speed describes the speed at a single moment.
Solving speed problems requires using consistent units and selecting the correct equation.
Understanding speed provides the foundation for studying velocity, acceleration, and more advanced concepts in mechanics.

4. Velocity

Learning outcomes

I can define velocity as speed in a specified direction.
I can distinguish between speed and velocity.
I can calculate average velocity using displacement and time.
I can describe velocity using both magnitude and direction.
I can interpret situations involving positive and negative velocities.

Introduction

Suppose two cars are travelling at 60 km/h. One is driving north, while the other is driving south. Although their speeds are the same, their motions are clearly different because they are travelling in different directions.

To fully describe motion, physicists use velocity instead of speed. Velocity tells us both how fast an object is moving and the direction in which it is moving. This makes velocity one of the most important quantities in mechanics and prepares us for studying acceleration, forces, and more advanced motion.

What is Velocity?

Velocity is speed in a specified direction.

More precisely, velocity is the rate of change of displacement with time.

Unlike speed, velocity tells us:

How fast an object is moving.
The direction in which it is moving.

Because it includes direction, velocity is a vector quantity.

Examples:

15 m/s east
80 km/h north
5 m/s downward

A value such as 20 m/s alone describes speed, not velocity.

Speed vs Velocity

Speed and velocity are closely related, but they are not the same.

Speed	Velocity
Distance travelled per unit time	Displacement per unit time
Scalar quantity	Vector quantity
Magnitude only	Magnitude and direction
Never negative	Can be positive, negative, or zero (depending on the chosen coordinate system)

Remember:

Speed uses distance.
Velocity uses displacement.

Calculating Average Velocity

Average velocity is calculated using displacement rather than distance.

The equation is:

$ Average \ Velocity = \frac{Displacement}{Time} $

Notice that this is different from the equation for average speed, which uses distance.

SI Units of Velocity

The SI unit of velocity is:

metres per second (m/s)

Other common units include:

kilometres per hour (km/h)
centimetres per second (cm/s)

Since velocity includes direction, the unit should always be accompanied by a direction whenever possible.

Examples:

12 m/s east
5.5 m/s upward
80 km/h north

Example 1: Straight-Line Motion

A cyclist rides 120 m east in 20 s.

Step 1

Write the known values.

Displacement = 120 m east

Time = 20 s

Step 2

Use the equation.

$ Average \ Velocity = \frac{120}{20} $

Step 3

Calculate.

$Average Velocity = 6.0 m/s east$

Answer: The cyclist's average velocity is 6.0 m/s east.

Example 2: Returning Toward the Starting Point

A student walks:

50 m east
Then 20 m west

Time taken = 35 s

Distance

$50 + 20 = 70 m$

Displacement

$50 - 20 = 30 m east$

Average Velocity

$ \frac{30}{35} $ $= 0.86 m/s east$

Although the student travelled 70 m, the average velocity depends only on the 30 m displacement.

Average Speed vs Average Velocity

These quantities are often confused.

Average Speed	Average Velocity
Uses total distance	Uses displacement
Scalar quantity	Vector quantity
No direction	Includes direction
Always positive	May be positive, negative, or zero

For a journey involving changes in direction:

Average speed is usually greater than the magnitude of the average velocity.

Positive and Negative Velocity

In one-dimensional motion, direction is often represented using positive and negative signs.

For example:

East = Positive (+)
West = Negative (−)

North = Positive (+)
South = Negative (−)

The choice is arbitrary, but it must remain consistent throughout the problem.

Example:

+8 m/s means 8 m/s east.
−8 m/s means 8 m/s west.

The negative sign indicates direction, not that the object is slowing down.

Interpreting Positive and Negative Velocities

Consider a car travelling along a straight road.

Velocity	Meaning
+20 m/s	Moving in the positive direction
+5 m/s	Moving more slowly in the positive direction
0 m/s	Stationary
–5 m/s	Moving in the opposite direction
–20 m/s	Moving faster in the negative direction

The sign tells us the direction of motion, while the magnitude tells us how fast the object is moving.

When are Speed and Velocity Equal?

The numerical values of speed and the magnitude of velocity are equal only when:

The object moves in a straight line.
The object never changes direction.

If an object changes direction:

Distance becomes greater than displacement.
Average speed becomes greater than the magnitude of average velocity.

Everyday Examples of Velocity

Velocity is important whenever direction matters.

Examples include:

A pilot flying northwest.
A ship sailing south.
Wind blowing eastward.
A football kicked toward the goal.
A satellite orbiting Earth.

In each case, both speed and direction are needed to fully describe the motion.

Why Understanding Velocity is Important

Velocity is one of the fundamental quantities in mechanics.

It is used to:

Calculate acceleration.
Predict future positions.
Analyse collisions.
Study projectile motion.
Design transport systems.
Navigate aircraft and spacecraft.

Many later topics in physics rely on understanding velocity.

Real-World Connections

Velocity plays a crucial role in science and engineering.

Examples include:

Pilots use velocity to navigate aircraft accurately.
Meteorologists measure wind velocity to forecast weather.
Engineers analyse the velocity of vehicles during crash testing.
Astronomers measure the velocities of stars and galaxies to study the expansion of the Universe.
GPS systems continuously calculate a vehicle's velocity to provide navigation and estimated arrival times.

Worked Example

A runner travels:

200 m north

25 s

Question

Calculate the runner's average velocity.

Solution

Step 1

Write the known values.

Displacement = 200 m north

Time = 25 s

Step 2

Use the equation.

$ Average \ Velocity = \frac{200}{25} $

Step 3

Calculate.

$Average Velocity = 8.0 m/s north$

Answer: The runner's average velocity is 8.0 m/s north.

Did You Know?

Although Earth's surface rotates at speeds of up to 1,670 km/h near the equator, your velocity is constantly changing because your direction changes as Earth rotates.
Astronomers use the Doppler effect to measure the velocities of distant stars and galaxies by observing changes in the wavelengths of light they emit.
A racing car may have a very high speed, but when it travels around a circular track, its velocity is constantly changing because its direction is continually changing.

Key Terms

Velocity — The rate of change of displacement with time; speed in a specified direction.

Average Velocity — The total displacement divided by the total time taken.

Speed — The rate at which distance is travelled.

Displacement — The straight-line change in position from the starting point to the ending point.

Vector Quantity — A quantity that has both magnitude and direction.

Scalar Quantity — A quantity that has magnitude only.

Key Takeaways

Velocity describes how fast an object moves and in what direction.
Velocity is calculated using displacement, while speed is calculated using distance.
Velocity is a vector quantity, meaning it has both magnitude and direction.
Positive and negative velocities indicate opposite directions of motion, not faster or slower movement.
Average velocity is found by dividing displacement by time.
Understanding velocity provides the foundation for studying acceleration, projectile motion, and many other areas of mechanics.

5. Motion in Everyday Life

Learning outcomes

I can identify examples of speed and velocity in real-world situations.
I can analyze motion in transportation, sports, and daily activities.
I can explain how motion measurements are used in technology.
I can collect and interpret simple motion data.
I can apply kinematic concepts to solve practical problems.

Introduction

Motion is all around us. Every day we walk, cycle, drive, ride buses, throw balls, climb stairs, and watch birds fly through the sky. Although these activities may seem ordinary, they all involve the same physical principles that scientists use to study the motion of planets, satellites, and spacecraft.

Understanding motion allows us to make accurate predictions, improve safety, design better vehicles, and enhance athletic performance. In this lesson, you will discover how the concepts of distance, displacement, speed, and velocity help us understand and solve problems in everyday life.

Motion Around Us

Motion occurs whenever an object's position changes over time.

Examples include:

A student walking to school.
A car travelling along a highway.
A football flying through the air.
A bird searching for food.
A train arriving at a station.
Earth orbiting the Sun.

Although these motions differ greatly in scale, they can all be described using the same kinematic concepts.

Speed in Everyday Life

Speed tells us how quickly an object travels.

Examples include:

A person walking at about 1.4 m/s.
A cyclist travelling at 25 km/h.
A car driving at the posted speed limit.
A high-speed train travelling over 300 km/h.
An aircraft cruising at approximately 900 km/h.

Speed helps us estimate:

Travel times.
Fuel use.
Arrival times.
Performance.

Velocity in Everyday Life

Velocity includes both speed and direction.

Examples include:

A boat travelling 15 km/h north.
Wind blowing 20 km/h west.
A football kicked toward the goal.
A spacecraft travelling toward Mars.

Direction is essential whenever we need to know where an object is moving, not just how fast.

Motion in Transportation

Transportation systems rely heavily on accurate motion measurements.

Examples include:

Cars

Drivers use:

Speedometers.
GPS navigation.
Cruise control.

These systems help maintain safe and efficient travel.

Aircraft

Pilots continuously monitor:

Airspeed.
Ground speed.
Direction.
Altitude.

Navigation systems use velocity to guide aircraft safely.

Trains

Rail operators monitor train speed to:

Maintain schedules.
Improve passenger comfort.
Ensure safe braking distances.

Ships

Captains use velocity measurements to compensate for:

Ocean currents.
Wind.
Waves.

Motion in Sports

Athletes and coaches use motion analysis to improve performance.

Examples include:

Sprinting

Scientists measure:

Running speed.
Reaction time.
Acceleration.

Cycling

Cyclists monitor:

Average speed.
Maximum speed.
Distance travelled.

Swimming

Motion analysis helps improve:

Stroke technique.
Turn efficiency.
Race times.

Ball Sports

Motion measurements help analyse:

Ball speed.
Throwing velocity.
Kick distance.
Player movement.

Professional teams often use motion-tracking cameras to collect detailed performance data.

Motion in Daily Activities

Many everyday tasks involve motion.

Examples include:

Walking to school.
Riding an elevator.
Throwing a ball.
Walking the dog.
Taking public transport.
Shopping.

Although these activities seem simple, they all involve measurable changes in position over time.

Motion and Technology

Modern technology depends on accurate motion measurements.

Examples include:

GPS Navigation

GPS receivers determine:

Position.
Speed.
Direction.
Estimated arrival time.

Fitness Trackers

Smart watches record:

Distance walked.
Running speed.
Cycling speed.
Calories burned.

Self-Driving Cars

Autonomous vehicles constantly measure:

Vehicle speed.
Surrounding traffic.
Distance to obstacles.
Direction of travel.

Radar Guns

Police use radar devices to measure vehicle speeds.

These measurements improve road safety.

Collecting Motion Data

Scientists collect motion data by measuring:

Distance.
Displacement.
Time.
Speed.
Velocity.

Simple equipment includes:

Measuring tape.
Stopwatch.
Metre stick.
Smartphone sensors.
GPS devices.

Careful measurements allow scientists to analyse motion accurately.

Interpreting Motion Data

Motion data can be organised into:

Tables.
Graphs.
Charts.

Example:

Time (s)	Distance (m)
0	0
2	6
4	12
6	18
8	24

From this table we observe:

Distance increases steadily.
The object travels equal distances in equal time intervals.
The speed is constant.

Interpreting data helps identify patterns in motion.

Applying Kinematics to Practical Problems

Kinematics allows us to solve many practical problems.

Examples include:

How long will a journey take?
Which route is the fastest?
How fast must an athlete run?
How long before a satellite reaches its destination?
How quickly should a driver brake?

These questions are answered using the concepts of:

Distance.
Displacement.
Speed.
Velocity.
Time.

Real-World Investigation

Suppose a student walks 80 m across the school playground in 50 s.

Question

Determine the student's average speed.

Solution

Distance = 80 m

Time = 50 s

$ Speed = \frac{80}{50} = 1.6 \ m/s $

The student's average walking speed is 1.6 m/s.

Now suppose the student walked to the playground and then returned to the starting point.

The distance would increase, but the displacement would become 0 m.

This illustrates why distance and displacement are different.

Why Studying Motion Matters

Understanding motion helps scientists and engineers:

Design safer vehicles.
Improve sports performance.
Launch satellites.
Predict weather patterns.
Navigate ships and aircraft.
Build robots.
Develop autonomous vehicles.

Nearly every branch of science and engineering depends on accurate measurements of motion.

Real-World Connections

Motion is one of the most widely used concepts in STEM careers.

Examples include:

Civil engineers calculate vehicle speeds when designing roads and bridges.
Mechanical engineers analyse moving parts in machines.
Biomedical engineers study human movement to improve prosthetic limbs.
Sports scientists use motion analysis to improve athletic performance.
Astronomers calculate the motion of planets, moons, and galaxies.

Whether studying a person walking or a spacecraft travelling through the Solar System, the same principles of kinematics apply.

Worked Example

A cyclist rides 15 km east in 30 minutes.

Question

Determine:

Average speed.
Average velocity.

Solution

Step 1: Convert the time

30 minutes = 0.5 hours

Step 2: Calculate average speed

$ \frac{15}{0.5} = 30 \ km/h $

Average speed = 30 km/h

Step 3: Calculate average velocity

The cyclist travelled in a straight line toward the east.

Displacement = 15 km east

$ \frac{15}{0.5} = 30 \ km/h \ east $

Average velocity = 30 km/h east

Since the cyclist never changed direction, the numerical values of average speed and average velocity are the same.

Did You Know?

Elite tennis serves can exceed 240 km/h, while a professional baseball pitch can reach speeds of over 160 km/h.
Modern smartphones use built-in accelerometers and GPS sensors to measure motion for navigation, fitness tracking, and gaming.
Engineers testing new cars collect thousands of motion measurements every second to improve vehicle safety, fuel efficiency, and performance.

Key Terms

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

Speed — The rate at which distance is travelled.

Velocity — The rate of change of displacement; speed in a specified direction.

Displacement — The straight-line change in position from the starting point to the ending point.

Kinematics — The branch of physics that describes motion without considering the forces that cause it.

Motion Data — Measurements such as distance, displacement, speed, velocity, and time used to analyse motion.

Key Takeaways

Motion can be described using distance, displacement, speed, velocity, and time.
Speed and velocity are used in transportation, sports, technology, and many everyday activities.
Motion measurements help improve safety, navigation, performance, and engineering design.
Scientists collect motion data using measuring tools, stopwatches, GPS systems, and electronic sensors, then analyse the results using tables and graphs.
Kinematic concepts allow us to solve practical problems involving travel, sports performance, and vehicle motion.
The same principles of motion apply to objects ranging from people walking across a playground to spacecraft exploring the Solar System.