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What Is Relativity?

When we describe an object's motion, an important question is:

Motion relative to what?

Imagine sitting on a train travelling at a constant velocity.

Relative to your seat:

you are at rest

Relative to the ground:

you are moving

Both descriptions are correct because motion is measured relative to a:

reference frame

This simple idea is at the heart of classical relativity.

---

Reference Frames

A reference frame is a coordinate system or viewpoint used to measure:

• position
• displacement
• velocity
• acceleration
• time

Suppose a car travels along a road at:

20 m/s

Someone standing beside the road measures the car's velocity as:

20 m/s

But a passenger sitting inside the car measures the car's velocity relative to themselves as:

0 m/s

The velocity depends on the:

reference frame

---

Relative Motion

Motion is generally described relative to another:

object or reference frame

Consider two cars travelling in the same direction.

Car A:

20 m/s

Car B:

15 m/s

Relative to the road, Car A travels at:

20 m/s

But relative to Car B:

20 − 15 = 5 m/s

So Car A moves away from Car B at:

5 m/s

This is an example of:

relative velocity

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Galilean Relativity

The classical principle of relativity is often associated with:

Galileo Galilei.

Galileo recognized that experiments involving ordinary mechanical motion behave the same way in reference frames moving at:

constant velocity

This became known as the:

principle of Galilean relativity

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Galileo's Ship

Galileo illustrated the idea using a thought experiment involving a:

moving ship

Imagine being inside a closed cabin on a ship travelling smoothly at:

constant velocity

Inside the cabin you might:

• drop a ball
• throw an object
• watch water drip
• observe insects flying

If the ship moves smoothly without accelerating, these experiments behave just as they would if the ship were:

stationary

From mechanical experiments inside the cabin alone, you cannot determine whether the ship is:

at rest or moving uniformly

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The Principle of Classical Relativity

The classical principle of relativity can be stated as:

The laws of mechanics are the same in all inertial reference frames.

This means there is no special inertial frame in which Newton's laws work:

better

than in another.

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What Is an Inertial Reference Frame?

An inertial reference frame is a reference frame that is not accelerating.

It is either:

at rest

or:

moving at constant velocity

relative to another inertial frame.

Newton's laws take their usual simple form in:

inertial frames

---

Newton's First Law and Inertial Frames

Isaac Newton described the behavior of objects when no resultant force acts.

Newton's First Law states that an object remains:

at rest

or:

moving at constant velocity

unless acted upon by a resultant external force.

This law effectively defines the idea of an:

inertial reference frame

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Example: Ball on a Train

Imagine a train moving at constant velocity.

A passenger throws a ball vertically upward.

What happens?

To the passenger:

the ball moves straight up and straight down

To an observer standing beside the railway:

the ball follows a curved path while also moving forward with the train

The observers disagree about the ball's:

path and velocity

but both can correctly apply:

Newton's laws

---

Why Does the Ball Return to the Passenger?

Before the ball is thrown, it is already moving horizontally with the:

train

When released, it retains this horizontal velocity.

Therefore, while it moves upward and downward, it also continues moving:

forward

with the train.

To the passenger, the horizontal motion is shared and therefore:

not apparent

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Relative Velocity

Classical relativity uses a simple rule for transforming velocities between reference frames.

Suppose:

• v = object's velocity measured by one observer
• u = velocity of another reference frame

Then the object's velocity relative to the moving frame can be written:

v′ = v − u

This is a Galilean velocity transformation.

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Worked Example 1

A train moves east at:

25 m/s

A passenger walks east through the train at:

2 m/s

Relative to the train:

v′ = 2 m/s east

Relative to the ground:

v = 25 + 2

v = 27 m/s east

---

Worked Example 2

The train still moves east at:

25 m/s

The passenger now walks west at:

2 m/s

Relative to the ground:

v = 25 − 2

v = 23 m/s east

The passenger walks backward relative to the train but still moves east relative to:

the ground

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Worked Example 3: Two Vehicles

Car A moves east at:

30 m/s

Car B moves east at:

22 m/s

Velocity of A relative to B:

30 − 22 = 8 m/s east

From someone inside Car B, Car A appears to move ahead at:

8 m/s

---

Opposite Directions

Suppose:

Car A travels east at:

20 m/s

Car B travels west at:

15 m/s

Their relative speed is:

20 + 15 = 35 m/s

They approach one another at:

35 m/s

The signs of velocity must therefore be handled carefully.

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Position Transformations

Classical relativity can also relate positions measured in different reference frames.

Suppose two reference frames move relative to each other at constant velocity:

u

If their origins coincide at:

t = 0

then:

x′ = x − ut

This is part of the:

Galilean transformation

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Time in Classical Physics

Classical physics makes an extremely important assumption:

time is absolute

This means all observers are assumed to agree about:

time intervals

Mathematically:

t′ = t

If one observer measures:

10 seconds

another inertial observer also measures:

10 seconds

regardless of their relative motion.

This seems completely reasonable in:

everyday experience

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Space and Time in Classical Mechanics

In classical mechanics:

space and time are treated separately

Observers may disagree about:

• position
• velocity

but they agree about:

• time intervals
• simultaneity

This classical picture works extremely well for:

ordinary speeds

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Absolute and Relative Motion

This distinction needs some care.

Relative Motion

Velocity and position depend on the chosen:

reference frame

There is no mechanically preferred inertial frame in Galilean relativity.

Absolute Motion

An absolute motion would mean motion measured relative to some universally preferred state of rest.

Classical mechanics historically used absolute space and absolute time in Newton's formulation, but Galilean relativity means that uniform mechanical motion cannot identify a unique:

absolute-rest frame

So in practical Newtonian mechanics, velocities are measured:

relative to a chosen frame

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Absolute Acceleration

Classical mechanics treats acceleration differently from constant velocity.

Suppose a car suddenly accelerates.

Passengers may feel themselves pushed backward against their:

seats

Acceleration produces observable physical effects.

Uniform velocity does not produce the same kind of effect.

Therefore:

constant velocity and acceleration are fundamentally different

in classical mechanics.

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Non-Inertial Reference Frames

An accelerating reference frame is called:

non-inertial

Examples include:

• an accelerating car
• a braking bus
• a rotating carousel
• a rapidly turning aircraft

Inside such a frame, objects may appear to behave in ways that cannot be explained using ordinary Newton's laws unless additional:

inertial or fictitious forces

are introduced.

---

Example: Accelerating Bus

You are standing inside a bus.

The bus suddenly accelerates forward.

You appear to move:

backward

relative to the bus.

From the ground frame, your body is resisting the change in motion because of:

inertia

The bus moves forward underneath you until forces from the floor and your body accelerate you with it.

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Newtonian Mechanics and Classical Relativity

Newtonian mechanics works consistently between inertial frames because acceleration is unchanged under a Galilean transformation.

If:

v′ = v − u

and u is constant, then:

a′ = a

Therefore, observers moving at constant velocity relative to one another agree on an object's:

acceleration

This is important because Newton's Second Law is:

F = ma

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The Laws of Mechanics Remain the Same

Suppose two observers move at constant velocity relative to each other.

They may disagree about:

velocity

but they agree about:

acceleration

Therefore, if they use the same mass and force:

F = ma

has the same form for both observers.

This is why Newtonian mechanics is compatible with:

Galilean relativity

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No Mechanical Experiment Reveals Uniform Motion

Imagine a perfectly smooth spacecraft far from planets and stars.

The windows are covered.

You perform mechanical experiments inside.

If the spacecraft moves at:

constant velocity

there is no ordinary mechanical experiment that can tell you its uniform velocity relative to some supposed:

absolute space

You can measure motion only relative to:

other objects or reference frames

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The Problem of Light

Classical relativity works extremely well for:

• cars
• trains
• aircraft
• projectiles
• most everyday mechanical systems

But physics encountered a major problem when considering:

light

Classical velocity addition suggests that velocities should simply:

add and subtract

But light does not behave according to this classical rule.

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Classical Prediction for Light

Suppose a spacecraft travels toward Earth at:

0.5c

where c is the speed of light.

It shines a beam of light forward.

Using classical velocity addition, someone might predict that an observer on Earth measures:

c + 0.5c = 1.5c

But this is:

not what special relativity predicts or experiments support

The measured speed of light in vacuum remains:

c

for all inertial observers.

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The Speed of Light

The speed of light in vacuum is approximately:

c = 3.00 × 10⁸ m/s

This is about:

300,000 km/s

According to special relativity, every inertial observer measures the same value of c for light in vacuum, regardless of the motion of:

the source or observer

This cannot be reconciled with ordinary Galilean velocity addition.

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Why Classical Relativity Fails at Very High Speeds

At everyday speeds:

v ≪ c

classical mechanics provides an excellent approximation.

But when speeds become a significant fraction of:

c

classical assumptions become inaccurate.

In particular, the assumptions of:

absolute time

and:

simple Galilean velocity addition

must be replaced.

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Einstein's Special Relativity

In 1905, Albert Einstein developed the theory of:

special relativity

It is based on two central postulates.

Postulate 1

The laws of physics are the same in all inertial reference frames.

This extends the relativity principle beyond mechanics to:

all laws of physics

Postulate 2

The speed of light in vacuum has the same value for all inertial observers.

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Galileo and Einstein

There is an important continuity between the two theories.

Galileo:

laws of mechanics are the same in inertial frames

Einstein:

laws of physics are the same in inertial frames

Einstein did not simply discard the principle of relativity.

He:

extended it

and changed our understanding of:

space and time

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Classical vs Einsteinian Relativity

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Absolute Time vs Relative Time

Classical physics assumes:

t′ = t

Einsteinian relativity does not.

Observers moving relative to one another can measure different:

time intervals

This leads to:

time dilation

A moving clock can be measured as running more slowly relative to a particular inertial observer's coordinate time.

This effect is negligible at ordinary speeds but becomes significant at:

relativistic speeds

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Length Is Also Frame-Dependent

Classical mechanics treats an object's length as independent of uniform motion.

Special relativity predicts:

length contraction

The measured length of an object along the direction of relative motion depends on the:

reference frame

Again, this effect is extremely small at:

ordinary speeds

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Simultaneity

Perhaps one of the deepest changes concerns events that happen:

at the same time

Classically, if two events occur simultaneously for one observer, they occur simultaneously for:

everyone

Special relativity shows that two spatially separated events that are simultaneous in one inertial frame may not be simultaneous in:

another inertial frame

This is called:

relativity of simultaneity

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Einstein's Train Thought Experiment

Imagine lightning strikes the front and back of a moving train.

An observer standing midway on the platform might receive light from both strikes at the:

same time

and conclude that the strikes were simultaneous in the platform frame.

An observer at the midpoint of the moving train is moving:

toward one flash

and:

away from the other

The two observers can therefore disagree about whether the spatially separated events were:

simultaneous

This disagreement is not caused by faulty instruments.

It follows from the structure of:

spacetime

---

Galilean Velocity Addition

For ordinary speeds:

v = u + v′

works extremely well.

Suppose a person runs at:

5 m/s

inside a train moving at:

20 m/s

Then:

v = 20 + 5

v = 25 m/s

No practical relativistic correction is needed.

---

Relativistic Velocity Addition

At very high speeds, velocities combine differently.

For motion in the same direction:

v = (u + v′) / (1 + uv′/c²)

This prevents the resulting velocity from exceeding:

c

for material objects or signals starting below c.

---

Worked Example: High-Speed Spacecraft

Suppose one spacecraft moves at:

0.70c

and launches an object forward at:

0.60c

relative to itself.

Classical prediction:

0.70c + 0.60c = 1.30c

This would exceed the speed of light.

Relativistically:

v = (0.70c + 0.60c) / (1 + (0.70)(0.60))

v = 1.30c / 1.42

v ≈ 0.915c

So the measured speed remains:

below c

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Why Classical Mechanics Still Works

If classical mechanics is not fundamentally accurate at all speeds, why do we still use it?

Because for:

v ≪ c

relativistic corrections are extremely small.

For:

• walking
• cars
• trains
• aircraft
• falling objects
• most engineering systems

Newtonian mechanics is:

extremely accurate and much simpler

A successful newer theory should reproduce the older theory in the region where the older theory was already successful.

---

The Correspondence Principle

At speeds much smaller than the speed of light:

special relativity approaches classical mechanics

This is an example of a broader scientific idea sometimes called the:

correspondence principle

The newer theory does not make ordinary Newtonian calculations useless.

Instead, Newtonian mechanics becomes an excellent:

low-speed approximation

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How Fast Is "Very Fast"?

The importance of relativistic corrections depends on the ratio:

v/c

At:

30 m/s

the ratio is extremely small.

At:

0.01c

relativistic corrections are still relatively small for many purposes.

At:

0.5c

they can no longer be ignored.

At:

0.9c

they are substantial.

Therefore, the relevant question is not simply:

"Is the object moving fast?"

but:

"How large is its speed compared with c?"

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The Lorentz Factor

Special relativity often uses the Lorentz factor:

γ = 1 / √(1 − v²/c²)

At low speeds:

v/c ≈ 0

so:

γ ≈ 1

Classical mechanics is therefore recovered approximately.

As v approaches c:

γ increases significantly

and relativistic effects become important.

---

Example: Everyday Speed

Suppose a car travels at:

30 m/s

Compared with:

c = 3.00 × 10⁸ m/s

we have approximately:

v/c = 1 × 10⁻⁷

This is tiny.

Therefore:

Newtonian mechanics is entirely adequate for ordinary driving calculations

---

Example: Spacecraft at 0.8c

For:

v = 0.8c

the Lorentz factor is:

γ = 1 / √(1 − 0.8²)

γ = 1 / √0.36

γ ≈ 1.67

This is very different from:

1

Relativistic effects therefore cannot be ignored.

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Classical Relativity in Everyday Life

Classical relativity explains many familiar situations.

Examples include:

• walking inside a moving train
• throwing a ball inside an aircraft
• comparing cars on a highway
• boats moving in flowing water
• aircraft flying through moving air

In each case, velocity depends on:

the chosen reference frame

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Boats and Rivers

Suppose a boat moves through water at:

5 m/s east

while the river flows:

2 m/s east

Relative to the riverbank:

5 + 2 = 7 m/s east

If the boat instead travels west through the water:

5 m/s west

then relative to the bank its speed is:

5 − 2 = 3 m/s west

This is a practical application of:

classical velocity addition

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Aircraft and Wind

An aircraft's velocity can be measured relative to:

the surrounding air

while its ground velocity is measured relative to:

Earth's surface

Wind changes the relationship between these velocities.

Pilots and navigation systems therefore use:

relative-motion calculations

to determine actual ground movement.

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Navigation and Reference Frames

Navigation always requires a clearly defined:

reference frame

For example:

• ship relative to water
• aircraft relative to air
• vehicle relative to road
• satellite relative to Earth
• planet relative to Sun

A statement such as:

"The object is travelling at 20 m/s"

is incomplete unless we know:

relative to what?

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Is Earth an Inertial Frame?

Strictly speaking:

not perfectly

Earth:

• rotates
• orbits the Sun
• experiences acceleration

Therefore, an Earth-fixed frame is not a perfect inertial frame.

However, for many everyday experiments over limited distances and times, Earth's surface can be treated as:

approximately inertial

This is another example of using an appropriate:

scientific approximation

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Absolute vs Relative: A Useful Summary

Position

Depends on:

reference frame

Velocity

Depends on:

reference frame

Acceleration in Galilean inertial frames

Observers agree on:

acceleration

Time in classical physics

Assumed to be:

absolute

Speed of light in special relativity

Measured as the same:

c

by all inertial observers.

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Worked Example 4: Train and Ball

A train moves east at:

18 m/s

A passenger throws a ball east at:

6 m/s

relative to the train.

Relative to the ground:

v = 18 + 6

v = 24 m/s east

If the passenger throws the ball west at 6 m/s:

v = 18 − 6

v = 12 m/s east

Even though the ball is thrown backward relative to the train, it is still moving:

east relative to the ground

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Worked Example 5: Relative Cars

Car A travels north at:

28 m/s

Car B travels north at:

20 m/s

Velocity of A relative to B:

28 − 20

= 8 m/s north

From Car B, Car A appears to move away at:

8 m/s

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Worked Example 6: Classical or Relativistic?

Which model should be used?

Baseball at 40 m/s

Use:

classical mechanics

Passenger aircraft at 250 m/s

Use:

classical mechanics

Spacecraft at 0.85c

Use:

special relativity

Electron moving at 0.95c

Use:

special relativity

The key consideration is:

speed relative to c

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Common Misconception: "Relative" Means Nothing Is Real

Relativity does not mean:

anything can be true

Measurements are made according to precise rules within:

reference frames

Different observers may measure different positions, velocities, lengths, or time intervals, but their measurements are connected by:

mathematical transformations

Relativity is therefore highly:

quantitative and predictive

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Common Misconception: There Must Be an Absolute State of Rest

Classical relativity provides no mechanical method for identifying a universally preferred:

inertial rest frame

If two inertial frames move uniformly relative to one another, the laws of mechanics work:

equally well in both

Neither can claim to be the uniquely:

stationary frame

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Common Misconception: Newton Was Simply Wrong

Newtonian mechanics is extraordinarily successful within its:

domain of applicability

It remains appropriate for:

• buildings
• bridges
• vehicles
• projectiles
• many planetary calculations
• everyday mechanics

Special relativity becomes necessary when:

relativistic effects are significant

Scientific theories often have:

domains where particular approximations work extremely well

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Common Misconception: Einstein Rejected Relativity

Einstein actually strengthened the:

principle of relativity

Galilean relativity applies the principle to:

mechanics

Special relativity extends it to:

all physical laws

including:

electromagnetism

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Common Misconception: Time Is Universal

This is an assumption of:

classical mechanics

Special relativity shows that measured time intervals can depend on:

relative motion

There is no universal clock giving the same elapsed time between arbitrary events for:

every observer

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Common Misconception: Velocities Always Add Normally

Classical addition:

v = u + v′

works extremely well at:

low speeds

But at relativistic speeds we need:

relativistic velocity addition

This ensures consistency with the invariant speed:

c

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From Galileo to Einstein

The development of relativity provides an excellent example of how science progresses.

Galileo established that:

uniform motion is relative

Newton developed powerful mathematical laws of:

mechanics

Later developments in electromagnetism and experiments concerning light exposed limitations in the classical framework.

Einstein then developed a broader theory that preserved the:

principle of relativity

while changing our understanding of:

space and time

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A Useful Comparison

Think of the relationship as:

Galileo → relative motion

↓

Newton → mechanics in inertial frames

↓

problem → light does not obey classical velocity addition

↓

Einstein → special relativity

↓

space and time become frame-dependent

The classical theory remains an excellent approximation when:

v ≪ c

---

Applying the Idea to an Unfamiliar Situation

Suppose you are inside a spacecraft moving smoothly through deep space.

You cannot see outside.

A ball floats beside you.

Can you determine your spacecraft's constant velocity through space using only ordinary mechanical experiments inside?

No.

If the spacecraft is an inertial frame, the laws of mechanics work normally.

You cannot determine a unique:

absolute uniform velocity

from internal mechanical experiments.

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Another Unfamiliar Situation

A spacecraft passes Earth at:

0.75c

and fires a probe forward at:

0.50c

relative to itself.

Should you calculate:

0.75c + 0.50c = 1.25c?

No.

At these speeds:

Galilean velocity addition fails

and:

relativistic velocity addition

must be used.

Recognizing when a model is no longer appropriate is an important scientific skill.

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Choosing the Correct Model

Ask:

Is the frame accelerating?

If yes:

ordinary inertial-frame analysis may need modification

Are speeds much smaller than c?

If yes:

classical mechanics is usually appropriate

Are speeds a significant fraction of c?

If yes:

special relativity may be required

Does the problem involve comparing measurements between observers?

Identify:

the reference frames

before calculating.

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Check Your Understanding

1. What is a reference frame?

2. Explain why a passenger can be at rest relative to a train but moving relative to the ground.

3. What is an inertial reference frame?

4. State the classical principle of relativity.

5. A train moves east at 30 m/s and a passenger walks east at 3 m/s. What is the passenger's velocity relative to the ground?

6. Two cars travel east at 25 m/s and 18 m/s. What is their relative velocity?

7. Why can a mechanical experiment inside a smoothly moving train not reveal the train's constant velocity?

8. What assumption does classical mechanics make about time?

9. Why does classical velocity addition create a problem when applied to light?

10. State the two basic postulates of special relativity.

11. Explain one important difference between Galilean and Einsteinian relativity.

12. Why does Newtonian mechanics still work extremely well for everyday objects?

13. A spacecraft travels at 0.8c. Explain why classical mechanics may no longer provide accurate results.

14. Explain the difference between an inertial and a non-inertial reference frame.

15. Explain how Einstein's theory extends rather than simply abandons the classical principle of relativity.

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Key Terms

• Reference frame: Coordinate system or viewpoint relative to which measurements are made.
• Relative motion: Motion described relative to another object or reference frame.
• Classical relativity: Principle that the laws of mechanics have the same form in all inertial reference frames.
• Galilean relativity: Classical description of relativity associated with Galileo.
• Inertial reference frame: Non-accelerating frame in which Newton's laws take their standard form.
• Non-inertial reference frame: Accelerating or rotating reference frame.
• Relative velocity: Velocity of one object measured from another reference frame.
• Galilean transformation: Classical equations relating position, velocity, and time between inertial frames.
• Absolute time: Classical assumption that time intervals are the same for all observers.
• Absolute motion: Hypothetical motion relative to a universally preferred state of rest.
• Speed of light: Invariant vacuum speed c ≈ 3.00 × 10⁸ m/s.
• Special relativity: Einstein's theory describing relationships between space, time, and motion in inertial frames.
• Lorentz transformation: Relativistic transformation relating space and time measurements between inertial frames.
• Time dilation: Difference in elapsed times measured between appropriate events by observers in relative motion.
• Length contraction: Frame-dependent reduction in measured length along the direction of relative motion.
• Relativity of simultaneity: Principle that spatially separated events simultaneous in one inertial frame need not be simultaneous in another.
• Lorentz factor: Factor γ = 1/√(1 − v²/c²) appearing throughout special relativity.
• Correspondence principle: Idea that a newer theory reproduces the successful predictions of an older theory within the older theory's valid domain.

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Key Takeaways

• Motion must always be described relative to a reference frame.
• An object can be stationary in one reference frame and moving in another.
• Position and velocity are relative quantities.
• An inertial reference frame moves at constant velocity and is not accelerating.
• The classical principle of relativity states that the laws of mechanics are the same in all inertial frames.
• There is no preferred inertial frame identifiable through ordinary internal mechanical experiments.
• Classical relative velocities can be calculated using Galilean velocity addition.
• Newtonian mechanics works consistently between Galilean inertial frames.
• Classical mechanics assumes that time is absolute.
• Classical mechanics treats space and time as separate.
• Newtonian mechanics works extremely well when v ≪ c.
• Classical relativity becomes inadequate when speeds become a significant fraction of the speed of light.
• Light does not follow ordinary Galilean velocity addition.
• The speed of light in vacuum is c ≈ 3.00 × 10⁸ m/s for every inertial observer.
• Einstein's special relativity extends the relativity principle from mechanics to all laws of physics.
• Special relativity replaces Galilean transformations with Lorentz transformations.
• In special relativity, time intervals, lengths, and simultaneity can depend on the observer's reference frame.
• Relativistic velocity addition prevents ordinary objects or signals starting below c from being transformed to speeds greater than c.
• Newtonian mechanics is not useless or obsolete; it is an extremely accurate low-speed approximation to relativistic mechanics.
• A central progression is Galilean relativity → Newtonian mechanics → limitations involving light → Einsteinian relativity.
• The most important question when choosing between classical and relativistic mechanics is often: How large is the speed compared with c?