What Is Gravity?

Gravity is an attractive interaction between objects that have mass.

Every object with mass gravitationally attracts every other object with mass.

This means gravity exists between:

• you and Earth
• Earth and the Moon
• Earth and the Sun
• two people standing beside each other
• planets and their moons
• stars and planets
• galaxies and other galaxies

The gravitational attraction between ordinary objects is extremely small, so we usually notice gravity only when at least one of the objects has a very large mass.

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Gravity Is Always Attractive

In ordinary physics, gravitational forces between masses are attractive.

This means two masses gravitationally pull toward one another.

For example, Earth attracts you toward its centre.

At the same time, you also gravitationally attract Earth.

The forces act in opposite directions along the line joining the centres of the two objects.

Because Earth's mass is enormous compared with a person's mass, Earth's resulting acceleration is immeasurably small in everyday situations.

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Gravity Acts at a Distance

Gravity is a non-contact interaction.

Earth does not need to physically touch the Moon to affect its motion.

The Sun does not need to touch Earth to keep Earth in orbit.

Gravity acts across space.

This distinguishes gravity from contact forces such as:

• friction
• normal force
• tension from a rope
• air resistance
• spring forces

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Gravitational Fields

A useful way to describe gravity is through a gravitational field.

A mass produces a gravitational field in the space around it.

Another mass placed within that field experiences a gravitational force.

For a spherical object such as Earth, the gravitational field points approximately toward the object's centre.

The field becomes weaker as distance from the object increases.

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Gravity on Earth

Earth has a very large mass:

approximately 5.97 × 10²⁴ kg

Its gravitational field strongly influences objects near its surface.

Gravity causes objects to:

• fall when released
• have weight
• accelerate downward
• follow curved paths when thrown
• remain near Earth's surface

Near Earth's surface, the gravitational field strength is approximately:

g = 9.8 N/kg

For many introductory calculations, this may be rounded to:

g ≈ 10 N/kg

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Mass and Weight

Mass and weight are related but different.

Mass measures the amount of matter or inertia of an object.

Unit:

kilogram (kg)

Weight is the gravitational force acting on an object.

Unit:

newton (N)

The relationship is:

W = mg

where:

• W = weight in newtons
• m = mass in kilograms
• g = gravitational field strength in N/kg

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Example: Weight on Earth

A student has a mass of:

60 kg

Using:

g = 9.8 N/kg

Weight:

W = mg

W = 60 × 9.8

W = 588 N

The student's mass is:

60 kg

The student's weight is approximately:

588 N

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Mass Does Not Depend on Location

Suppose an astronaut has a mass of:

70 kg

On Earth:

mass = 70 kg

On the Moon:

mass = 70 kg

In orbit:

mass = 70 kg

Mass does not change simply because the astronaut moves somewhere else.

Weight can change because gravitational field strength changes.

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Weight on the Moon

The Moon has less mass than Earth, so its surface gravitational field is weaker.

The Moon's surface gravitational field strength is approximately:

1.6 N/kg

For a 60 kg person:

W = mg

W = 60 × 1.6

W = 96 N

The person's mass remains 60 kg, but their weight is much smaller.

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Falling Objects

If an object is released near Earth's surface, gravity causes it to accelerate downward.

Ignoring air resistance, the acceleration is approximately:

9.8 m/s²

This is called the acceleration due to gravity.

The symbol:

g

is commonly used for both gravitational field strength and acceleration due to gravity.

Numerically near Earth's surface:

g ≈ 9.8 N/kg

and:

g ≈ 9.8 m/s²

These units describe closely related aspects of the same gravitational field.

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Do Heavier Objects Fall Faster?

Ignoring air resistance, objects near Earth's surface experience the same gravitational acceleration regardless of their mass.

A heavy ball and a light ball released together in a vacuum fall with the same acceleration.

Why?

A more massive object experiences a greater gravitational force, but it also has greater inertia.

These effects balance so that the acceleration is the same when other forces are negligible.

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Air Resistance Changes What We Observe

A feather and a metal ball do not usually fall together through Earth's atmosphere.

The reason is not that gravity stops working normally.

The difference comes from:

air resistance

The feather experiences air resistance that is large compared with its weight.

In a vacuum, where air resistance is absent, they fall together.

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Gravity and Projectile Motion

Throw a ball horizontally.

Gravity immediately begins accelerating it downward.

The ball therefore follows a curved path.

Without gravity, the ball would continue moving approximately in a straight line at constant velocity if no other forces acted.

Gravity continually changes the vertical component of its velocity.

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Gravity Extends Far Beyond Earth

Earth's gravitational field does not suddenly stop at the edge of the atmosphere.

It extends far into space.

As distance increases, the gravitational attraction becomes weaker, but it does not abruptly become zero.

This is why Earth's gravity can influence:

• the Moon
• artificial satellites
• spacecraft
• nearby objects in space

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Newton's Universal Law of Gravitation

Newton described gravitational attraction mathematically.

For two masses:

F = Gm₁m₂/r²

where:

• F = gravitational force
• G = universal gravitational constant
• m₁ = first mass
• m₂ = second mass
• r = distance between their centres

This equation is called Newton's law of universal gravitation.

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What Does the Gravitation Equation Tell Us?

The equation:

F = Gm₁m₂/r²

reveals two major relationships.

Gravitational force depends on:

1. The masses of the objects

Greater masses produce stronger gravitational attraction.

2. The distance between them

Greater distance produces weaker gravitational attraction.

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Effect of Mass

Suppose one object's mass doubles while everything else stays constant.

Since:

F ∝ m

the gravitational force doubles.

If both masses double:

2 × 2 = 4

the gravitational force becomes:

4 times greater

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Effect of Distance

Gravity follows an inverse-square relationship.

F ∝ 1/r²

If distance doubles:

F becomes 1/2² = 1/4

of its original value.

If distance triples:

F becomes 1/3² = 1/9

of its original value.

If distance becomes four times greater:

F becomes 1/16

of its original value.

Gravity weakens rapidly with increasing distance.

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Why Don't We Notice Gravity Between People?

You and another person gravitationally attract each other.

However, human masses are relatively small.

The gravitational attraction between two people is therefore tiny compared with forces such as:

• Earth's gravity
• friction
• normal forces
• muscular forces

Gravity becomes especially important when astronomical masses are involved.

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Gravity and the Moon

Earth gravitationally attracts the Moon.

So why does the Moon not simply fall directly into Earth?

Because the Moon also has a large sideways velocity.

Gravity continually bends the Moon's motion toward Earth.

Instead of travelling in a straight line, the Moon follows an orbit.

The Moon can therefore be thought of as continually falling around Earth.

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Newton's Cannon Thought Experiment

Imagine firing a cannonball horizontally from a very high mountain.

At low speed, it travels forward and falls to Earth.

At greater speed, it travels farther before hitting Earth.

At a sufficiently high horizontal speed, Earth's surface curves away beneath it at the same rate that it falls.

The object enters orbit.

This thought experiment shows the connection between:

falling objects and orbiting objects

Both are influenced by gravity.

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Gravity Provides the Centripetal Force

An orbiting object continuously changes direction.

A force directed toward the centre of its curved path is required.

For a satellite orbiting Earth, gravity provides this centripetal force.

For a simplified circular orbit:

Fgravity = Fcentripetal

Therefore:

GMm/r² = mv²/r

This relationship connects gravity with orbital motion.

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Orbital Speed

Simplifying the circular-orbit equation gives:

v = √(GM/r)

This shows that orbital speed depends on:

• the mass of the central object
• the orbital distance

For satellites around the same planet, a satellite in a lower circular orbit generally moves faster than one in a higher circular orbit.

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Artificial Satellites

Artificial satellites orbit Earth for many purposes.

Examples include:

• communications
• weather monitoring
• navigation
• Earth observation
• scientific research

Gravity continually accelerates these satellites toward Earth while their sideways motion carries them forward.

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Astronauts in Orbit

Astronauts aboard an orbiting spacecraft often appear to be weightless.

This does not mean there is no gravity.

Earth's gravity is still acting strongly enough to maintain the spacecraft's orbit.

The astronauts and spacecraft are falling together.

This condition is called free fall.

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Apparent Weightlessness

When standing on Earth, the ground pushes upward on you with a normal force.

You experience this support force as part of your apparent weight.

In an orbiting spacecraft:

• the spacecraft falls toward Earth
• the astronaut falls toward Earth
• both accelerate together

There is little or no normal force supporting the astronaut.

The astronaut therefore experiences apparent weightlessness.

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Gravity and the Solar System

The Sun contains most of the mass in the Solar System.

Its gravitational influence governs the large-scale orbital motion of:

• planets
• dwarf planets
• asteroids
• comets
• many smaller objects

Planets have sideways velocities while the Sun's gravity continually changes their direction.

The result is orbital motion.

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Planetary Orbits

Planetary orbits are not perfect circles.

They are approximately elliptical.

The Sun lies at one focus of a planet's elliptical orbit.

The planet's speed changes as its distance from the Sun changes.

This behaviour was described by Kepler's laws of planetary motion and later explained using Newtonian gravitation.

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Moons and Planets

Planets can also gravitationally control the motion of nearby moons.

For example:

Earth's gravity governs the Moon's orbit.

Jupiter's gravity governs the orbits of its many moons.

Saturn's gravity governs its moons and strongly influences the particles forming its rings.

Gravity therefore creates systems within larger gravitational systems.

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Gravity and Tides

The Moon's gravity affects Earth as well.

Differences in the Moon's gravitational pull across Earth contribute strongly to ocean tides.

The Sun also contributes to tides.

Tides provide an everyday example of gravitational effects between astronomical objects.

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Gravity and Stars

Gravity is essential to stars.

A star contains an enormous amount of matter.

Gravity pulls the star's material inward.

Inside an active star, this inward gravitational effect is balanced by pressure associated with the hot gas and energy generated in the star.

Gravity therefore plays a major role in:

• star formation
• stellar structure
• stellar evolution

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Gravity and Star Formation

Stars form from enormous clouds of gas and dust.

If part of a cloud becomes sufficiently dense, gravitational attraction can cause material to collapse inward.

As the material collapses:

• density increases
• pressure increases
• temperature increases

Eventually, conditions may become sufficient for nuclear fusion to begin.

Gravity therefore plays a fundamental role in creating stars.

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Gravity on Larger Scales

Gravity also affects structures far larger than the Solar System.

It influences:

• star clusters
• galaxies
• galaxy groups
• galaxy clusters

Because gravity acts over enormous distances and is always attractive in ordinary matter, its effects accumulate on astronomical scales.

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Gravity and the Four Fundamental Interactions

Modern physics describes four fundamental interactions:

• gravitational
• electromagnetic
• strong nuclear
• weak nuclear

Gravity is one of these fundamental interactions.

The others behave very differently.

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Gravity vs Electromagnetism

Gravity:

• acts between masses/energy
• is attractive between ordinary masses
• has effectively unlimited range
• is comparatively very weak at the particle scale
• dominates many astronomical systems

Electromagnetism:

• acts between electrically charged particles
• can attract or repel
• has unlimited range
• is much stronger than gravity at atomic scales
• governs electricity, magnetism, chemistry, and many contact forces

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Gravity vs the Strong Interaction

The strong interaction acts on subatomic particles and is responsible for binding quarks and, through the residual strong interaction, helping bind protons and neutrons in atomic nuclei.

It is:

• extremely strong at nuclear scales
• effective only over very short distances

Gravity is much weaker at the particle level but acts across enormous distances.

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Gravity vs the Weak Interaction

The weak interaction is involved in processes such as certain forms of radioactive decay.

It operates over extremely short distances.

Gravity, by contrast, has long-range effects and becomes dominant when dealing with very large astronomical masses.

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Why Can the Weakest Force Dominate Space?

At the particle scale, gravity is extremely weak compared with electromagnetism and the nuclear interactions.

So why does gravity dominate planets and stars?

One major reason is that large astronomical objects are generally close to electrically neutral overall.

Positive and negative electric charges tend to cancel.

Mass does not cancel in the same way.

As enormous amounts of matter accumulate, their gravitational effects also accumulate.

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Gravity Is Not the Same as Magnetism

Gravity and magnetism are sometimes confused because both can act without direct contact.

However, they are different interactions.

Gravity:

acts between masses

Magnetic forces:

are associated with magnetic fields, moving charges, and magnetic materials

A rock falls toward Earth because of gravity, not because Earth is acting like a magnet on the rock.

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Gravity Is Not the Normal Force

A book resting on a table experiences gravity downward.

But it does not accelerate downward because the table provides an upward normal force.

If the book is stationary:

Fnet = 0

and approximately:

N = W

The normal force does not replace gravity. Both forces act simultaneously.

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Gravity Is Not Air Resistance

A falling object may experience:

weight downward

and:

air resistance upward

Gravity and air resistance are different forces.

Gravity exists even in a vacuum.

Air resistance requires interaction with particles in a gas.

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Gravitational Force Between Two Objects

Consider two objects:

m₁ = 5 kg

m₂ = 10 kg

separated by some distance.

Each object gravitationally attracts the other.

According to Newton's third law:

the force on object 1 due to object 2 is equal in magnitude and opposite in direction to the force on object 2 due to object 1.

Even if the masses are very different, the interaction forces form an equal-and-opposite pair.

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Worked Example 1: Weight

A 50 kg student stands on Earth.

Using:

g = 9.8 N/kg

Calculate weight:

W = mg

W = 50 × 9.8

W = 490 N

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Worked Example 2: Different Planet

A 40 kg object is placed where:

g = 3.7 N/kg

Calculate its weight:

W = 40 × 3.7

W = 148 N

Its mass remains:

40 kg

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Worked Example 3: Doubling Mass

Two objects experience gravitational force F.

If one object's mass doubles while everything else remains constant:

Fnew = 2F

The gravitational force doubles.

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Worked Example 4: Doubling Both Masses

If both masses double:

Fnew = (2)(2)F

Therefore:

Fnew = 4F

The gravitational force becomes four times larger.

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Worked Example 5: Doubling Distance

Two masses experience force F at distance r.

If the distance becomes:

2r

then:

Fnew = F/2²

Therefore:

Fnew = F/4

The gravitational force becomes one quarter as large.

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Worked Example 6: Tripling Distance

If the distance becomes:

3r

then:

Fnew = F/3²

Therefore:

Fnew = F/9

The gravitational force becomes one ninth as large.

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Worked Example 7: Combining Changes

Suppose both masses double and their separation also doubles.

Mass effect:

2 × 2 = 4

Distance effect:

1/2² = 1/4

Combine:

4 × 1/4 = 1

Therefore:

Fnew = F

The gravitational force remains unchanged.

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Worked Example 8: Orbital Motion

A satellite travels sideways around Earth.

Without gravity, it would tend to continue along a straight-line path.

Earth's gravity continuously changes the satellite's direction toward Earth.

The combination produces:

orbital motion

The satellite is continually falling toward Earth while moving forward.

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Worked Example 9: Apparent Weightlessness

An astronaut and spacecraft are orbiting Earth.

Both experience Earth's gravitational field.

Both accelerate toward Earth together.

Because the astronaut is not being supported by a surface in the usual way, the astronaut experiences:

apparent weightlessness

This does not mean:

gravity = 0

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Worked Example 10: Comparing Forces

A book sits on a desk.

Forces on the book include:

gravity downward

and:

normal force upward

These forces may be equal in magnitude when the book is stationary.

However, they are different types of forces:

• gravity is a long-range fundamental interaction
• the normal force is a contact force arising from electromagnetic interactions between matter

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Gravity in Everyday Situations

Gravity is important when:

• an object falls
• a ball is thrown
• a person jumps
• rain falls
• water flows downhill
• an object has weight
• a pendulum swings
• ocean tides occur

Although gravity is always present, other forces may balance or oppose it.

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Gravity in Space

Gravity is important when:

• planets orbit stars
• moons orbit planets
• satellites orbit Earth
• comets travel through the Solar System
• stars form
• galaxies interact
• spacecraft change trajectories

There is no sharp boundary beyond which gravity simply disappears.

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Common Mistakes

Mistake 1: "There is no gravity in space."

Incorrect.

Gravity acts throughout space. Astronauts in orbit appear weightless because they are in continuous free fall.

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Mistake 2: "Heavy objects always fall faster."

Ignoring air resistance, objects experience the same gravitational acceleration near Earth's surface.

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Mistake 3: "Mass and weight are the same."

Mass is measured in:

kg

Weight is a force measured in:

N

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Mistake 4: "Gravity only acts downward."

"Downward" near a planet means:

toward the planet's centre

In space, gravitational force points toward the attracting mass.

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Mistake 5: "Satellites are beyond Earth's gravity."

Satellites orbit precisely because Earth's gravity still acts on them.

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Mistake 6: "Orbiting objects have no acceleration."

An orbiting object's direction is continuously changing.

A change in velocity means there is acceleration.

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Error Analysis

A student says:

"The Moon is not falling because it stays the same distance from Earth."

This is misleading.

The Moon is continually accelerated toward Earth by gravity.

However, it also has sideways velocity.

The combination produces its orbit.

The Moon can therefore be understood as continually falling around Earth rather than falling directly into it.

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Another Error Analysis

A student says:

"An astronaut has no weight in orbit because Earth has no gravitational field there."

This is incorrect.

Earth's gravitational field extends into space.

The astronaut appears weightless because the astronaut and spacecraft are falling together.

The correct concept is:

free fall and apparent weightlessness

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A Useful Gravity Problem-Solving Strategy

When analysing a gravitational situation:

Step 1: Identify the objects interacting.

Step 2: Identify which mass is producing the important gravitational field.

Step 3: Determine the direction of gravitational force.

Step 4: Decide whether other forces are present.

Step 5: Choose the appropriate relationship, such as W = mg or Newton's gravitation equation.

Step 6: Consider how mass and distance affect the force.

Step 7: Check whether the result makes physical sense.

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Did You Know?

The same basic gravitational interaction helps explain phenomena ranging from a falling apple to the motion of moons, planets, stars, and galaxies.

Newton's great insight was that the gravity causing objects to fall near Earth's surface and the gravity governing the Moon's orbit could be understood as manifestations of the same universal interaction.

This connected terrestrial physics and astronomical motion within one mathematical framework.

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Key Terms

• Gravity: Attractive interaction associated with mass and energy.
• Gravitation: Gravitational interaction between masses.
• Gravitational force: Attractive force between masses in Newtonian physics.
• Gravitational field: Region in which a mass experiences gravitational influence.
• Gravitational field strength: Gravitational force per unit mass, measured in N/kg.
• Mass: Measure of an object's inertia, measured in kilograms.
• Weight: Gravitational force acting on an object.
• Free fall: Motion when gravity is the only significant force acting.
• Orbit: Curved path of one object around another due to gravity and its motion.
• Satellite: Object orbiting another object.
• Natural satellite: Naturally occurring orbiting object, such as the Moon.
• Artificial satellite: Human-made object placed into orbit.
• Centripetal force: Net inward force required for circular motion.
• Inverse-square relationship: Relationship in which a quantity varies as 1/r².
• Apparent weightlessness: Condition experienced when an object and its surroundings are in free fall together.
• Fundamental interaction: One of the basic interactions used to describe physical phenomena.

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Key Equations

Weight:

W = mg

Newton's law of universal gravitation:

F = Gm₁m₂/r²

For circular orbital motion:

Fgravity = Fcentripetal

and:

GMm/r² = mv²/r

Circular orbital speed:

v = √(GM/r)

Near Earth's surface:

g ≈ 9.8 N/kg

and:

g ≈ 9.8 m/s²

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Key Relationships

If one mass doubles:

F → 2F

If both masses double:

F → 4F

If distance doubles:

F → F/4

If distance triples:

F → F/9

If distance becomes four times greater:

F → F/16

Gravity therefore:

increases with mass

and:

decreases with the square of distance

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Key Takeaways

• Gravity is an attractive interaction between objects with mass.
• Every mass gravitationally attracts every other mass.
• Gravity is a non-contact interaction.
• A mass creates a gravitational field in the space around it.
• Near a spherical planet, the gravitational field points approximately toward the planet's centre.
• Gravity affects objects both on Earth and throughout space.
• Earth's gravity gives objects weight.
• Mass and weight are different quantities.
• Mass is measured in kilograms.
• Weight is measured in newtons.
• Weight can be calculated using W = mg.
• Mass normally remains constant when an object moves to another location, while weight can change.
• Near Earth's surface, gravitational field strength is approximately 9.8 N/kg.
• Ignoring air resistance, objects near Earth fall with the same gravitational acceleration.
• Gravity follows an inverse-square relationship with distance in Newtonian physics.
• Increasing mass increases gravitational attraction.
• Increasing separation decreases gravitational attraction.
• Gravity does not suddenly disappear in space.
• Astronauts in orbit experience apparent weightlessness because they and their spacecraft are in continuous free fall.
• The Moon remains in orbit because gravity continually changes its direction while it has sideways velocity.
• Artificial satellites orbit Earth for the same fundamental reason.
• Gravity provides the centripetal force required for many orbital motions.
• The Sun's gravity governs the large-scale orbital motion of planets and many smaller Solar System objects.
• Planetary orbits are approximately elliptical rather than perfectly circular.
• Gravity also influences tides, star formation, galaxies, and galaxy clusters.
• Gravity is one of the four fundamental interactions.
• Gravity differs from electromagnetic, strong, and weak interactions in its properties and relative strength.
• Contact forces such as the normal force and friction are not forms of gravity.
• Gravity is comparatively weak at particle scales but becomes dominant across many astronomical systems.
• The same gravitational principles connect falling objects on Earth with the motion of planets, moons, and satellites.