Compare Gravity Scales

Explore how gravitational force works at every scale: from objects on a planet's surface, to moons orbiting planets, to stars orbiting the center of our galaxy.

Earth & Space Science

What Is Gravity?

Gravity is an attractive force between any two objects that have mass. Every piece of matter in the universe pulls on every other piece of matter. You feel Earth's gravity holding you to the ground, but the same force also keeps the Moon in orbit around Earth and Earth in orbit around the Sun.

Newton's Law of Universal Gravitation

Newton discovered that the gravitational force between two objects depends on exactly two things: their masses and the distance between them.

Mass vs Distance: Two Levers of Gravity

Three Scales of Gravitational Interaction

Planetary Surface

Objects pulled toward a planet's center. Your weight on Earth, Mars, or Jupiter.

Solar System Orbits

Moons orbit planets. Planets orbit the Sun. Gravity keeps them on curved paths.

Galactic Scale

Billions of stars orbit the center of the Milky Way, held together by enormous combined gravity.

Surface Gravity: Weight on Different Worlds

Your weight is the gravitational force a planet exerts on you at its surface. It depends on the planet's mass and radius. A planet with more mass or a smaller radius has stronger surface gravity.

Mass vs Weight: A Key Distinction

Mass

The amount of matter in an object. Measured in kilograms. Stays the same everywhere in the universe.

Weight

The gravitational force on an object: \( W = m \times g \). Measured in newtons. Changes depending on where you are.

Solar System Orbits

At the solar-system scale, gravity keeps moons orbiting planets and planets orbiting the Sun. The Sun contains about 99.8% of the solar system's total mass, so its gravitational pull dominates.

An Orbit Is Falling Around an Object

An orbiting body moves forward fast enough that, as gravity curves its path downward, it continuously "misses" the object below. The result is a stable, repeating loop.

1

Gravity pulls inward toward the center of the massive body (Sun, planet).

2

The orbiting body moves sideways (tangentially) fast enough to avoid crashing.

3

These two motions combine into a curved orbital path, an ellipse.

The Milky Way: Gravity at Galactic Scale

Our Milky Way galaxy contains 100 to 400 billion stars, plus gas, dust, and dark matter. All of this mass creates a combined gravitational field that holds the galaxy together. Stars, including our Sun, orbit the galactic center.

Comparing Gravity Across Scales

How to Compare Gravitational Interactions

1

Identify the two objects interacting gravitationally (e.g., Earth and Moon, Sun and Jupiter, star and galactic center).

2

Compare their masses. Larger masses produce stronger gravitational pull.

3

Compare the distance between them. Greater distance means much weaker force (it drops with \( r^2 \)).

4

Consider the outcome. Surface gravity affects weight. Orbital gravity determines orbital speed and period.

5

Use the universal law \( F = G \frac{m_1 m_2}{r^2} \) if you need to calculate or compare quantitatively.

Example 1: Weight on Different Planets

A rock has a mass of 10 kg. Compare its weight on Earth (\( g = 9.8 \text{ m/s}^2 \)) and on Mars (\( g = 3.7 \text{ m/s}^2 \)).

1

Use \( W = m \times g \).

2

On Earth: \( W = 10 \times 9.8 = \) 98 N

3

On Mars: \( W = 10 \times 3.7 = \) 37 N

Example 2: Doubling the Distance

Two asteroids float in space. If the distance between them doubles, how does the gravitational force change?

Example 3: Sun vs Earth Pulling on the Moon

Both the Sun and Earth pull on the Moon. The Sun is about 333,000 times more massive than Earth, but it is about 390 times farther from the Moon. Which pull is stronger?

1

Mass effect: Sun's extra mass makes its pull \( \sim 333{,}000 \times \) stronger.

2

Distance effect: Being 390× farther weakens it by \( 390^2 = 152{,}100 \times \).

3

Net ratio: \( \dfrac{333{,}000}{152{,}100} \approx \) 2.2. The Sun's pull on the Moon is roughly 2.2× stronger than Earth's.

Common Mistakes

✕ Confusing Mass and Weight

Mass (kg) is the amount of matter and never changes. Weight (N) is a force and changes with gravity. They are not the same.

✕ Forgetting the Square

Doubling distance doesn't halve gravity: it reduces it to one-quarter. The \( r^2 \) in the formula is crucial.

✕ Thinking Gravity Only Pulls Down

Gravity pulls toward the center of mass. "Down" is just the direction toward Earth's center. In space, gravity pulls objects toward each other.

Summary: Gravity at Every Scale

Property	Surface	Solar System	Galaxy
Objects involved	Person & planet	Planet & Sun	Stars & galactic center
Typical distance	Thousands of km	Millions of km	Thousands of light-years
Typical mass	\( \sim 10^{24} \text{ kg} \)	\( \sim 10^{30} \text{ kg} \)	\( \sim 10^{42} \text{ kg} \)
Observable effect	Weight, falling	Orbits, seasons	Galactic rotation
Governing law	\( F = G \frac{m_1 m_2}{r^2} \) (same everywhere)

Question 1 of 5

According to Newton's law of universal gravitation, what happens to the gravitational force if the distance between two objects is tripled?

The force follows an inverse-square law. Tripling the distance means dividing by \( 3^2 = 9 \), so the force becomes one-ninth as strong.

Question 2 of 5

An astronaut has a mass of 70 kg. Which statement is true about the astronaut on the Moon, where \( g = 1.6 \text{ m/s}^2 \)?

Mass never changes regardless of location: it stays 70 kg. Weight = \( m \times g = 70 \times 1.6 = 112 \) N, which is much less than on Earth (686 N).

Question 3 of 5

What keeps the planets in orbit around the Sun instead of flying off into space?

Gravity from the Sun continuously pulls each planet inward, curving its otherwise straight-line path into an elliptical orbit. There is no "push" from space, and magnetism does not hold orbits together.

Question 4 of 5

Jupiter has about 318 times Earth's mass but its surface gravity is only 2.5 times greater. Why isn't Jupiter's surface gravity 318 times greater?

Surface gravity depends on both mass and radius: \( g = G \frac{M}{R^2} \). Jupiter's radius is about 11 times Earth's, so \( R^2 \approx 121 \). Dividing 318 by 121 gives roughly 2.5, matching the observed ratio.

Question 5 of 5

Which best explains why stars in the Milky Way orbit the galactic center instead of scattering in random directions?

The enormous combined mass near the galactic center, including stars, gas, dust, and dark matter, creates a gravitational pull strong enough to hold hundreds of billions of stars in orbit. The same universal law applies at this immense scale.