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+---- Thread: CHAPTER 2 — GRAVITY AND ORBITS (/showthread.php?tid=177)
CHAPTER 2 — GRAVITY AND ORBITS - Leejohnston - 11-15-2025
Chapter 2 — Gravity and Orbits
Gravity is the fundamental force that shapes the structure of the universe.
It governs the motion of planets, stars, galaxies, and even light itself.
Understanding gravity is the first step to understanding how the cosmos moves and evolves.
This chapter explains how gravity works, why orbits are stable, and what keeps planets and moons
from flying away or falling into the objects they orbit.
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2.1 The Nature of Gravity
Gravity is an attractive force between any two objects that have mass.
The more massive an object is, the stronger its gravitational pull.
Examples:
• The Earth pulls you downward.
• The Moon pulls the oceans, creating tides.
• The Sun pulls the planets into orbit.
• A black hole pulls so strongly that not even light can escape.
Gravity weakens with distance, but it never completely disappears.
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2.2 Why Orbits Exist
Planets do not simply fall into the Sun.
Moons do not simply fall into their planets.
Spacecraft do not float freely forever.
Objects in orbit are in a constant state of *falling* — but they move forward fast enough that
they keep missing what they are falling toward.
This balance creates a stable orbit.
Orbit = Forward motion + Gravitational pull
If the Earth suddenly stopped moving sideways, it would fall straight into the Sun.
If it moved too fast, it would escape into space.
---
2.3 Circular and Elliptical Orbits
Most orbits are not perfect circles.
They are ellipses (oval-shaped), where the object moves:
• faster when it is closer to the Sun or planet
• slower when it is farther away
This happens because gravity is stronger at close distances and weaker farther out.
This behaviour was discovered by Johannes Kepler and later explained by Isaac Newton.
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2.4 Orbital Speed
For an object to stay in orbit, it must travel at the correct speed.
Examples:
• A satellite 300 km above Earth must travel at about 7.8 km per second.
• The Earth orbits the Sun at about 30 km per second.
• The Moon orbits Earth at about 1 km per second.
If an object goes too slow, it spirals inward.
If it goes too fast, it escapes the orbit completely.
---
2.5 Why Astronauts Float
Astronauts in orbit are not floating because "there is no gravity" —
gravity at the International Space Station is still about 90 percent as strong as on the surface.
They float because they are constantly falling around the Earth, not toward it.
This is what we call "microgravity".
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2.6 The Gravity of Different Planets
Gravity depends on mass and size.
Some examples:
• Earth: 9.8 m/s²
• Moon: 1.6 m/s²
• Mars: 3.7 m/s²
• Jupiter: 24.8 m/s²
A person weighing 70 kg on Earth would feel much lighter on the Moon and much heavier on Jupiter.
---
2.7 Escape Velocity
Escape velocity is the speed required for an object to break free from the gravitational pull
of a planet or star.
Examples:
• Earth: about 11.2 km/s
• Moon: about 2.4 km/s
• Sun: about 617 km/s
Rockets reach orbit using a combination of speed, height, and engine thrust.
---
2.8 How Gravity Shapes the Universe
Gravity is the main architect of the cosmos:
• It pulls gas together to form stars.
• It binds stars together into galaxies.
• It bends the path of light.
• It controls the motion of planets and moons.
• It directs the evolution of galaxies over billions of years.
Without gravity, the universe would be chaotic and structureless.
---
Chapter Summary
• Gravity is an attractive force between all masses.
• Orbits form when forward motion balances gravitational pull.
• Most orbits are elliptical.
• Orbital speed determines whether an object stays in orbit or escapes.
• Astronauts float because they are in continuous free-fall.
• Each planet has a different gravitational strength.
• Escape velocity is the speed needed to break free from gravity.
• Gravity shapes stars, galaxies, and the entire universe.
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Practice Questions
1. Why do planets not fall into the Sun?
2. Why do astronauts experience microgravity even when gravity is still strong in orbit?
3. What happens if an object moves too slowly to maintain orbit?
4. What happens if it moves too fast?
5. Which planet listed has the weakest gravity, and why does this matter for exploration?
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Written and Compiled by Lee Johnston — Founder of The Lumin Archive