Introduction
Have you ever wondered why you stay firmly on the ground, but the Moon stays floating above us? Or why astronauts seem to float inside their spaceship even though gravity is always pulling?
It all comes down to a force that affects everything with mass—gravity.
In this chapter, we’ll dive into what gravity really is, how it affects our daily life, and how it’s used to launch satellites and space missions. Don’t worry—it’s not as hard as it sounds. We’ll break it down in the simplest way, with everyday examples, relatable explanations, and visual ideas to help you understand it easily.
Let’s explore the invisible but powerful force of gravitation and how it shapes life on Earth and beyond.
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Newton’s Universal Law of Gravitation
What the Law Says:
“Every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.”
Formula:
F = G × (m₁ × m₂) / r²
Where:
- F = gravitational force (in newtons, N)
- G = gravitational constant = 6.674 × 10⁻¹¹ Nm²/kg²
- m₁ and m₂ = masses of the two objects (kg)
- r = distance between their centers (m)
Key Point: Gravity is stronger between heavier objects and weaker when they are farther apart.
Real-Life Example:
- You and the Earth are pulling on each other, but the Earth is so much more massive, so it feels like only you’re being pulled down.
- The Moon stays in orbit around Earth due to this same gravitational pull.
Gravity on Earth
Gravity is what gives us weight and keeps everything anchored to Earth’s surface. Without it, we’d all be floating into space!
Acceleration Due to Gravity (g)
- Symbol: g
- Average value on Earth: 9.8 m/s²
- Meaning: Every second, the speed of a freely falling object increases by 9.8 meters per second.
Important Note:
All objects fall at the same rate under gravity, regardless of mass (ignoring air resistance). Yes—even a feather and a hammer (as proved on the Moon!).
Free Fall – What Happens When Gravity Acts Alone
A body is in free fall when only gravity is acting on it—no air resistance, no push, no pull.
Formulas for Free Fall:
- v = u + g × t
- s = ut + ½ g × t²
- v² = u² + 2gs
Where:
- v = final velocity
- u = initial velocity
- s = distance fallen
- t = time
Mass vs Weight – What’s the Difference?
| Property | Mass | Weight |
| Definition | Amount of matter in an object | Force of gravity acting on mass |
| Symbol | m | W |
| Unit | Kilogram (kg) | Newton (N) |
| Constant? | Yes, everywhere | Changes depending on gravity |
| Formula | — | W = m × g |
Key Idea:
- Your mass is the same on Earth, Moon, or Mars.
- Your weight changes depending on local gravity.
Thrust and Pressure – How Forces Work on Surfaces
- Thrust = Force applied perpendicular to a surface
- Pressure = Thrust / Area
Unit of Pressure: Pascal (Pa)
Example:
A sharp knife cuts easily because it applies the same force over a smaller area → more pressure!
Archimedes’ Principle and Floatation
Archimedes’ Principle:
When a body is immersed in a fluid, it experiences an upward force (buoyant force) equal to the weight of the fluid displaced.
This is why some objects float while others sink.
Floatation Rule:
- If an object’s density < fluid → it floats
- If an object’s density > fluid → it sinks
Example:
- Ice floats in water.
- A heavy ship floats because it displaces enough water to balance its weight.
Satellite Motion – Staying in Orbit
What Are Satellites?
Satellites are objects that revolve around planets due to gravity.
They can be:
- Natural – like the Moon
- Artificial – built by humans for GPS, weather, communication, and more
Why Don’t Satellites Fall?
Because they are in a constant state of free fall—falling toward Earth while moving forward fast enough to miss it!
This forms a curved path—called an orbit.
Orbital Speed and Period
Orbital Speed (v):
v = √(GM / r)
Where:
- G = gravitational constant
- M = mass of Earth
- r = distance from Earth’s center
This speed keeps satellites moving in orbit without crashing down.
Time Period (T):
The time taken to complete one orbit.
- For geostationary satellites, this is 24 hours—they stay fixed above one location on Earth.
Escape Velocity – How Rockets Leave Earth
What Is Escape Velocity?
It is the minimum speed needed to break free from Earth’s gravitational pull.
Formula:
vₑ = √(2gR)
- For Earth: approximately 11.2 km/s
Used when launching rockets, space probes, or reaching other planets.
Core Concepts Summary Table
| Concept | Meaning |
| Gravitational Force | Attraction between any two masses |
| Free Fall | Motion under gravity alone |
| Acceleration due to g | Rate at which objects speed up when falling (9.8 m/s²) |
| Mass | Amount of matter (kg), remains constant |
| Weight | Force of gravity on mass (N), changes with gravity |
| Buoyant Force | Upward force in fluids |
| Satellite Motion | Objects staying in orbit due to balance of gravity and speed |
| Escape Velocity | Speed needed to break free from Earth’s pull |
Frequently Asked Questions
Q1. What causes objects to fall?
Gravity pulls all objects toward Earth.
Q2. Why does a satellite stay in orbit?
It’s moving forward fast enough while being pulled down by gravity—this creates a circular motion.
Q3. Why does a ship float but a nail sinks?
The ship displaces more water and has lower average density.
Q4. What is the value of g on the Moon?
About 1.63 m/s² (nearly 1/6th of Earth’s gravity).
Q5. Why do astronauts feel weightless?
Because they’re in free fall along with their spacecraft.
Fun and Surprising Facts
- The Moon doesn’t fall to Earth—it’s always falling, but never lands!
- You weigh slightly less at the equator than at the poles.
- The first artificial satellite was Sputnik 1, launched in 1957.
- Geostationary satellites orbit the Earth once every 24 hours—matching Earth’s rotation.
Conclusion
Gravitation isn’t just about falling—it’s about holding the entire universe together. It’s the reason you stay on Earth, the Moon orbits us, and we can send rockets into space.
Understanding gravity helps explain how ships float, why astronauts float, how we weigh things, and how technology like GPS and weather satellites works.
So next time you drop something, look at the Moon, or see a rocket launch on TV—just remember, it’s all gravity in action.
