GRAVITATION
Gravity is the force that pulls objects towards the Earth (or any planet).
Gravitation is the general force of attraction between any two objects in the universe.
Simple difference:
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Gravity = attraction towards Earth.
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Gravitation = attraction between any two masses (like Earth and Moon, Sun and Earth, etc.).
1. What are the effects of a force acting on an object?
When a force is applied to an object, it can cause one or more of the following effects:
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Change in motion: If the object was not moving, force can make it start moving.
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Change in speed: If the object is already moving, force can make it move faster or slower.
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Change in direction: A moving object can change its direction when a force acts on it.
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Change in shape or size: For example, when you press a sponge, it gets compressed.
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Stop a moving object: Like when you use brakes on a bicycle.
In short: A force can start, stop, speed up, slow down, change direction, or change the shape of an object.
2. What types of forces are you familiar with?
There are many types of forces we experience or study. Some common ones are:
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Muscular force: The force applied by muscles (like lifting a bag).
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Frictional force: The force that slows things down when they rub against each other (like sliding a book on a table).
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Gravitational force: The force that pulls everything towards Earth (like when an apple falls).
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Magnetic force: The force between magnets or magnetic materials (like a magnet pulling iron nails).
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Electrostatic force: The force between charged objects (like when your hair stands up after rubbing a balloon).
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Tension force: The force through a string or rope when pulled from both ends.
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Spring force: The force in stretched or compressed springs.
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Normal force: The force from a surface that supports an object (like a table pushing up on a book).
3. What do you know about the gravitational force?
Gravitational force is a natural force of attraction between any two objects that have mass. The most common example is Earth pulling everything towards its centre, which is why things fall down.
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It is an invisible force that acts at a distance.
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The greater the mass, the stronger the gravitational force.
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It keeps us on the ground and prevents us from floating in the air.
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It also keeps the Moon in orbit around the Earth and planets in orbit around the Sun.
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It acts between all objects in the universe, but we usually feel Earth’s gravity because Earth is huge.
In short: Gravitational force is the pulling force that acts between all objects, and it's the reason why things fall down and stay on the ground.
What is Gravitation?
Gravitation is a natural force of attraction. It means that every object in the universe pulls every other object towards itself.
You have already learned that this force is universal, which means it works everywhere in the universe, not just on Earth. It works between any two objects that have mass – even if they are far away in space.
How was Gravitation Discovered?
The discovery of gravitation is connected to a famous scientist named Sir Isaac Newton.
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One day, Newton was sitting under a tree and he saw an apple fall straight down from the tree.
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He started thinking:
Why did the apple fall vertically (straight down) and not sideways or upward? -
This question made him wonder about a force that might be pulling the apple towards the ground.
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After thinking a lot, Newton understood that the Earth must be pulling the apple towards its centre.
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That pulling force is what we now call gravitational force.
This force always pulls things in the vertical direction, which means from the object straight towards the centre of the Earth. So, the apple doesn’t move sideways — it goes downward because that’s the direction of the Earth's pull.
The Vertical Direction
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The line from the apple to the center of the Earth is what we call the vertical.
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That is why any object hanging or falling moves along this vertical line — because that’s the path towards the center of the Earth.
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This is the reason why we always say things “fall down” — they are pulled by Earth's gravity in the vertical direction.
Force on the Moon and Other Planets
Newton didn’t stop there. He thought:
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If this force pulls apples down from a tree, does it also pull things that are much farther away?
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He wondered:
Does this gravitational force also act on the Moon?
Does it act on the planets and the Sun too?
He realised that the same force that pulls the apple also keeps the Moon in orbit around the Earth.
In fact, this force also works between planets, the Sun, and everything else in space.
So, Newton concluded that gravity is not just for things on Earth — it works everywhere in the universe.
In Short:
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Gravitation is a universal force — it works everywhere, not just on Earth.
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Newton discovered gravity by thinking about why an apple falls down.
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He understood that the Earth pulls all objects towards its centre.
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The same force also acts on the Moon, planets, and even the Sun.
Newton’s three laws of motion:
1. Newton’s First Law of Motion – Law of Inertia
Statement:
An object will remain at rest or keep moving in a straight line at constant speed unless a force is applied to change its state.
In simple words:
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If something is not moving, it will stay still until you push or pull it.
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If something is already moving, it will keep moving the same way unless something slows it down, speeds it up, or changes its direction.
Example:
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A ball on the ground won’t move unless you kick it.
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A rolling ball will eventually stop because of friction, which is a force.
2. Newton’s Second Law of Motion – Force and Acceleration
Statement:
The force acting on an object is equal to the mass of the object multiplied by its acceleration.
Formula: F = m × a
In simple words:
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A stronger force makes an object move faster (more acceleration).
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A heavier object needs more force to move.
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If you apply the same force to a small object and a heavy object, the small one will move more easily.
Example:
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It’s easier to push an empty cart than a full cart.
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If you push harder, the cart moves faster.
3. Newton’s Third Law of Motion – Action and Reaction
Statement:
For every action, there is an equal and opposite reaction.
In simple words:
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If you push something, it pushes back with the same amount of force in the opposite direction.
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The forces always come in pairs.
Example:
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When you jump, your legs push the ground downward (action), and the ground pushes you upward (reaction).
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When a balloon is released, the air goes out backward, and the balloon moves forward.
Summary Table:
| Law | Name | Key Idea |
|---|---|---|
| 1st | Law of Inertia | Objects stay still or keep moving unless forced to change. |
| 2nd | F = m × a | More force = more acceleration; heavier objects need more force. |
| 3rd | Action = Reaction | Every action has an equal and opposite reaction. |
Force and Motion
We know that motion means the movement of an object.
We also know that an object can move faster, slower, or even change direction.
But how do these changes happen?
👉 The answer is: A force is needed.
What does force do to motion?
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If an object is at rest (not moving), a force can start its motion.
Example: A football starts rolling when you kick it. -
If an object is already moving, a force can:
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Increase its speed (make it go faster)
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Decrease its speed (slow it down or stop it)
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Change its direction (like when a car takes a turn)
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Important point:
Without a force, the object will keep moving in the same way or stay at rest.
So, to change the speed or direction of anything that is moving (or to start movement), we must apply a force.
Great Scientists: Sir Isaac Newton
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Sir Isaac Newton (1642–1727) was one of the most important scientists in history.
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He was born in England.
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He wrote a famous book called Principia, in which he gave:
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Laws of motion
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Equations of motion
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Theory of gravity
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Before Newton, Kepler had explained how planets move, but not why.
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Newton explained the reason for planetary motion using his theory of gravity and proved Kepler’s laws using mathematics.
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Newton also made important contributions in other subjects like:
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Light
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Heat
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Sound
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Mathematics
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He invented a completely new branch of mathematics called Calculus, which is very useful in science and maths.
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Newton was the first person to build a reflecting telescope (a telescope that uses mirrors).
Laws of Motion
These are three rules given by Sir Isaac Newton that explain how and why objects move:-
An object stays at rest or keeps moving unless a force acts on it.
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Force = Mass × Acceleration (F = m × a)
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Every action has an equal and opposite reaction.
Equations of Motion
These are formulas used to calculate the motion of an object when it moves with constant acceleration:-
v = u + at (Final speed)
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s = ut + ½at² (Distance covered)
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v² = u² + 2as (Speed and distance relation)
Where:
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u = initial speed
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v = final speed
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a = acceleration
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t = time
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s = distance
Theory of Gravity
Newton said that every object in the universe pulls every other object with a force called gravity.
This force depends on:-
The masses of the objects
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The distance between them
Larger mass = more gravity
More distance = less gravity
Centripetal Force and Motion of Celestial Bodies-
When you tie a stone to a string and rotate it in a circle, you are applying a force on the stone.
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This force is always pulling the stone towards the centre of the circle.
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This inward force is necessary to keep the stone moving in a circular path.
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The force you apply through the string is called a centripetal force (which means “centre-seeking” force).
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If you suddenly release the string, the force stops acting on the stone.
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The stone will then move along a straight line in the direction it was moving at that instant (this line is called the tangent to the circle).
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This happens because an object in motion keeps moving in a straight line unless a force changes its direction.
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A similar thing happens when a 5-rupee coin placed on a rotating disk slides off—moving in a straight line.
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This proves that a force is needed to keep any object moving in a circular path.
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Now consider the moon moving around the Earth.
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The moon's direction and speed keep changing, which means a force must be acting on it.
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This force is gravitational force from the Earth, pulling the moon towards it.
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The direction of this force is towards the centre of the Earth.
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If this force stopped, the moon would fly off in a straight line, just like the stone.
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The same idea applies to the planets going around the Sun.
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The Sun pulls all the planets towards itself using gravitational force.
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This force also acts as a centripetal force for the planets.
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It keeps them moving in orbits around the Sun.
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Without this force, planets would not move in curved paths.
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Instead, they would fly off into space in straight lines.
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The gravitational force from the Earth acts on the moon just like the string’s pull acts on the stone.
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The gravitational force from the Sun acts on all the planets in the solar system.
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That’s why planets, including Earth, stay in their orbits.
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In all these cases, the force is directed towards the centre of the circular path.
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This force, which keeps objects in circular motion, is always a centripetal force.--
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