While sitting under an apple tree with Isaac Newton, an apple fell from the tree and inspired Newton to think about why apples and other objects always fall straight down, not sideways or up. This led him to develop his theory of universal gravitation, which states that every object in the universe attracts every other object with a force proportional to their masses and inversely proportional to the square of the distance between them. Newton showed that the same force that causes apples and other falling objects on Earth also governs the motion of celestial objects in space.

2. Story behind Newton`s apple
After dinner the weather being warm, we
went into the garden & drank tea under the
shade of some apple trees, only he and
myself . He told me he was just in the same
situation as when formerly the notion of
gravitation came to his mind “why should
that apple always descend perpendicularly
to that ground”, “why should it not go
sideways or upward?”

3. GRAVITATIONAL FORCE
Gravitational force is a force that
attracts any object with mass
Every object is being pulled each
other
Is there a force between two objects
around you? If so then why don`t
they move towards each other?

4. Newton`s Universal law of
gravitation
Every object in the universe attracts
every other object in the universe
with a force that is proportional to
their masses & inversely
proportional to the square of
distance between them.

5. Consider two objects of mass M1 & M2
lies at a distance of “d” from each other
as shown in the figure.

6. According to universal law of
gravitation ,
F α M1 × M2
F α 1⁄d²
Combining the equations ,
F α M1×M2 ⁄ d²
F ₌ G(M1×M2) ⁄ d²
 where G is the constant of
proportionality & called universal
gravitation constant.

7. Gravitational constant
G ₌ Fd² ⁄ M1× M2
Value of G was found out by Henry
Cavendish
Unit of G is Nm²Kg¯²