Gravity
What is Gravity?
Gravity is a fundamental force of nature that causes objects with mass (or energy) to attract each other.
In simple words: Gravity is why things fall down, planets orbit, stars exist, and the universe sticks together.
Why Does Gravity Exist?
Because mass and energy affect space and time.
There are two main ways scientists explain gravity:
Newton’s Gravity (Classical View)
Isaac Newton said:
Every object in the universe attracts every other object.
Newton’s Law of Universal Gravitation
G m1 m2
F=-------------------
r2
Where:
F = gravitational force
G = gravitational constant
m₁, m₂ = masses of the two objects
r = distance between them
Key ideas:
More mass → more gravity
More distance → less gravity
Gravity is always attractive, never repulsive
Newton’s model works GREAT for:
Falling objects
Planets orbiting
Everyday physics
But it’s not perfect for extreme stuff like black holes.
Einstein’s Gravity (Modern View – Relativity)
Albert Einstein said gravity is not a force.
Instead:
Gravity is the curvature of spacetime caused by mass and energy.
What is a Curvature of spacetime?
Curvature of spacetime is the fundamental concept in Einstein’s general relativity that defines gravity not as a force, but as a geometric warping of the four-dimensional fabric of space and time by mass and energy.
Gravity in Space
Astronauts aren’t “gravity-free”
They’re in free fall, constantly missing Earth as they orbit
Gravity exists everywhere in the universe
Gravity on Earth
Earth’s gravity pulls everything toward its center
Acceleration due to gravity:
g=9.8 m/s2
That means:
Every second, falling speed increases by 9.8 m/s
Mass vs Weight
Important difference
Mass
Amount of matter
Same everywhere
Measured in kg
Weight
Force due to gravity
Changes with gravity
Measured in newtons
On the Moon :
Mass = same
Weight = less (gravity is weaker)
Gravity and Orbits
Why don’t planets fall into the Sun?
Because:
Gravity pulls them in
Their forward speed keeps them moving sideways
Result = orbit
Same idea for:
Extreme Gravity: Black Holes
Formed when massive stars collapse
Gravity so strong that not even light escapes
Boundary called event horizon
Inside? Physics breaks. We don’t fully know
Gravitational Waves
Predicted by Einstein
Ripples in spacetime
Caused by massive events (black hole collisions)
First detected in 2015