Gravitation Notes – Class 9 CBSE
1. What is Gravitation?
Gravitation is a force that pulls objects toward each other. It exists everywhere in the universe. For example:
- Earth pulls objects toward it (why things fall down).
- The Sun pulls planets toward it, keeping them in orbit.
2. Discovery of Gravitation
Sir Isaac Newton, a famous scientist, discovered gravitation. He observed an apple falling from a tree and realized that Earth pulls the apple down. This pulling force is called gravitational force.
3. Universal Law of Gravitation
This law explains that every object in the universe attracts every other object. The formula is:
F = G (m1 × m2) / r2
- F: Gravitational force (pull between two objects)
- G: Universal Gravitational Constant (
6.67 × 10-11 Nm2/kg2) - m1 and m2: Masses of two objects
- r: Distance between the objects
4. Gravity on Earth
Earth’s gravity pulls everything toward its center. This is why objects fall to the ground and why we don’t float in the air.
5. Free Fall
When an object falls only because of gravity and no other forces (like air), it is called free fall. For example, astronauts in space appear weightless because they are in free fall around Earth.
6. Acceleration Due to Gravity (g)
On Earth, objects falling under gravity increase their speed at a fixed rate. This increase is called acceleration due to gravity, denoted by g.
Value of g on Earth: 9.8 m/s2.
Formula to calculate g:
g = GM / R2
- G: Gravitational constant
- M: Mass of Earth
- R: Radius of Earth
7. Difference Between Mass and Weight
- Mass:
- How much matter is in an object.
- Same everywhere (does not change).
- Measured in kilograms (kg).
- Weight:
- The force of gravity on an object.
- Changes based on the planet’s gravity.
- Measured in Newtons (N).
- Formula:
W = m × g
8. Gravitation and the Moon
The Moon has less mass than Earth, so its gravity is weaker. On the Moon, you would weigh only about 1/6th of your weight on Earth.
9. Tides and Gravitation
The Moon and the Sun pull on Earth’s water, causing tides:
- High tides: Water is pulled strongly.
- Low tides: Water is pulled less.
10. Satellites and Orbits
Satellites are objects that move around planets due to gravity. They stay in orbit because:
- Gravity pulls them toward the planet.
- Their high speed keeps them moving forward.
Examples:
- The Moon is a natural satellite of Earth.
- Artificial satellites (like communication satellites) are used for TV, weather, and internet.
11. Escape Velocity
The speed needed to leave Earth’s gravity is called escape velocity. On Earth, this speed is about 11.2 km/s.
12. Importance of Gravitation
- Keeps us on Earth.
- Makes planets orbit the Sun.
- Creates tides in oceans.
- Helps satellites and spacecraft stay in space.
13. Applications of Gravitation
- Daily Life: Things fall due to gravity.
- Astronomy: Explains orbits of planets, moons, and stars.
- Space Exploration: Helps launch rockets and study space.
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