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General Science: Gravitation (Physics)
📅 Date: 16 December 2025 (Tuesday)
This is a significant chapter. Gravitation connects the motion we studied earlier to the forces of the universe. For your exam, this topic is less about calculating the force between two planets and more about understanding phenomena (tides, satellite motion, why we don't float away) and principles (Archimedes, buoyancy).
1. The Universal Law of Gravitation
Newton didn't just discover that apples fall; he realized the force making the apple fall is the same force keeping the Moon in orbit.
The Law: Every object in the universe attracts every other object.
The Formula: F = G × (M × m) / d2
- F = Force of attraction.
- M, m = Masses of the two objects.
- d = Distance between their centers.
- G = Universal Gravitational Constant (Fixed value everywhere).
- Mass Relationship: If you double the mass of one object, the Force doubles. (Directly proportional).
- Inverse Square Law: If you double the distance (d), the Force drops to 1/4th. If you halve the distance, the Force increases 4 times.
- Tides: Caused by the gravitational pull of the Moon and the Sun on Earth’s water.
2. Centripetal Force & Free Fall
Centripetal Force (Circular Motion)
The Moon moves in a circle around Earth. To move in a circle, a force must pull the object towards the center. This is Centripetal Force. If gravity disappeared, the Moon would fly off in a straight line.
Free Fall and 'g'
Acceleration due to Gravity (g): The rate at which falling objects speed up. (Average on Earth = 9.8 m/s2).
The Galileo Concept: 'g' does not depend on the mass of the falling object. In a vacuum, a bowling ball and a feather hit the ground at the same time.
The Earth is not a perfect sphere; it bulges at the Equator and is flattened at the Poles.
- At Poles: Radius is smaller → Gravity (g) is Stronger.
- At Equator: Radius is larger → Gravity (g) is Weaker.
- Result: You weigh slightly more at the Poles than at the Equator.
3. Mass vs. Weight (Don't Confuse Them!)
This is the most common trap in General Science questions.
- Mass (m): Amount of matter. Constant everywhere. SI Unit: kg. Can never be zero.
- Weight (W): The force with which Earth attracts the object. Formula: W = m × g. SI Unit: Newton (N). Can be zero (in space).
Example: Weight on the Moon
Gravity on Moon = 1/6th of Earth.
If your Mass is 60 kg:
On Earth: Weight = 60 × 9.8 ≈ 600N.
On Moon: Mass is still 60 kg. Weight is 600 / 6 = 100N.
4. Thrust and Pressure
Thrust: Force acting perpendicular to a surface.
Pressure: Thrust per unit Area. (P = F / A). SI Unit: Pascal (Pa).
Logic: Smaller Area → Higher Pressure
- Knives: Sharp edges (Small area) → High pressure → Cuts easily.
- School bags: Wide straps (Large area) → Less pressure on shoulders.
- Camels: Broad feet (Large area) → Less pressure → Don't sink in sand.
5. Buoyancy and Archimedes’ Principle
Why do heavy steel ships float, but a small iron nail sinks?
Buoyancy (Upthrust): The upward force exerted by a fluid on an immersed object.
- Density Rule:
If Object Density < Liquid Density → Floats (Cork, Ice).
If Object Density > Liquid Density → Sinks (Iron nail).
Archimedes’ Principle
"When a body is immersed in a fluid, it experiences an upward force equal to the weight of the fluid displaced by it."
- Designing Ships and Submarines.
- Lactometers: Used to test the purity of milk.
- Hydrometers: Used to determine the density of liquids.
Relative Density: Ratio of density of substance to density of water. It has NO UNIT.
6. Mentor’s Final Drill (Exam-Ready Questions)
A: It becomes 1/9th. (Inverse square of 3).
Q: Where is the value of 'g' maximum on Earth?
A: At the Poles.
Q: An astronaut has a mass of 70kg on Earth. What is his mass on the Moon?
A: 70kg. (Mass never changes).
Q: Which instrument is based on Archimedes' Principle to test milk purity?
A: Lactometer.
Q: Why does a mug of water feel lighter inside a bucket of water?
A: Due to the Buoyant Force (Upthrust) acting upwards.
Study Strategy: Focus on the "Why" questions. Why do ships float? Why do we have tides? These logic-based questions are the favorites of the commission. You have successfully navigated Mechanics! Keep it up!