Gravitation
Universal gravitation, free fall, mass vs weight, and thrust-pressure.
Chapter Overview
Universal gravitation explains why objects fall and planets orbit. Newton's law of gravitation, free fall, acceleration due to gravity g, mass vs weight, and buoyancy (Archimedes' principle) are covered.
Newton's Law of Gravitation
Gravitational force
F = G m₁m₂/r² ; G = 6.67 × 10⁻¹¹ N·m²/kg²
Free fall
v = u + gt ; s = ut + ½gt² ; v² = u² + 2gs
Weight
W = mg (N)
Buoyancy
Archimedes' principle
Buoyant force = weight of fluid displaced
Float/sink
Float if density_object < density_fluid
Solved Examples
Step-by-step solutions — read each step before checking the final answer.
Weight on Earth
Mass 5 kg on Earth (g=9.8 m/s²). Find weight.
- 1W = mg = 5 × 9.8
Answer
49 N
Free fall
Stone dropped from rest from 20 m height. Time to hit ground?
- 1s=½gt² → 20=½(9.8)t²
- 2t²≈4.08 → t≈2 s
Answer
≈ 2 s
Key Points to Remember
- ✓ g decreases with altitude and depth
- ✓ Mass is constant; weight varies with g
- ✓ Thrust on immersed body equals weight of displaced fluid
Exam Tips
- • Use g=10 m/s² if not specified
- • Distinguish mass (kg) and weight (N)
- • Draw force diagram for buoyancy problems
Practice MCQs — Gravitation
Test your understanding with topic-wise multiple choice questions. Explanations appear after each answer.
SI unit of G is: