Topics Covered
10 key topics in this chapter
Study Resources
Key Concepts
01. Universal Law of Gravitation
Every object in the universe attracts every other object with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them. F = Gm₁m₂/r². G = 6.674 × 10⁻¹¹ N·m²/kg².
02. Acceleration due to Gravity (g)
g = GM_Earth / R²Earth ≈ 9.8 m/s² at Earth's surface. g decreases as altitude increases (r increases). g on moon ≈ 1.63 m/s² (≈ g_Earth/6). Free fall: any object falling under gravity alone (ignoring air resistance).
03. Mass vs Weight
Mass (m) is the amount of matter — scalar, constant everywhere. Weight (W = mg) is the gravitational force on a body — vector, varies with g. An astronaut's mass is constant; weight changes on Moon or in space.
04. Thrust & Pressure
Thrust is the force acting perpendicular to a surface. Pressure = Thrust / Area (P = F/A). SI unit: Pascal (Pa = N/m²). Pressure depends on area — sharp knives cut better (small area → large pressure).
05. Archimedes' Principle
When a body is immersed in a fluid (partially or fully), it experiences an upward buoyant force equal to the weight of the fluid displaced. Buoyancy = weight of fluid displaced. This explains why ships float and hot air balloons rise.
06. Relative Density
Relative density = density of substance / density of water. If RD > 1, object sinks; RD < 1, object floats. RD of water = 1 (at 4 °C). RD has no unit (it is a ratio).
Formulas at a Glance
| # | Name | Expression | Notes |
|---|---|---|---|
| 01 | Gravitational Force | F = Gm₁m₂/r² |
G = 6.674 × 10⁻¹¹ N·m²/kg² |
| 02 | g at surface | g = GM/R² |
g on Earth ≈ 9.8 m/s² |
| 03 | Weight | W = mg |
SI unit: Newton (N) |
| 04 | Pressure | P = F/A |
SI unit: Pascal (Pa = N/m²) |
| 05 | Buoyant Force | F_b = ρ_fluid × V_displaced × g |
Archimedes' principle |
| 06 | Relative Density | RD = ρ_substance / ρ_water |
Dimensionless ratio |
Important Notes
Exam Tips & Common Mistakes
Always state the universal law in words first, then give the formula F = Gm₁m₂/r² with all symbols defined.
"What is the weight of a body of mass 5 kg on Moon?" → W = m × g_moon = 5 × 1.63 ≈ 8.15 N. A standard 2-mark numerical.
Pressure: smaller area → higher pressure. Explain camel's feet (large area → low pressure on sand) as an example.
"Why does a boat float but an iron nail sink?" — boat has hollow shape displacing more water; weight < buoyancy. Iron nail is solid; weight > buoyancy.
Distinguish between G (constant, universal) and g (varies with location, acceleration). This distinction appears in 1-mark and 3-mark questions.