📚 1. Universal Law of Gravitation
✅ Key Definition
Every object in the universe attracts every other object with a force called gravitational force.
🌱 Explanation
• This law was given by .
• The force of attraction depends on:
- Mass of the two objects
- Distance between them
• The force increases if masses increase.
• The force decreases if distance increases.
• Mathematical formula:
F = G \frac{m_1 m_2}{r^2}
Where:
F = Gravitational force
G = Universal gravitational constant
m₁, m₂ = masses
r = distance between centres
📚 Subtopics
• Value of G = 6.67 × 10⁻¹¹ N m²/kg²
• SI unit of G → N m²/kg²
🖊 Diagram Description
Draw two spheres with centres marked. Show distance “r” between their centres. Draw arrows showing force of attraction between them.
🧠 Memory Trick
“More mass → More force”
“More distance → Less force (square relation)”
📚 2. Importance of Universal Law of Gravitation
🌱 Explanation
This law explains:
• Why we are bound to Earth
• Why the Moon revolves around Earth
• Why planets revolve around the Sun
• Why tides occur in oceans
📚 3. Free Fall
✅ Key Definition
When an object falls towards Earth under the influence of gravity alone, it is said to be in free fall.
🌱 Explanation
• Only gravity acts on the object.
• Acceleration during free fall is called acceleration due to gravity (g).
• Near Earth’s surface, g = 9.8 m/s².
• All objects fall with same acceleration (ignoring air resistance).
📚 Equations of Motion for Free Fall
v = u + gt
s = ut + ½gt²
v² = u² + 2gs
🧠 Important Point
In free fall, “a” is replaced by “g”.
📚 4. Acceleration Due to Gravity (g)
✅ Key Definition
The acceleration produced in a body due to Earth’s gravitational pull is called acceleration due to gravity.
🌱 Explanation
• g = 9.8 m/s²
• g is maximum at poles
• g is minimum at equator
• g decreases as we go up from Earth’s surface
🧠 Memory Trick
“g changes with place”
📚 5. Mass and Weight
✅ Mass
Amount of matter contained in a body.
✅ Weight
The force with which Earth attracts a body.
🌱 Explanation
• Mass is constant everywhere.
• Weight changes from place to place.
• Weight = mg
📊 Difference Between Mass and Weight
| Mass | Weight |
|---|---|
| Amount of matter | Force of gravity |
| SI unit: kg | SI unit: Newton |
| Constant everywhere | Changes with g |
| Scalar quantity | Vector quantity |
🖊 Diagram Description
Draw a body on Earth and label downward force as “Weight (W = mg)”.
🧠 Memory Trick
“M for Matter, W for Weight (pull)”
📚 6. Thrust and Pressure
✅ Thrust
Force acting perpendicular to a surface.
✅ Pressure
Thrust per unit area.
Formula:
Pressure = Force / Area
SI unit: Pascal (Pa)
🌱 Explanation
• Smaller area → More pressure
• Larger area → Less pressure
📊 Example Table
| Situation | Area | Pressure |
|---|---|---|
| Sharp knife | Small | High |
| Camel’s feet | Large | Low |
🧠 Important Point
Pressure inversely depends on area.
📚 7. Pressure in Liquids
🌱 Explanation
• Liquids exert pressure in all directions.
• Pressure increases with depth.
• Dam walls are thicker at bottom.
🖊 Diagram Description
Draw a container with three holes at different depths. Show water flowing farthest from lowest hole.
📚 8. Buoyancy
✅ Key Definition
The upward force exerted by a liquid on an immersed object is called buoyant force.
🌱 Explanation
• Objects experience upward force in liquids.
• If buoyant force > weight → object floats.
• If buoyant force < weight → object sinks.
🧠 Memory Trick
“Upward force = Buoyancy”
📚 9. Archimedes’ Principle
✅ Key Definition
When a body is immersed in a fluid, it experiences an upward force equal to the weight of fluid displaced.
Given by .
🌱 Explanation
• Apparent loss in weight = Weight of displaced liquid
• Used in designing ships and submarines
🖊 Diagram Description
Draw a beaker with water. Show object immersed and water level rising. Label displaced water.
📚 10. Relative Density
✅ Key Definition
Relative density is the ratio of density of a substance to density of water.
Formula:
Relative Density = Density of substance / Density of water
🌱 Explanation
• It has no unit.
• If relative density < 1 → object floats.
• If relative density > 1 → object sinks.
🧠 Memory Trick
“Compare with water = Relative density”
🌟 Quick Revision Summary
🔥 Every object attracts every other object (Universal Law of Gravitation).
🔥 Force ∝ masses and inversely ∝ square of distance.
🔥 g = 9.8 m/s².
🔥 Weight = mg.
🔥 Mass constant, weight variable.
🔥 Pressure = Force/Area.
🔥 Liquid pressure increases with depth.
🔥 Buoyant force acts upward.
🔥 Archimedes’ Principle → Upthrust = Weight of displaced fluid.
🔥 Relative density has no unit.
