📘 Chapter 10 – Gravitation
🧠 Detailed & Easy Notes
🔹 1. What is Gravitation?
- Gravitation is the force of attraction between two objects in the universe.
- It acts even without contact.
🔹 2. Universal Law of Gravitation
“Every object in the universe attracts every other object with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.”
📌 Formula:
F = G × (m₁ × m₂) / r²
Where:
- F = gravitational force
- G = universal gravitational constant (6.67 × 10⁻¹¹ Nm²/kg²)
- m₁, m₂ = masses of objects
- r = distance between them
🔹 3. Importance of Gravity
- Binds us to Earth
- Keeps planets in orbit
- Causes tides
- Helps in formation of stars and galaxies
🔹 4. Free Fall
- When only gravity acts on a body, it’s called free fall.
- Acceleration due to gravity = g
- g = 9.8 m/s² on Earth
📌 Formula:
v = u + gt
s = ut + ½gt²
v² = u² + 2gs
(Where u = initial velocity, v = final velocity, s = height, t = time)
🔹 5. Mass and Weight
- Mass: Quantity of matter. (kg)
- Weight: Force by which Earth pulls object. (N)
📌 W = mg - Weight changes with gravity, mass doesn’t.
🔹 6. Thrust and Pressure
- Thrust: Force perpendicular to surface
- Pressure = Thrust / Area
📌 Unit of pressure: Pascal (Pa) - Pressure ∝ 1/Area
🔹 7. Pressure in Fluids
- Liquids and gases exert pressure in all directions.
- Increases with depth.
🔹 8. Buoyancy and Upthrust
- Buoyant Force: Upward force by fluid on object.
- Heavier objects sink, lighter float.
🔹 9. Archimedes’ Principle
“When an object is immersed in fluid, it experiences upward buoyant force equal to the weight of displaced fluid.”
📌 Used in:
- Designing ships, submarines
- Measuring purity
- Finding volume of irregular objects
🔹 10. Density and Relative Density
- Density = Mass / Volume (kg/m³)
- Relative Density = Density of substance / Density of water
- No unit (ratio)
✅ MCQs
- The value of G is:
a) 6.67 × 10⁻¹¹ Nm²/kg²
b) 9.8 m/s²
c) 10 m/s²
d) 6.67 Nm²/kg - Weight = ?
a) mg²
b) mv
c) mg
d) mass × distance - Unit of pressure:
a) Newton
b) Pascal
c) Joule
d) Watt - Which force pulls objects toward Earth?
a) Magnetic
b) Electric
c) Friction
d) Gravitational - Pressure in a fluid increases with:
a) Height
b) Depth
c) Speed
d) Area - Buoyant force is always:
a) Downward
b) Upward
c) Forward
d) Backward - Archimedes’ principle is used to measure:
a) Temperature
b) Speed
c) Density
d) Distance - If an object floats, its density is:
a) More than water
b) Equal to water
c) Less than water
d) Can’t say - SI unit of density is:
a) kg
b) m/s
c) kg/m³
d) g/cm³ - Relative density has:
a) SI unit
b) No unit
c) m/s²
d) m³ - What is thrust?
a) Force parallel to surface
b) Force perpendicular to surface
c) Weight
d) Pressure - Acceleration due to gravity on Earth:
a) 6.67 m/s²
b) 10 m/s²
c) 9.8 m/s²
d) 9 m/s - In free fall, only force acting is:
a) Friction
b) Gravity
c) Tension
d) Applied force - Mass of an object on moon is:
a) 1/6th
b) Double
c) Same
d) Zero - Buoyant force depends on:
a) Shape
b) Type of fluid
c) Area
d) Velocity
✅ Assertion & Reason
A: Weight of object changes on Moon.
R: g on Moon is 1/6th of Earth.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true
A: Objects sink if they are heavier than water.
R: Water can’t exert enough upward force.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true
A: Buoyant force acts upward.
R: Fluids apply upward thrust on submerged object.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true
A: G (universal constant) changes with location.
R: It depends on height and place.
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true
A: Mass changes with gravity.
R: Weight = mg
a) A and R true, R explains A
b) A and R true, R doesn’t explain A
c) A true, R false
d) A false, R true
✅ Case-Based Study
📘 Passage:
A metal ball is dropped from a tall building. As it falls, its velocity increases due to gravity. Another ball is placed in water and it floats. A third heavier object sinks in water.
- What causes metal ball to accelerate?
a) Friction
b) Gravity
c) Wind
d) Upthrust - What is value of g on Earth?
a) 10 m/s²
b) 8.9 m/s²
c) 9.8 m/s²
d) 9 m/s² - Which object will float in water?
a) Higher density
b) Lower density
c) Equal mass
d) More mass - What is the reason for floating?
a) Upward buoyant force > weight
b) Downward force
c) No gravity
d) Shape only - Why does heavier object sink?
a) High pressure
b) More area
c) Weight > buoyant force
d) Fluid less
✅ Reason-Based Questions
- Why does a stone fall faster than paper?
Because air resistance is less on stone. - Why do objects float or sink?
Because of relation between weight and buoyant force. - Why is G called universal constant?
Because it remains same everywhere. - Why do we feel weightless in free fall?
Because no contact force is acting. - Why is pressure more in deep sea?
Because pressure increases with depth. - Why is water stored in overhead tanks?
To create more pressure by height. - Why do ships float despite being heavy?
Because of large volume displacing enough water. - Why does ice float on water?
Because its density is less than water. - Why does moon have less gravity?
Because its mass is less than Earth. - Why is Archimedes’ principle useful?
To find density and purity of substances. - Why does weight decrease on moon?
Because g is 1/6th of Earth. - Why can’t we measure mass using spring balance?
Because spring balance measures weight, not mass. - Why do bubbles rise in water?
Because of upward buoyant force. - Why is thrust more if area is small?
Because pressure = force/area. - Why does parachute slow down fall?
Because it increases air resistance.
✅ Final Answer Key
MCQs:
1–a, 2–c, 3–b, 4–d, 5–b, 6–b, 7–c, 8–c, 9–c, 10–b, 11–b, 12–c, 13–b, 14–c, 15–b
Assertion & Reason:
16–a, 17–a, 18–a, 19–d, 20–d
Case-Based:
21–b, 22–c, 23–b, 24–a, 25–c