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Are you a Class 9 ICSE student struggling with Floatation and Archimedes’ Principle? At Dhingra Classes, Nashik, we understand that this chapter can feel tricky — but with the right guidance and practice, it becomes one of the easiest scoring topics in Physics.
This comprehensive guide covers 50 solved MCQs, 15 solved short answer questions, and 12 solved long answer questions based on the ICSE Class 9 Physics syllabus. Every answer is explained step-by-step so you can master Floatation, Archimedes’ Principle, Buoyancy, Density, and Relative Density with confidence.
📚 What is Floatation?
Floatation is the phenomenon where an object remains on the surface of a liquid or gas without sinking. Whether an object floats or sinks depends on the upthrust (buoyant force) acting on it and the density of the object compared to the liquid.
🔑 Key Concepts:
Thrust: The force acting perpendicular to the surface of an object.
Pressure: Thrust per unit area (P = F/A).
Buoyant Force (Upthrust): The upward force exerted by a fluid on a body immersed in it.
Archimedes’ Principle: A body immersed in a fluid experiences an upthrust equal to the weight of the fluid displaced.
Density: Mass per unit volume (ρ = m/V).
Relative Density: Ratio of the density of a substance to the density of water.
🧪 Archimedes’ Principle – Statement & Formula
Archimedes’ Principle states:
When a body is wholly or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced by it.
📐 Formula:
Upthrust (F_B) = Weight of fluid displaced = V × ρ × g
Where:
V = Volume of fluid displaced
ρ = Density of the fluid
g = Acceleration due to gravity (9.8 m/s²)
🧠 Law of Floatation:
A body floats in a liquid if the weight of the body is equal to the weight of the liquid displaced by it.
Conditions:
If density of body < density of liquid → Body floats
If density of body = density of liquid → Body remains suspended
If density of body > density of liquid → Body sinks
📝 SECTION 1: 50 SOLVED MCQs on Floatation and Archimedes’ Principle
Q1. The SI unit of thrust is:
(a) Newton (b) Pascal (c) Joule (d) Watt
Ans: (a) Newton
Q2. The SI unit of pressure is:
(a) Newton (b) Pascal (c) Joule (d) Watt
Ans: (b) Pascal
Q3. Archimedes’ Principle is related to:
(a) Heat (b) Light (c) Buoyancy (d) Sound
Ans: (c) Buoyancy
Q4. The upthrust on a body immersed in a liquid depends on:
(a) Mass of the body (b) Volume of the body (c) Density of the liquid (d) Both (b) and (c)
Ans: (d) Both (b) and (c)
Q5. A body floats in a liquid when:
(a) Weight of body > Upthrust (b) Weight of body < Upthrust (c) Weight of body = Upthrust (d) None of these
Ans: (c) Weight of body = Upthrust
Q6. The relative density of water is:
(a) 0 (b) 1 (c) 1000 (d) 9.8
Ans: (b) 1
Q7. The density of water in SI unit is:
(a) 1 g/cm³ (b) 1000 kg/m³ (c) Both (a) and (b) (d) 100 kg/m³
Ans: (c) Both (a) and (b)
Q8. A piece of iron sinks in water because:
(a) Its density is less than water (b) Its density is more than water (c) It is heavy (d) None of these
Ans: (b) Its density is more than water
Q9. A ship floats on water because:
(a) It is light (b) Its average density is less than water (c) It is made of wood (d) None of these
Ans: (b) Its average density is less than water
Q10. The upthrust on a body is equal to:
(a) Weight of the body (b) Weight of the fluid displaced (c) Volume of the body (d) Mass of the body
Ans: (b) Weight of the fluid displaced
Q11. The buoyant force acts in which direction?
(a) Downward (b) Upward (c) Sideways (d) None of these
Ans: (b) Upward
Q12. A hydrometer is used to measure:
(a) Density of solids (b) Relative density of liquids (c) Pressure (d) Temperature
Ans: (b) Relative density of liquids
Q13. The relative density of a substance is:
(a) Density of substance × Density of water (b) Density of substance / Density of water (c) Density of water / Density of substance (d) None of these
Ans: (b) Density of substance / Density of water
Q14. When a body is fully immersed in a liquid, the upthrust is:
(a) Maximum (b) Minimum (c) Zero (d) Infinite
Ans: (a) Maximum
Q15. A body weighs 50 N in air and 40 N in water. The upthrust is:
(a) 90 N (b) 10 N (c) 50 N (d) 40 N
Ans: (b) 10 N
Q16. The apparent weight of a body in water is:
(a) More than actual weight (b) Less than actual weight (c) Equal to actual weight (d) Zero
Ans: (b) Less than actual weight
Q17. Ice floats on water because:
(a) It is cold (b) Its density is less than water (c) It is solid (d) None of these
Ans: (b) Its density is less than water
Q18. The upthrust on a body depends on:
(a) Depth of the liquid (b) Shape of the body (c) Density of the liquid (d) Mass of the body
Ans: (c) Density of the liquid
Q19. A balloon filled with helium rises in air because:
(a) It is light (b) Density of helium is less than air (c) It is round (d) None of these
Ans: (b) Density of helium is less than air
Q20. The principle used in the working of a submarine is:
(a) Archimedes’ Principle (b) Pascal’s Law (c) Newton’s Law (d) Ohm’s Law
Ans: (a) Archimedes’ Principle
Q21. The relative density of a substance has:
(a) No unit (b) Unit kg/m³ (c) Unit g/cm³ (d) Unit N
Ans: (a) No unit
Q22. A body of density 0.8 g/cm³ is placed in water. It will:
(a) Sink (b) Float (c) Remain suspended (d) Dissolve
Ans: (b) Float
Q23. The weight of the liquid displaced by a floating body is equal to:
(a) Volume of body (b) Weight of body (c) Mass of body (d) Density of body
Ans: (b) Weight of body
Q24. The upthrust on a body immersed in a liquid is independent of:
(a) Density of liquid (b) Volume of body (c) Depth of liquid (d) Acceleration due to gravity
Ans: (c) Depth of liquid
Q25. A body weighs 100 N in air and 80 N in a liquid. The buoyant force is:
(a) 180 N (b) 20 N (c) 100 N (d) 80 N
Ans: (b) 20 N
Q26. The density of a substance is 2.5 g/cm³. Its relative density is:
(a) 2.5 (b) 0.4 (c) 25 (d) 250
Ans: (a) 2.5
Q27. A floating body displaces a liquid whose weight is:
(a) Less than the body (b) More than the body (c) Equal to the body (d) Zero
Ans: (c) Equal to the body
Q28. The upthrust on a body is maximum when:
(a) Partially immersed (b) Fully immersed (c) Just touching the surface (d) None of these
Ans: (b) Fully immersed
Q29. The instrument used to measure the density of milk is:
(a) Barometer (b) Lactometer (c) Thermometer (d) Ammeter
Ans: (b) Lactometer
Q30. The buoyant force on a body in a liquid is:
(a) Directly proportional to volume (b) Inversely proportional to volume (c) Independent of volume (d) None of these
Ans: (a) Directly proportional to volume
Q31. A body of volume 100 cm³ is immersed in water. The upthrust is (g = 10 m/s²):
(a) 1 N (b) 10 N (c) 100 N (d) 0.1 N
Ans: (a) 1 N
Q32. The density of a floating body is:
(a) Greater than liquid (b) Less than liquid (c) Equal to liquid (d) None of these
Ans: (b) Less than liquid
Q33. The apparent loss in weight of a body in water is equal to:
(a) Weight of body (b) Upthrust (c) Volume of body (d) Density of body
Ans: (b) Upthrust
Q34. A body floats with 1/4 of its volume above water. The density of the body is:
(a) 0.25 g/cm³ (b) 0.75 g/cm³ (c) 1 g/cm³ (d) 1.25 g/cm³
Ans: (b) 0.75 g/cm³
Q35. The upthrust on a body is equal to the weight of:
(a) The body (b) The liquid displaced (c) The container (d) None of these
Ans: (b) The liquid displaced
Q36. A body of mass 500 g and volume 200 cm³ is placed in water. It will:
(a) Float (b) Sink (c) Remain suspended (d) None of these
Ans: (b) Sink
Q37. The relative density of a substance can be measured using:
(a) Barometer (b) Hydrometer (c) Thermometer (d) Ammeter
Ans: (b) Hydrometer
Q38. A body weighs 60 N in air and 50 N in a liquid. The relative density of the liquid is (volume of body = 1000 cm³):
(a) 1 (b) 1.2 (c) 0.8 (d) 1.5
Ans: (a) 1
Q39. The upthrust on a body is directly proportional to:
(a) Depth (b) Density of liquid (c) Shape of body (d) None of these
Ans: (b) Density of liquid
Q40. A body of density 1.2 g/cm³ is placed in water. It will:
(a) Float (b) Sink (c) Remain suspended (d) None of these
Ans: (b) Sink
Q41. The law of floatation was given by:
(a) Newton (b) Archimedes (c) Pascal (d) Einstein
Ans: (b) Archimedes
Q42. The weight of a body in air is 80 N. Its weight in water is 60 N. The upthrust is:
(a) 140 N (b) 20 N (c) 80 N (d) 60 N
Ans: (b) 20 N
Q43. The density of a substance is 0.9 g/cm³. Its relative density is:
(a) 0.9 (b) 9 (c) 90 (d) 0.09
Ans: (a) 0.9
Q44. A body floats in water with 60% of its volume submerged. The density of the body is:
(a) 0.4 g/cm³ (b) 0.6 g/cm³ (c) 1 g/cm³ (d) 1.6 g/cm³
Ans: (b) 0.6 g/cm³
Q45. The upthrust on a body in a liquid depends on:
(a) Volume of body immersed (b) Density of liquid (c) Acceleration due to gravity (d) All of these
Ans: (d) All of these
Q46. A body of volume 500 cm³ is fully immersed in water. The upthrust is (g = 10 m/s²):
(a) 5 N (b) 50 N (c) 0.5 N (d) 500 N
Ans: (a) 5 N
Q47. The relative density of a substance is 2. Its density is:
(a) 2 g/cm³ (b) 2000 kg/m³ (c) Both (a) and (b) (d) 0.2 g/cm³
Ans: (c) Both (a) and (b)
Q48. A body floats in a liquid of density 1.2 g/cm³ with 1/3 of its volume above the liquid. The density of the body is:
(a) 0.4 g/cm³ (b) 0.8 g/cm³ (c) 1.2 g/cm³ (d) 1.6 g/cm³
Ans: (b) 0.8 g/cm³
Q49. The buoyant force on a body in a liquid is:
(a) Directly proportional to density of liquid (b) Inversely proportional to density of liquid (c) Independent of density of liquid (d) None of these
Ans: (a) Directly proportional to density of liquid
Q50. A body weighs 100 N in air and 70 N in a liquid. The relative density of the liquid is (volume of body = 3000 cm³):
(a) 1 (b) 1.5 (c) 2 (d) 0.5
Ans: (a) 1
📝 SECTION 2: 15 SOLVED SHORT ANSWER QUESTIONS on Floatation and Archimedes’ Principle
Q1. Define thrust and pressure. Give their SI units.
Ans:
Thrust: The force acting perpendicular to the surface of an object. SI unit: Newton (N).
Pressure: Thrust per unit area. SI unit: Pascal (Pa).
Formula: Pressure = Thrust / Area
Q2. State Archimedes’ Principle.
Ans:
Archimedes’ Principle states that when a body is wholly or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced by it.
Q3. What is buoyant force? On what factors does it depend?
Ans:
Buoyant force (upthrust) is the upward force exerted by a fluid on a body immersed in it. It depends on:
Volume of the body immersed in the fluid.
Density of the fluid.
Acceleration due to gravity (g).
Q4. Define density and relative density. Give their SI units.
Ans:
Density: Mass per unit volume (ρ = m/V). SI unit: kg/m³.
Relative Density: Ratio of the density of a substance to the density of water. It has no unit.
Q5. Why does a ship float on water while an iron nail sinks?
Ans:
A ship floats because its average density (including the hollow space inside) is less than the density of water. An iron nail sinks because the density of iron is greater than the density of water.
Q6. State the Law of Floatation.
Ans:
The Law of Floatation states that a body floats in a liquid if the weight of the body is equal to the weight of the liquid displaced by it.
Q7. What is the apparent weight of a body in a liquid?
Ans:
Apparent weight = Actual weight − Upthrust
It is the weight of the body when immersed in a liquid.
Q8. A body weighs 80 N in air and 60 N in water. Find the upthrust.
Ans:
Upthrust = Weight in air − Weight in water
= 80 N − 60 N = 20 N
Q9. Define the term “upthrust”. What is its direction?
Ans:
Upthrust (buoyant force) is the upward force exerted by a fluid on a body immersed in it. Its direction is vertically upward.
Q10. Why does an iceberg float with most of its volume submerged?
Ans:
An iceberg floats because the density of ice (0.92 g/cm³) is less than the density of seawater (1.03 g/cm³). About 90% of its volume remains submerged because the weight of the displaced seawater equals the weight of the iceberg.
Q11. What is a hydrometer? Where is it used?
Ans:
A hydrometer is an instrument used to measure the relative density of liquids. It is used in:
Measuring the density of milk (lactometer).
Measuring the density of battery acid.
Q12. A body of volume 200 cm³ is immersed in water. Find the upthrust (g = 10 m/s²).
Ans:
Upthrust = V × ρ × g
= 200 × 10⁻⁶ m³ × 1000 kg/m³ × 10 m/s²
= 2 N
Q13. Why is it easier to lift a heavy stone in water than in air?
Ans:
It is easier to lift a heavy stone in water because the upthrust exerted by water reduces the apparent weight of the stone.
Q14. What is the relation between relative density and density?
Ans:
Relative Density = Density of substance / Density of water
Since density of water = 1000 kg/m³ (or 1 g/cm³), relative density is a pure number with no unit.
Q15. A body floats with 3/5 of its volume submerged in water. Find its density.
Ans:
Density of body = (Volume submerged / Total volume) × Density of water
= (3/5) × 1 g/cm³ = 0.6 g/cm³
📝 SECTION 3: 12 SOLVED LONG ANSWER QUESTIONS on Floatation and Archimedes’ Principle
Q1. State Archimedes’ Principle. Describe an experiment to verify it.
Ans:
Archimedes’ Principle: When a body is wholly or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced.
Experiment:
Take a solid cylinder and weigh it in air (W₁).
Fill a displacement vessel with water up to the spout.
Immerse the cylinder in water and collect the displaced water.
Weigh the cylinder in water (W₂).
Weigh the displaced water (W₃).
Observation: W₁ − W₂ = W₃
Conclusion: Upthrust = Weight of fluid displaced. Hence, Archimedes’ Principle is verified.
Q2. Derive the expression for upthrust on a body immersed in a liquid.
Ans:
Consider a body of volume V immersed in a liquid of density ρ.
Weight of liquid displaced = V × ρ × g
According to Archimedes’ Principle, Upthrust = Weight of liquid displaced
F_B = V × ρ × g
Where:
V = Volume of body immersed
ρ = Density of liquid
g = Acceleration due to gravity
Q3. Explain the conditions for a body to float, sink, or remain suspended in a liquid.
Ans:
Floats: If density of body < density of liquid (e.g., wood in water).
Sinks: If density of body > density of liquid (e.g., iron in water).
Remains Suspended: If density of body = density of liquid (e.g., egg in salt water of matching density).
Q4. A body weighs 120 N in air and 90 N in water. Find:
(a) Upthrust
(b) Volume of the body
(c) Relative density of the body (g = 10 m/s²)
Ans:
(a) Upthrust = 120 − 90 = 30 N
(b) Upthrust = V × ρ × g
30 = V × 1000 × 10
V = 30/10000 = 0.003 m³ = 3000 cm³
(c) Relative density = Weight in air / Upthrust = 120/30 = 4
Q5. Explain the working of a submarine based on Archimedes’ Principle.
Ans:
A submarine has ballast tanks that can be filled with water or air.
To dive: Water is let into the ballast tanks, increasing the submarine’s weight and density. It sinks.
To surface: Compressed air is pumped into the tanks, expelling water. The submarine becomes lighter and rises.
This works on Archimedes’ Principle and the Law of Floatation.
Q6. A body of volume 500 cm³ and mass 400 g is placed in water. Will it float or sink? Find the volume of the body above water.
Ans:
Density of body = 400/500 = 0.8 g/cm³
Since 0.8 < 1 (density of water), the body will float.
Volume submerged = (Density of body / Density of water) × Total volume
= 0.8 × 500 = 400 cm³
Volume above water = 500 − 400 = 100 cm³
Q7. Define relative density. How is it measured using a hydrometer?
Ans:
Relative Density: The ratio of the density of a substance to the density of water.
Hydrometer:
It consists of a weighted bulb and a graduated stem.
When placed in a liquid, it floats with a certain portion submerged.
The reading on the stem at the liquid surface gives the relative density.
Higher relative density → hydrometer floats higher.
Q8. A body floats with 2/3 of its volume submerged in a liquid of density 1.2 g/cm³. Find the density of the body.
Ans:
Density of body = (Volume submerged / Total volume) × Density of liquid
= (2/3) × 1.2 = 0.8 g/cm³
Q9. Explain why a balloon filled with hydrogen rises in air.
Ans:
Hydrogen has a lower density than air.
The weight of the air displaced by the balloon is greater than the weight of the balloon.
The upthrust (weight of air displaced) exceeds the weight of the balloon.
Hence, the net upward force makes the balloon rise.
Q10. A body weighs 200 N in air and 150 N in a liquid of density 0.8 g/cm³. Find:
(a) Upthrust
(b) Volume of the body
(c) Density of the body (g = 10 m/s²)
Ans:
(a) Upthrust = 200 − 150 = 50 N
(b) Upthrust = V × ρ × g
50 = V × 800 × 10
V = 50/8000 = 0.00625 m³ = 6250 cm³
(c) Density = Mass/Volume
Mass = 200/10 = 20 kg
Density = 20/0.00625 = 3200 kg/m³
Q11. Describe an experiment to determine the relative density of a solid using Archimedes’ Principle.
Ans:
Weigh the solid in air (W₁).
Weigh the solid in water (W₂).
Upthrust = W₁ − W₂
Relative Density = W₁ / (W₁ − W₂)
This gives the relative density of the solid.
Q12. A piece of wood floats in water with 40% of its volume above water. Find the density of wood.
Ans:
Volume submerged = 60% = 0.6
Density of wood = 0.6 × Density of water
= 0.6 × 1 = 0.6 g/cm³
🎯 Exam Tips for Class 9 ICSE Physics Students
🌊 Understand Archimedes’ Principle — it is the foundation of floatation
📐 Memorise formulas for upthrust, density, and relative density
🧮 Practise numerical problems regularly
📊 Draw diagrams for experiments and derivations
✅ Verify units in all calculations
🔗 Connect concepts to real-life examples like ships, submarines, and balloons
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