🌍 Gravitation – Class 9 Science MCQs

Section 4: Archimedes’ Principle, Buoyancy & Numericals/HOTS (Q76–Q100)

Q76. The upward force experienced by a body when immersed in fluid is called:

A. Pressure
B. Thrust
C. Buoyant force
D. Normal force

✅ Correct Answer: C. Buoyant force

Explanation:

• A. Pressure = force/area.
• B. Thrust = force on surface.
• C. Correct, buoyant force acts upward in fluid.
• D. Normal = contact force on solids.

Q77. Archimedes’ Principle states that:

A. Fluid exerts pressure equally in all directions
B. Upthrust on body = weight of displaced fluid
C. Pressure = force/area
D. Density of fluid = mass/volume

✅ Correct Answer: B. Upthrust on body = weight of displaced fluid

Q78. When an object floats in water, its weight is:

A. Greater than upthrust
B. Equal to upthrust
C. Less than upthrust
D. Zero

✅ Correct Answer: B. Equal to upthrust

Q79. When an object sinks in water, its weight is:

A. Less than upthrust
B. Equal to upthrust
C. Greater than upthrust
D. Zero

✅ Correct Answer: C. Greater than upthrust

Q80. Which of the following will float in water? (Density of water = 1 g/cm³)

A. Density = 0.8 g/cm³
B. Density = 1.2 g/cm³
C. Density = 2 g/cm³
D. Density = 5 g/cm³

✅ Correct Answer: A. Density = 0.8 g/cm³

Q81. Why does a ship float on water?

A. Its weight is zero
B. Density of ship < water
C. Average density of ship < water
D. Weight = thrust

✅ Correct Answer: C. Average density of ship < water

Q82. Which of these is used to measure relative density?

A. Thermometer
B. Hygrometer
C. Hydrometer
D. Barometer

✅ Correct Answer: C. Hydrometer

Q83. Relative density is ratio of:

A. Mass of substance / volume
B. Density of substance / density of water
C. Weight of substance / weight of water
D. Mass / weight

✅ Correct Answer: B. Density of substance / density of water

Q84. Relative density has:

A. SI unit kg/m³
B. SI unit N/m²
C. No unit
D. SI unit g/cm³

✅ Correct Answer: C. No unit

Explanation:

• Being a ratio of densities, it is dimensionless.

Q85. A body floats if:

A. Density of body < density of fluid
B. Density of body > density of fluid
C. Densities are equal
D. No relation

✅ Correct Answer: A. Density of body < density of fluid

Q86. A stone weighs 50 g in air and 30 g in water. Loss of weight = ?

A. 20 g
B. 30 g
C. 50 g
D. 80 g

✅ Correct Answer: A. 20 g

Explanation:

• Loss of weight = weight in air – weight in water = 50 – 30 = 20 g.

Q87. Loss of weight of body in fluid = ?

A. Weight of object
B. Volume of fluid
C. Weight of displaced fluid
D. Density of object

✅ Correct Answer: C. Weight of displaced fluid

Q88. Which is denser?

A. Cork
B. Ice
C. Iron
D. Water

✅ Correct Answer: C. Iron

Q89. Why is it easier to lift heavy stone under water?

A. Density decreases
B. Buoyant force acts upward
C. Mass decreases
D. Friction reduces

✅ Correct Answer: B. Buoyant force acts upward

Q90. Which of the following floats partly immersed?

A. Wood block
B. Iron nail
C. Stone
D. Coin

✅ Correct Answer: A. Wood block

Q91. If density of liquid increases, buoyant force:

A. Decreases
B. Increases
C. Constant
D. Zero

✅ Correct Answer: B. Increases

Explanation:

• Buoyant force ∝ ρ (density of fluid).

Q92. Why does ice float on water?

A. Ice is lighter
B. Density of ice < water
C. Shape of ice
D. Cold temperature

✅ Correct Answer: B. Density of ice < water

Q93. A body sinks completely when:

A. Density of body < liquid
B. Density of body = liquid
C. Density of body > liquid
D. None

✅ Correct Answer: C. Density of body > liquid

Q94. A body of mass 100 g displaces 40 g water when floating. Relative density = ?

A. 0.4
B. 0.6
C. 1.0
D. 2.5

✅ Correct Answer: D. 2.5

Explanation:

• RD = mass of body / mass of displaced water = 100/40 = 2.5.

Q95. A solid weighs 10 N in air and 8 N in water. Buoyant force = ?

A. 2 N
B. 8 N
C. 10 N
D. 12 N

✅ Correct Answer: A. 2 N

Explanation:

• Loss of weight = 10 – 8 = 2 N.

Q96. A body floats with ¼ volume outside water. Density of body = ? (density of water = 1 g/cm³)

A. 0.25 g/cm³
B. 0.5 g/cm³
C. 0.75 g/cm³
D. 1 g/cm³

✅ Correct Answer: C. 0.75 g/cm³

Explanation:

• Fraction immersed = density of body / density of water = ¾ → density = 0.75.

Q97. If upthrust is greater than weight, body will:

A. Float partially
B. Float fully
C. Rise upwards
D. Sink

✅ Correct Answer: C. Rise upwards

Q98. Density of gold = 19.3 g/cm³. Density of water = 1 g/cm³. Relative density of gold = ?

A. 9.3
B. 19.3
C. 20.3
D. 193

✅ Correct Answer: B. 19.3

Q99. Submarine can float or sink in water because:

A. It is hollow
B. It is heavy
C. It changes its average density by filling/emptying tanks
D. It is pushed by water

✅ Correct Answer: C. It changes its average density by filling/emptying tanks

Q100. Hot air balloon rises up because:

A. It is light
B. Buoyant force > weight of balloon
C. Air pressure decreases
D. Gravity decreases

✅ Correct Answer: B. Buoyant force > weight of balloon

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