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Gravitation & Pressure — Homi Bhabha Class 9 Physics MCQs with Solutions
Free Homi Bhabha Class 9 Physics Gravitation & Pressure MCQs with step-by-step solutions covering Universal Law of Gravitation, Variation in g, Mass vs Weight, Pressure & Fluid Thrust, Archimedes' Principle & Buoyancy. Practise online on Prepizo — no login needed.
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Sample questions with solutions
Q1 — Universal Law of Gravitation · easy · theory
According to the universal law of gravitation, if the distance between two objects is doubled while their masses stay the same, the gravitational force between them becomes:
A. 1/2 of the original force
B. 4 times the original force
C. 2 times the original force
D. 1/4 of the original force ✓ Correct
Solution: Force is proportional to 1/r². When r is doubled, the force becomes 1/(2²) = 1/4 of the original value.
Q2 — Variation in g · easy · theory
The value of acceleration due to gravity, g, on the Earth is maximum at:
A. the centre of the Earth
B. the poles ✓ Correct
C. the equator
D. the top of a tall mountain
Solution: The Earth is slightly flattened at the poles, so the polar radius is smallest. Since g is proportional to 1/R², g is largest at the poles.
Q3 — Mass vs Weight · easy · theory
Which of the following quantities remains the same for a body everywhere in the universe?
A. Both mass and weight
B. Mass ✓ Correct
C. Weight
D. Neither mass nor weight
Solution: Mass is the amount of matter in a body and does not change with location. Weight depends on g and changes from place to place.
Q4 — Pressure & Fluid Thrust · easy · theory
Pressure exerted on a surface is defined as:
A. mass per unit volume
B. force per unit length
C. thrust (force) acting per unit area ✓ Correct
D. force multiplied by area
Solution: Pressure = thrust (the force acting perpendicular to a surface) divided by the area over which it acts, P = F/A.
Q5 — Pressure & Fluid Thrust · easy · numerical
A force of 200 N acts perpendicularly on a surface of area 4 m². The pressure exerted is:
A. 204 Pa
B. 50 Pa ✓ Correct
C. 0.02 Pa
D. 800 Pa
Solution: P = F/A = 200/4 = 50 pascal (1 Pa = 1 N/m²).
Q6 — Archimedes' Principle & Buoyancy · easy · theory
The upward force exerted by a fluid on an object placed in it is called:
A. gravitational pull
B. thrust
C. buoyant force (upthrust) ✓ Correct
D. pressure
Solution: The upward force a fluid exerts on a body placed in it is the buoyant force, also called upthrust.
Q7 — Archimedes' Principle & Buoyancy · easy · theory
An object floats on the surface of a liquid when:
A. its weight is greater than the buoyant force
B. its weight equals the buoyant force acting on it ✓ Correct
C. its weight is much greater than the upthrust
D. it loses all its weight
Solution: A body floats when the upward buoyant force balances its weight, so weight = buoyant force.
Q8 — Universal Law of Gravitation · hard · numerical
If the distance between two bodies is doubled, the gravitational force between them becomes:
A. double
B. one-fourth ✓ Correct
C. one-half
D. four times
Solution: Since $F \propto \dfrac{1}{r^2}$, doubling $r$ makes $F$ become $\dfrac{1}{2^2} = \dfrac{1}{4}$ of the original.
Q9 — Universal Law of Gravitation · hard · numerical
If both masses are doubled while the distance between them is kept the same, the gravitational force becomes:
A. 2 times
B. unchanged
C. 4 times ✓ Correct
D. half
Solution: $F \propto m_1 m_2$; doubling both masses multiplies the force by $2 \times 2 = 4$.
Q10 — Universal Law of Gravitation · hard · numerical
If the distance between two masses is reduced to half, the gravitational force becomes:
A. 2 times
B. 4 times ✓ Correct
C. one-fourth
D. half
Solution: $F \propto \dfrac{1}{r^2}$; halving $r$ multiplies the force by $\dfrac{1}{(1/2)^2} = 4$.
Q11 — Variation in g · hard · theory
The value of g is greatest at the:
A. top of a mountain
B. poles ✓ Correct
C. centre of the Earth
D. equator
Solution: The Earth is slightly flattened, so the poles are closer to the centre, giving a larger g than at the equator.
Q12 — Variation in g · hard · theory
The value of acceleration due to gravity at the centre of the Earth is:
A. maximum
B. zero ✓ Correct
C. equal to that at the surface
D. infinite
Solution: At the centre the mass pulls equally in all directions, so the net gravitational acceleration is zero.
Q13 — Mass vs Weight · hard · numerical
A person has a mass of 60 kg. On the Moon (where $g = 1.6$ m/s²) his weight is:
A. 96 N ✓ Correct
B. 37.5 N
C. 60 N
D. 600 N
Solution: Weight on Moon $= mg = 60 \times 1.6 = 96$ N (mass stays 60 kg).
Q14 — Mass vs Weight · hard · theory
A beam balance is used to measure mass and works the same everywhere, whereas a spring balance measures weight, because:
A. a beam balance compares masses while a spring balance responds to the force of gravity ✓ Correct
B. a spring balance measures mass directly
C. both measure the same quantity
D. a beam balance uses springs
Solution: A beam balance compares the unknown mass with standard masses (independent of g), while a spring balance reads the gravitational force (weight).
Q15 — Mass vs Weight · hard · numerical
A body weighs 490 N on Earth where $g = 9.8$ m/s². Its mass is:
A. 98 kg
B. 4802 kg
C. 490 kg
D. 50 kg ✓ Correct
Solution: Mass $= \dfrac{W}{g} = \dfrac{490}{9.8} = 50$ kg.
Q16 — Pressure & Fluid Thrust · hard · numerical
The same force is applied first over an area of 2 m² and then over 0.5 m². The pressure in the second case compared to the first is:
A. 4 times greater ✓ Correct
B. the same
C. 2 times greater
D. 4 times smaller
Solution: For a fixed force $P \propto \dfrac{1}{A}$; reducing area from 2 to 0.5 m² (a factor of 4) makes the pressure 4 times greater.
Q17 — Pressure & Fluid Thrust · hard · numerical
The pressure due to a column of water 10 m deep (density $1000$ kg/m³, $g = 10$ m/s²) is:
A. 100000 Pa ✓ Correct
B. 110000 Pa
C. 1000 Pa
D. 10000 Pa
Solution: Pressure $= h\rho g = 10 \times 1000 \times 10 = 100000$ Pa.
Q18 — Archimedes' Principle & Buoyancy · hard · numerical
A body immersed in water displaces 200 cm³ of water. The buoyant force on it (density of water $1$ g/cm³, $g = 10$ m/s²) is:
A. 0.2 N
B. 200 N
C. 2 N ✓ Correct
D. 20 N
Solution: Mass of water displaced $= 200$ g $= 0.2$ kg; buoyant force $= 0.2 \times 10 = 2$ N.
Q19 — Archimedes' Principle & Buoyancy · hard · theory
A ship made of iron floats on water although iron is denser than water because:
A. the water pushes it up with a magnetic force
B. its hollow shape makes its average density less than that of water ✓ Correct
C. iron floats on water
D. the ship has no weight
Solution: The hull encloses air, so the ship's average density is below that of water and it displaces enough water to float.
Q20 — Archimedes' Principle & Buoyancy · hard · numerical
A body weighs 50 N in air and 40 N when fully immersed in water. The buoyant force acting on it is:
A. 40 N
B. 10 N ✓ Correct
C. 90 N
D. 50 N
Solution: Buoyant force = apparent loss in weight $= 50 - 40 = 10$ N.
Q21 — Universal Law of Gravitation · hard · numerical
The gravitational force between two masses is F. If one of the masses is tripled and the distance between them is halved, the new force is:
A. 12F ✓ Correct
B. 1.5F
C. 6F
D. 3F
Solution: F is proportional to m/r². Tripling a mass multiplies the force by 3, and halving the distance multiplies it by 2² = 4. Net factor = 3 × 4 = 12, so the new force is 12F.
Q22 — Variation in g · hard · numerical
On the surface of the Earth g = 9.8 m/s². At a height equal to the radius of the Earth (h = R), the value of g becomes:
A. 4.9 m/s²
B. 9.8 m/s²
C. 1.225 m/s²
D. 2.45 m/s² ✓ Correct
Solution: g at height h is g R²/(R+h)². With h = R this is g R²/(2R)² = g/4 = 9.8/4 = 2.45 m/s².
Q23 — Variation in g · hard · numerical
A planet has twice the mass of the Earth and twice its radius. The acceleration due to gravity on the surface of this planet is:
A. one-fourth of g on Earth
B. equal to g on Earth
C. twice g on Earth
D. half of g on Earth ✓ Correct
Solution: g is proportional to M/R². Here M is 2 times and R² is 2² = 4 times, so g is 2/4 = 1/2, that is half of g on Earth.
Q24 — Mass vs Weight · hard · numerical
An object weighs 240 N on the Earth (g = 10 m/s²). What is its mass, and what would it weigh on a planet where g = 4 m/s²?
A. mass 24 kg and weight 240 N on the planet
B. mass 24 kg and weight 96 N on the planet ✓ Correct
C. mass 24 kg and weight 60 N on the planet
D. mass 96 kg and weight 24 N on the planet
Solution: Mass = W/g = 240/10 = 24 kg and is unchanged. On the planet, weight = m × g = 24 × 4 = 96 N.
Q25 — Pressure & Fluid Thrust · hard · numerical
A brick of dimensions 20 cm × 10 cm × 5 cm weighs 15 N. The maximum pressure it can exert while resting on a table is:
A. 1500 Pa
B. 3000 Pa ✓ Correct
C. 300 Pa
D. 750 Pa
Solution: Maximum pressure comes from the smallest face, 10 cm × 5 cm = 50 cm² = 0.005 m². P = 15/0.005 = 3000 Pa.
Q26 — Archimedes' Principle & Buoyancy · hard · numerical
A block of volume 500 cm³ is fully immersed in water (density 1000 kg/m³, g = 10 m/s²). The buoyant force on it is:
A. 0.5 N
B. 5 N ✓ Correct
C. 50 N
D. 500 N
Solution: Buoyant force = density × volume × g = 1000 × (500×10⁻⁶) × 10 = 1000 × 0.0005 × 10 = 5 N.
Q27 — Universal Law of Gravitation · medium · theory
According to Newton's universal law of gravitation, the force between two masses is:
A. directly proportional to the distance between them
B. independent of the distance between them
C. directly proportional to the product of the masses and inversely proportional to the square of the distance ✓ Correct
D. inversely proportional to the product of the masses
Solution: $F = \dfrac{G m_1 m_2}{r^2}$ — proportional to $m_1 m_2$ and inversely proportional to $r^2$.
Q28 — Universal Law of Gravitation · medium · theory
We do not notice the gravitational force of attraction between everyday objects because:
A. gravity acts only on large planets
B. friction cancels gravity
C. the gravitational constant G is extremely small ✓ Correct
D. small objects have no mass
Solution: G $\approx 6.67\times10^{-11}$ is so tiny that the force between ordinary masses is negligible.
Q29 — Universal Law of Gravitation · medium · theory
The gravitational force between two bodies is always:
A. repulsive
B. zero
C. sometimes attractive, sometimes repulsive
D. attractive ✓ Correct
Solution: Gravitation is always an attractive force between masses.
Q30 — Variation in g · medium · theory
As we go higher above the surface of the Earth, the value of acceleration due to gravity (g):
A. remains constant
B. increases
C. becomes zero immediately
D. decreases ✓ Correct
Solution: Since $g = \dfrac{GM}{r^2}$, increasing the distance $r$ from the centre decreases $g$.