CBSE Important Questions Class 9 Science Chapter 9 – Gravitation

Class 9 Science Chapter 9 Gravitation is part of the CBSE Class 9 Science syllabus. It explains the universal law of gravitation, free fall and the acceleration due to gravity, the difference between mass and weight, and why all bodies fall at the same rate when there is no air resistance. The chapter also covers thrust and pressure, buoyancy, Archimedes' principle and relative density.

These CBSE important questions for Class 9 Science Chapter 9 cover how fishes float, the three conditions when a solid is immersed in a fluid, comparing the density of water and glycerine, why a crumbled paper falls faster, the hammer and feather experiment on the Moon, forces on a skydiver, gravity inside a spherical shell, mass and weight, and Archimedes' principle. Every question has a clear step-by-step answer in simple English with figures, so students can practise on their own, parents can check the working, and teachers can use them for class tests.

Q1. (i) How do fishes float in water?
(ii) Write the conditions that take place when a solid is immersed in a fluid.

An object sinks when its weight is more than the buoyant force

Answer: (i) Floating of fishes in water: Fishes have an air-filled floating tube (swim bladder) that runs along the body. The air in this tube lowers the apparent density of the fish, so that the weight of the fish becomes equal to the weight of the water it displaces, and the fish floats. When the fish wants to dive, it squeezes the air out of the tube. Its density then increases and it sinks towards the river bed.
(ii) When a solid is immersed in a fluid, one of three things can happen:
(a) If the weight of the body is more than the buoyant force, the body sinks.
(b) If the weight of the body is equal to the weight of the fluid displaced, the body floats fully submerged.
(c) If the weight of the body is less than the weight of the fluid displaced, the body floats with a part of it above the surface.

Q2. A student is given three experimental setups as shown in the figure. From the observations, the density of
(a) water is highest
(b) water is the same as glycerine
(c) water is more than glycerine
(d) glycerine is highest

Spring balance readings for a block in air, in glycerine and in water

Answer: (d) glycerine is highest
The spring balance shows the least reading when the block is in glycerine. A smaller reading means a greater loss of weight, which means a greater buoyant force. Since the buoyant force depends on the density of the liquid, glycerine must be denser than water.

Q3. Renthabile has taken two pieces of paper of the same mass. She crumbles one piece into a ball and then lets both of them fall to the ground. Which paper will reach the ground first?

Answer: The crumbled paper ball will reach the ground first.
Both pieces have the same mass, so the force of gravity on both is the same. But the flat sheet has a much larger surface area, so a lot more air resistance acts on it in the upward direction and slows it down. The crumbled ball has a small surface area, faces less air resistance and therefore falls faster.
(In a vacuum, where there is no air resistance, both would land at the same time.)

Q4. If one heavy object and another light object fall from the same height of 50 m in a vacuum, which object will reach the ground first?

Answer: Both objects will reach the ground at the same time.
In a vacuum there is no air resistance, so the only force acting on each object is its own weight. The acceleration due to gravity (g = 9.8 m/s²) is the same for every object, whatever its mass or size. Since both start from the same height with the same acceleration, they take the same time to fall.

Q5. A skydiver takes his dive and opens the parachute after some time. Describe the forces acting on the diver just after opening the parachute, and explain how the skydiver lands safely on the ground.

Air resistance acting upwards and weight acting downwards on a skydiver

Answer: Two forces act on the skydiver:
1. His weight, acting downwards.
2. The air resistance, acting upwards.
Just after the parachute opens, its large surface area makes the air resistance much greater than the weight. The net upward force slows the diver down. As the speed falls, the air resistance also falls, until it becomes exactly equal to the weight. Now the net force is zero and the diver moves down with a constant, low speed called the terminal velocity. At this slow steady speed he lands safely.

Q6. A skydiver opens his parachute after some time of his dive. Just after opening the parachute his speed will
(a) Increase
(b) Zero
(c) Decrease
(d) Constant

Answer: (c) Decrease
The open parachute has a large surface area, so the air resistance on it becomes much larger than the diver's weight. This gives a net upward force, which slows the diver down. Only later, when the air resistance becomes equal to the weight, does the speed become constant.

Q7. In mission Apollo 15, commander David Scott dropped a hammer and a feather at the same time on the Moon and observed that both reached the surface together. Explain the reason behind it.

Answer: There is no atmosphere on the Moon, so there is no air resistance there.
In the absence of air resistance, the only force acting on a falling body is the force of gravity, and the acceleration due to gravity is the same for all bodies whatever their mass. So the hammer and the feather fell with exactly the same acceleration and reached the Moon's surface at the same time. On Earth the feather falls slowly only because air resistance acts on it.

Q8. A point mass m is placed inside a spherical shell of radius R and mass M at a distance R/2 from the centre of the shell. The gravitational force exerted by the shell on the point mass is
(a) GMm/R²
(b) −GMm/R²
(c) zero
(d) 4GMm/R²

Answer: (c) zero
The gravitational pulls from all the different parts of a uniform spherical shell cancel each other out for any point inside the shell. So wherever the point mass is placed inside, the net gravitational force on it due to the shell is zero.

Q9. If the earth loses its gravity, then for a body
(a) weight becomes zero, but not mass
(b) mass becomes zero, but not weight
(c) both mass and weight are zero
(d) neither mass nor weight becomes zero

Answer: (a) weight becomes zero, but not mass
Mass is the amount of matter in a body. It stays the same everywhere and does not depend on gravity.
Weight is the force with which the earth attracts the body, W = mg. If gravity disappears, g = 0, so the weight becomes zero, but the mass stays the same.

Q10. Imagine a new planet having the same density as the earth but 3 times bigger than the earth in size. If the acceleration due to gravity on the surface of the earth is g and that on the new planet is g′, then
(a) g′ = g/9
(b) g′ = 27g
(c) g′ = 9g
(d) g′ = 3g

Answer: (d) g′ = 3g
g = GM/R². For a sphere of density ρ, M = ρ × (4/3)πR³.
So g = G × ρ × (4/3)πR³ / R² = (4/3)πGρR.
This shows that for the same density, g is directly proportional to the radius R.
Since the new planet has 3 times the radius, g′ = 3g.

Q11. State the universal law of gravitation and write its mathematical form. Define the universal gravitational constant G.

Answer: Universal law of gravitation: Every body in the universe attracts every other body with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Mathematically, F = G m1m2 / d²
where m1 and m2 are the masses, d is the distance between their centres, and G is the universal gravitational constant.
G is the force of attraction between two bodies of unit mass each kept at a unit distance apart. Its value is 6.673 × 10−11 N m²/kg².

Q12. State Archimedes' principle. Write any two of its applications.

Answer: Archimedes' principle: When a body is immersed fully or partly in a liquid, it experiences an upward force (buoyant force) that is equal to the weight of the liquid displaced by the body.
Applications:
1. It is used in designing ships and submarines, which float because they displace a large weight of water.
2. It is used in making lactometers, which check the purity of milk, and hydrometers, which measure the density of liquids.

Related Study Material on Extramarks

Q.1 A ship sinks to different levels, if its journey takes it to different regions. This is due to the

i. Variation in amount of mineral salts dissolved in sea water and the temperature due to the climatic conditions

ii. Constancy in amount of mineral salts dissolved in sea water and the temperature due to the climatic conditions

iii. The variation in the weight of the ship with temperature

iv. The consistency in the density of seawater at various regions

Marks:1
Ans

variation in amount of mineral salts dissolved in sea water and the temperature due to the climatic conditions

Q.2 A piece of iron is totally immersed in water. If its density is 7.8 x 103 kg/m3 and volume is 10-4 m3 then calculate
(i) The upthrust 
(ii) Apparent weight of iron piece in water.

Marks:5
Ans

Density of iron, d = 7.8 x 103 kg/m3;

Volume of iron piece, V = 10-4 m3

Mass of iron piece, M = V x d

= (10-4 m3) x (7.8 x 103 kg/m3)

= 0.78 kg

(i) Upthrust, = weight of displaced water

So, = Volume of displaced water x density of water x g

[Where g is the acceleration due to gravity]

So, = Volume of iron piece x density of water x g

[Volume of displaced water = volume of iron piece]

Hence, = (10-4 m3) (1000 kg/m3) (10 m/s2)

= 1 N

(ii) Apparent weight,
= true weight of iron piece upthrust on iron piece in water

True weight of iron piece = M x g

= (0.78 kg) (10 m/s2)

= 7.8 N

Thus, = 7.8 N 1 N

= 6.8 N

Q.3 The density of gold is 19.3 × 103 kg/m3. Find its relative density.

Marks:1
Ans


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