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Gravitation Class 9 Science Chapter 9 Important Questions, Download PDF

Explore Gravitation Class 9 Science Chapter 9 Important Questions with detailed answers to strengthen exam preparation. Download the PDF and revise key concepts effectively.

Important Questions Class 9 Science Chapter 9: Gravitation is an important part of physics that helps us understand how things attract each other in the universe.

This chapter is very important for exams, as it includes both simple concepts and numerical questions. To score well, students should clearly understand ideas like gravitational force, acceleration due to gravity, and buoyancy. Many exam questions also need the use of formulas and basic theory. These class 9 science chapter 9 important questions help students revise and prepare better.

Gravitation Class 9 Important Questions and Answers

This section presents a curated list of important questions covering the essential aspects of gravitation, including universal law, free fall, and buoyancy.

Each question is accompanied by a precise answer to aid your understanding and exam readiness. This compilation is particularly useful for students preparing for important questions of gravitation class 9:

Question 1: State Universal law of gravitation.

Answer: Universal law of gravitation states that 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. The force is along the line joining the centres of the two objects.

Question 2: What is the SI unit of G?

Answer: The SI unit of G is .

Question 3: Write the formula to find the magnitude of gravitational force between earth and an object on the surface of earth.

Answer: The formula for the magnitude of gravitational force is where is the mass of Earth, is the mass of the object, and is the radius of Earth.

Question 4: What is the relation between G and g?

Answer: The relation between G (universal gravitational constant) and g (acceleration due to gravity) is .

Question 5: What is free fall?

Answer: Free fall is the state of motion when an object falls solely under the influence of gravity, with no other forces like air resistance acting upon it.

Question 6: How does the value of g change at the poles and at the equator?

Answer: The Earth is not a perfect sphere; it's flattened at the poles and bulges at the equator. Consequently, the radius at the poles is slightly less than at the equator. Since , and is smaller at the poles, the value of is greater at the poles compared to the equator.

Question 7: How will the weight of an object change if the Earth suddenly stops rotating?

Answer: The rotation of Earth causes a slight reduction in the apparent weight of objects, especially at the equator, due to the centrifugal force. If the Earth were to stop rotating, this centrifugal effect would disappear, and the apparent weight of objects would increase, particularly at the equator.

Question 8: Define weight of an object.

Answer: The weight of an object is the force with which the Earth attracts it. It is given by W = mg, where m is the mass of the object and g is the acceleration due to gravity. Its SI unit is Newton (N).

Question 9: What is buoyancy?

Answer: Buoyancy refers to the upward force exerted by a fluid that opposes the weight of an immersed object. This upward push acts against gravity, making objects appear lighter in a fluid.

Question 10: State Archimedes' principle.

Answer: Archimedes' principle states that when an object is wholly or partially immersed in a fluid, it experiences an upward force (buoyant force) which is equal to the weight of the fluid displaced by the object.

Question 11: What is relative density?

Answer: Relative density is the ratio of the density of a substance to the density of water. It is a dimensionless quantity. Relative density = (Density of a substance) / (Density of water).

Question 12: Why does an object float or sink when placed on the surface of water?

Answer: An object floats if its density is less than the density of water, or if the buoyant force exerted by the water is greater than or equal to the weight of the object. It sinks if its density is greater than the density of water, causing the buoyant force to be less than the object's weight

Question 13: The earth and the moon are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater or smaller or the same as the force with which the moon attracts the earth? Why?

Answer: The Earth attracts the Moon with the same force with which the Moon attracts the Earth. This is due to Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. Gravitational force is a mutual force.

Question 14: What is the magnitude of the gravitational force between the Earth and a 1 kg object on its surface? (Mass of Earth = $6 \times 10^{24} \text{ kg}$ , Radius of Earth = $6.4 \times 10^6 \text{ m}$ , G = $6.7 \times 10^{-11} \text{N m}^2 \text{kg}^{-2}$ )

Answer:

Given:

$M_e = 6 \times 10^{24} \text{ kg}$

m = 1 kg

$R_e = 6.4 \times 10^6 \text{ m}$

$G = 6.7 \times 10^{-11} \text{N m}^2 \text{kg}^{-2}$ Using the formula $F = G \frac{M_e m}{R_e^2}$

$F = (6.7 \times 10^{-11}) \times \frac{(6 \times 10^{24}) \times 1}{(6.4 \times 10^6)^2}$

$F = (6.7 \times 10^{-11}) \times \frac{6 \times 10^{24}}{40.96 \times 10^{12}}$

$F = \frac{40.2 \times 10^{13}}{40.96 \times 10^{12}}$

$F \approx 9.8 \text{ N}$

Question 15: The Earth's gravitational force causes an acceleration of $5 \text{ m/s}^2$ on an object. What is the acceleration of the object due to the Moon's gravitational force, if the mass of the Moon is times that of Earth and its radius is times that of Earth?

Answer:

Let be acceleration due to gravity on Earth, be acceleration due to gravity on Moon.

$g_e = G \frac{M_e}{R_e^2} = 5 \text{ m/s}^2$

$M_m = \frac{1}{81} M_e$

$R_m = \frac{1}{4} R_e$ $g_m = G \frac{M_m}{R_m^2} = G \frac{\frac{1}{81} M_e}{(\frac{1}{4} R_e)^2}$

$g_m = G \frac{\frac{1}{81} M_e}{\frac{1}{16} R_e^2} = \frac{16}{81} G \frac{M_e}{R_e^2}$

$g_m = \frac{16}{81} g_e$

$g_m = \frac{16}{81} \times 5 \text{ m/s}^2$

$g_m = \frac{80}{81} \text{ m/s}^2 \approx 0.987 \text{ m/s}^2$

Question 16: An object is thrown vertically upwards with a velocity of 49 m/s. Calculate:

(i) the maximum height to which it rises,

(ii) the total time it takes to return to the surface of the Earth. ( $g = 9.8 \text{ m/s}^2$ )

Answer:

Given: u = 49 m/s, $a = -g = -9.8 \text{ m/s}^2$

(i) At maximum height, final velocity v = 0.

Using the equation of motion:

$0^2 - (49)^2 = 2 \times (-9.8) \times s$

-2401 = -19.6s

$s = \frac{2401}{19.6} = 122.5 \text{ m}$

Maximum height = 122.5 m

(ii) Time to reach maximum height ():

Using v = u + at

$t_1 = \frac{49}{9.8} = 5 \text{ s}$ Total time to return to the surface of the Earth ( $t_{total}$ ):

The time of ascent is equal to the time of descent.

$t_{total} = 2 \times t_1 = 2 \times 5 = 10 \text{ s}$

Question 17: A stone is released from the top of a tower of height 19.6 m. Calculate its final velocity just before touching the ground. ( $g = 9.8 \text{ m/s}^2$ )

Answer:

Given: u = 0

s = 19.6 m

$a = g = 9.8 \text{ m/s}^2$ Using the equation of motion:

$v^2 - 0^2 = 2 \times 9.8 \times 19.6$

$v^2 = 19.6 \times 19.6$

$v = \sqrt{(19.6)^2} = 19.6 \text{ m/s}$

Final velocity = 19.6 m/s

Question 18: A stone is thrown vertically upward with an initial velocity of 40 m/s from the top of a 200 m high tower. Calculate its velocity when it hits the ground. ( $g = 10 \text{ m/s}^2$ )

Answer:

Given: u = +40 m/s

s = -200 m

$a = -g = -10 \text{ m/s}^2$ Using the equation of motion:

$v^2 - (40)^2 = 2 \times (-10) \times (-200)$

$v = \sqrt{5600} = 20 \sqrt{14} \text{ m/s}$ So, $v \approx -74.8 \text{ m/s}$ (downward direction).

The velocity when it hits the ground is approximately 74.8 m/s downwards.

Question 19: An object has mass 20 kg on Earth. What will be its weight on the Moon and on the Earth? (Given g on Earth = $10 \text{ m/s}^2$ , g on Moon = g on Earth)

Answer:

Given: Mass m = 20 kg

$g_e = 10 \text{ m/s}^2$

$g_m = \frac{1}{6} g_e = \frac{1}{6} \times 10 = \frac{10}{6} \approx 1.67 \text{ m/s}^2$ Weight on Earth ():

$W_e = m \times g_e = 20 \text{ kg} \times 10 \text{ m/s}^2 = 200 \text{ N}$ Weight on Moon ():

$W_m = m \times g_m = 20 \text{ kg} \times \frac{10}{6} \text{ m/s}^2 = \frac{200}{6} = \frac{100}{3} \approx 33.33 \text{ N}$

Question 20: A ball is thrown up and reaches a maximum height of 19.6 m. Calculate the velocity with which it was thrown upwards and the time taken by it to reach the highest point. ( $g = 9.8 \text{ m/s}^2$ )

Answer:

Given: s = 19.6 m, v = 0

$a = -g = -9.8 \text{ m/s}^2$ (i) Initial velocity (u):

Using

$0^2 - u^2 = 2 \times (-9.8) \times 19.6$

$-u^2 = -19.6 \times 19.6$

u = 19.6 m/s

Velocity with which it was thrown upwards is 19.6 m/s.

(ii) Time taken to reach the highest point (t):

Using v = u + at

0 = 19.6 + (-9.8)t

9.8t = 19.6

$t = \frac{19.6}{9.8} = 2 \text{ s}$

Time taken is 2 s.

Question 21: A stone is allowed to fall from the top of a tower 100 m high and at the same instant another stone is projected vertically upwards from the ground with a velocity of 25 m/s. Calculate when and where the two stones will meet. ( $g = 10 \text{ m/s}^2$ )

Answer:

Let the two stones meet after time t.

Let be the distance covered by the first stone (falling from top) and be the distance covered by the second stone (projected upwards).

Total height H = 100 m.

For the first stone (falling):

$a_1 = g = 10 \text{ m/s}^2$

$s_1 = u_1 t + \frac{1}{2} a_1 t^2 = 0 \times t + \frac{1}{2} \times 10 \times t^2 = 5t^2$ For the second stone (projected upwards):

$u_2 = 25 \text{ m/s}$

$a_2 = -g = -10 \text{ m/s}^2$

$s_2 = u_2 t + \frac{1}{2} a_2 t^2 = 25t + \frac{1}{2} \times (-10) \times t^2 = 25t - 5t^2$ When they meet, the sum of their distances from their respective starting points will be equal to the total height of the tower.

25t = 100

$t = \frac{100}{25} = 4 \text{ s}$

They meet after 4 seconds.

Now, calculate where they meet:

$s_1 = 5t^2 = 5 \times (4)^2 = 5 \times 16 = 80 \text{ m}$

So, they meet 80 m from the top of the tower.

Alternatively, from the ground: $s_2 = 25t - 5t^2 = 25 \times 4 - 5 \times (4)^2 = 100 - 80 = 20 \text{ m}$

They meet 20 m from the ground.

Question 22: In what direction does the buoyant force act?

Answer: The buoyant force acts vertically upwards, opposing the force of gravity.

Question 23: Why do objects float or sink?

Answer: An object floats if the buoyant force acting on it is equal to or greater than its weight, which occurs if the object's average density is less than or equal to the fluid's density. Conversely, an object sinks if its weight exceeds the buoyant force, meaning its average density is greater than the fluid's density.

Question 24: You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?

Answer: Your actual mass is slightly more than 42 kg. This is because the weighing machine measures your apparent weight, which is slightly less than your true weight due to the upward buoyant force of the air and the slight reduction from Earth's rotation (centrifugal effect).

Question 25: A block of wood is held underwater.

(a) What is the magnitude of the buoyant force on the block?

(b) What is the magnitude of the gravitational force on the block?

(c) What is the net force on the block?

Answer:

(a) The magnitude of the buoyant force on the block is equal to the weight of the water displaced by the volume of the submerged block.

(b) The magnitude of the gravitational force on the block is equal to its weight, which is its mass times the acceleration due to gravity (W = mg).

(c) The net force on the block is the difference between the buoyant force (upwards) and the gravitational force (downwards). If the block is held underwater, it implies an external force is applied. If only considering buoyant and gravitational forces, the net force determines if it would sink or rise if released.

Question 26: What is the relation between pressure and thrust?

Answer: Thrust is the force acting perpendicularly on a surface. Pressure is the thrust per unit area. Mathematically, $\text{Pressure} = \frac{\text{Thrust}}{\text{Area}}$ .

Question 27: What factors determine the buoyant force experienced by an object?

Answer: The buoyant force experienced by an object depends on two main factors:

Volume of the object immersed in the fluid: The larger the submerged volume, the greater the buoyant force.
Density of the fluid: A denser fluid exerts a greater buoyant force for the same immersed volume.

Question 28: You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, is one actually heavier than the other? If so, which one and why?

Answer: In reality, the bag of cotton is slightly heavier than the iron bar, even though they show the same mass on a weighing machine. This is because the cotton bag occupies a much larger volume than the iron bar. The larger volume of cotton displaces more air, experiencing a greater buoyant force from the air. Since the weighing machine measures apparent weight (actual weight - buoyant force), for the apparent weight to be the same, the actual weight of the cotton must be slightly higher to compensate for the larger upward buoyant force. Therefore, the cotton bag has a greater actual mass.

Important Questions Class 9 Science Chapter 9 PDF

The PDF link for the Important Questions for Class 9 Science Chapter 9 is available below. This PDF is created by PW to provide students with important questions and answers covering all the crucial topics in the chapter on Gravitation.

By referring to this PDF students can enhance their understanding and improve their preparation for exams. Download the PDF from the link below for easy access to these important questions.

Important Questions Class 9 Science Chapter 9 PDF

Benefits of Important Questions Class 9 Science Chapter 9

Focused Study : Important questions help students focus on key topics, making their study time more effective.

Know the Exam Style : Practicing these questions helps students understand the format of the exam and the types of questions that might appear.

Better Time Management : By working on important questions students can learn how to manage their time better during the exam.

Memory Improvement : Repeatedly studying important questions can help students remember information more easily when they need it.

Spotting Weak Areas : Important questions can show students where they need to practice more, helping them improve before the exam.

Smart Study Planning : Knowing which topics are important allows students to create a study plan that helps them do well in the exam.

Increased Confidence : Practicing important questions can make students feel more confident and prepared for the exam.

Less Exam Stress : Being well-prepared with important questions can help reduce anxiety and make students feel more secure.

Practice Like the Real Exam : Attempting important questions under timed conditions simulates the actual exam, helping students get used to the pressure.

Improved Answering Skills : Working on important questions helps students develop better ways to answer different types of questions, like multiple-choice or problem-solving.

Explore More Chapters

Chapter 1 - Matter in Our Surroundings Questions	Chapter 2 - Is Matter around us Pure? Questions
Chapter 3 - Atoms and Molecules Questions	Chapter 4 - Structure of The Atom Questions
Chapter 5 - The Fundamental Unit of Life Questions	Chapter 6 - Tissues Questions
Chapter 7 - Motion Questions	Chapter 8 - Force and Laws of Motion Questions
Chapter 9 - Gravitation Questions	Chapter 10 - Work and Energy Questions
Chapter 11 - Sound Questions	Chapter 12 - Improvement in Food Resources Questions

Gravitation Class 9 Science Chapter 9 Important Questions FAQs

Does gravity act in space?

Yes, gravity exists in space. It is the force that keeps planets orbiting the Sun and governs the motion of all celestial bodies.

What causes the tides on Earth?

Tides are caused by the gravitational pull of the Moon and the Sun on Earth's oceans. The Moon has a greater effect because it is closer to Earth.

Why don't we feel the gravitational force between everyday objects?

The gravitational force between everyday objects is very small because the masses involved are relatively small compared to celestial bodies like planets or stars.

Why do astronauts feel weightless in space?

Astronauts feel weightless because they are in a state of continuous free fall towards Earth. Since they are falling at the same rate as their spacecraft, they experience the sensation of weightlessness, even though gravity is still acting on them.

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