CBSE Class 9 Science Chapter-9 Important Questions - Free PDF Download
Important Questions for CBSE Class 9 Science Chapter 9 - Gravitation is provided in this article in the form of free-to-download pdf notes.
Table of Content
Gravity, or gravitation, is a physical phenomenon that is observed for all objects which have mass. It is the phenomenon that governs the movement of heavenly bodies such as stars and planets and keeps one stuck on the ground and makes objects fall from a height. Gravity or Gravitation is one of the four fundamental forces of nature. Solving the important questions of gravitation of Class 9 will help the students boost their confidence and get ready for their final exam. The questions on gravitation for Class 9 Science will provide the students, the essential insights covered in the chapter.
So, download the free pdf and start solving the important questions of CBSE Class 9 Science Chapter 9 Gravitation.
CBSE Class 9 Science Chapter 9 - Gravitation - Topics Covered
Following are the topics that are covered in the CBSE Class 9 Science Chapter 9:
Gravitation
Universal Law of Gravitation
Free Fall
Acceleration Due to Gravity (g)
Difference between G and g
Mass and Weight
Weight of an Object on the Surface of the Moon
FAQs on Important Questions for CBSE Class 9 Science Chapter 9 - Gravitation
1. What is the Universal Law of Gravitation and why is this law considered 'universal'?
The Universal Law of Gravitation, formulated by Isaac Newton, states that every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. It is called 'universal' because it applies to all objects with mass, regardless of their size or composition, from tiny atoms to massive galaxies, anywhere in the universe.
2. What are some frequently asked differences between 'g' (acceleration due to gravity) and 'G' (Universal Gravitational Constant) for a 3-mark question?
For the CBSE Class 9 exam, the key differences between g and G are:
- Definition: 'g' is the acceleration experienced by a body in free fall due to Earth's gravitational pull. 'G' is the gravitational force between two bodies of unit mass separated by a unit distance.
- Value: The value of 'g' varies depending on location (approx. 9.8 m/s² on Earth's surface). The value of 'G' is constant everywhere in the universe (6.674 × 10⁻¹¹ Nm²/kg²).
- Units: The SI unit for 'g' is metres per second squared (m/s²). The SI unit for 'G' is Newton-metre squared per kilogram squared (Nm²/kg²).
- Nature: 'g' is a vector quantity as it has both magnitude and direction (towards the centre of the Earth). 'G' is a scalar quantity.
3. An object has a mass of 20 kg. What would be its weight on Earth and on the Moon? (Assume g on Earth = 9.8 m/s²)
The relationship between mass and weight is Weight = mass × acceleration due to gravity (W = m × g).
On Earth:
- Mass (m) = 20 kg
- g on Earth = 9.8 m/s²
- Weight on Earth = 20 kg × 9.8 m/s² = 196 Newtons (N).
On the Moon:
- The acceleration due to gravity on the Moon is about 1/6th of that on Earth.
- g on Moon ≈ 9.8 / 6 ≈ 1.63 m/s²
- Weight on Moon = 20 kg × 1.63 m/s² = 32.6 Newtons (N).
The mass of the object remains 20 kg on both the Earth and the Moon, but its weight changes.
4. State Archimedes' Principle and list two of its practical applications.
Archimedes' Principle states that when a body is immersed fully or partially in a fluid (a liquid or a gas), it experiences an upward force that is equal to the weight of the fluid displaced by it. This upward force is known as the buoyant force.
Two practical applications are:
- Ships and Submarines: A ship floats because the weight of the water it displaces is equal to its own weight. Submarines use ballast tanks to take in or expel water, changing their weight to sink or rise based on this principle.
- Lactometers: These devices are used to check the purity of milk. A lactometer sinks more in purer milk (which is less dense) and floats higher in adulterated milk (which is often denser), indicating the water content based on buoyancy.
5. If the Earth's gravity pulls on the Moon, why doesn't the Moon fall and crash into the Earth?
The Moon is constantly falling towards the Earth due to gravity, but it never hits it because it also has a high orbital velocity (sideways motion). This tangential velocity is perfectly balanced with Earth's gravitational pull. The gravity provides the necessary centripetal force that continuously pulls the Moon into a curved path, which results in it orbiting the Earth instead of flying off into space or crashing into the planet.
6. Why is it much easier to lift a heavy stone underwater than it is in the air?
It is easier to lift a heavy stone underwater because the water exerts an upward force on the stone called the buoyant force. According to Archimedes' principle, this buoyant force is equal to the weight of the water displaced by the stone. This upward force counteracts a portion of the stone's weight (the downward gravitational force), reducing the net force required to lift it. In air, the buoyant force from the air is negligible, so you have to overcome the stone's full weight.
7. For the CBSE Class 9 exam 2025-26, what kind of 5-mark numerical problems can be expected from the Gravitation chapter?
A typical 5-mark question from this chapter often involves multiple steps or combines different concepts. You can expect problems such as:
- Calculating the gravitational force between two objects, and then determining how the force changes if the mass of one object is doubled and the distance between them is halved.
- Solving for the acceleration due to gravity ('g') on another planet given its mass and radius relative to Earth.
- Using the equations of motion (v = u + at, s = ut + ½at², v² = u² + 2as) for an object in free fall, for instance, to calculate the time taken to reach the ground or the velocity just before impact.
- Problems involving the calculation of weight on the Moon or another planet, which first requires finding the object's mass from its weight on Earth.
8. A stone is thrown vertically upwards. What is its velocity and acceleration at the highest point of its journey?
This is a common conceptual question. At the highest point of its journey:
- The velocity of the stone is momentarily zero (0 m/s). It stops for an instant before it begins to fall back down.
- The acceleration of the stone is 9.8 m/s² directed downwards. The acceleration due to gravity ('g') continues to act on the stone throughout its flight, even at the very top. It is this constant downward acceleration that causes the stone to slow down on its way up and speed up on its way down.
