Gravitation is one of the most important topics in Class 9 Science, as it explains the fundamental force that governs planetary motion, tides, and free-fall motion. Understanding the laws of gravitation helps students develop strong conceptual knowledge and problem-solving skills.
In this topic, we provide important questions on gravitation for Class 9 along with their answers. These questions cover theory, numerical problems, and real-life applications. This guide will help students prepare effectively for their exams.
Key Topics Covered
Before diving into the questions, let’s review some essential concepts related to gravitation:
1. Newton’s Law of Universal Gravitation
The force of attraction between two masses is given by:
where:
- F = gravitational force
- G = universal gravitational constant ( $6.674 times 10^{-11} Nm2/kg2$ )
- m_1, m_2 = masses of two objects
- r = distance between the centers of the objects
2. Acceleration Due to Gravity ( g )
The acceleration due to gravity on Earth is approximately ** $9.8 m/s^2$ **.
3. Free Fall Equations
For an object falling freely:
where:
- u = initial velocity
- v = final velocity
- s = distance covered
- t = time
4. Weight and Mass
where:
- W = weight
- m = mass
- g = acceleration due to gravity
Important Questions on Gravitation for Class 9
1. Conceptual Questions
Q1: What is gravitation?
A: Gravitation is the force of attraction that exists between any two objects in the universe due to their masses. It is responsible for keeping planets in orbit around the Sun and causing objects to fall toward the Earth.
Q2: State Newton’s Law of Universal Gravitation.
A: Newton’s Law of Universal Gravitation 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 centers.
Q3: Why does the moon not fall into the Earth?
A: The moon does not fall into the Earth because it has a tangential velocity that keeps it in orbit. The gravitational pull of the Earth acts as a centripetal force, keeping the moon in circular motion around the Earth.
Q4: What is the difference between mass and weight?
A:
- Mass is the amount of matter in an object and remains constant everywhere. It is measured in kilograms (kg).
- Weight is the force exerted by gravity on an object. It changes based on location and is measured in newtons (N) using the formula ** W = mg **.
Q5: Why do astronauts experience weightlessness in space?
A: Astronauts experience weightlessness in space because they are in a state of free fall inside the spacecraft. Since the spacecraft and astronauts fall at the same rate, they do not feel any normal force acting on them, creating the sensation of floating.
2. Numerical Problems on Gravitation
Q6: Calculate the gravitational force between two objects of masses 80 kg and 200 kg separated by a distance of 5 meters.
Solution:
Using the formula:
So, the gravitational force is ** $4.27 times 10^{-8} N$ **.
Q7: A stone is dropped from a height of 50 meters. How long will it take to reach the ground?
Solution:
Using the equation:
Since u = 0 :
So, the stone takes 3.2 seconds to reach the ground.
Q8: A person weighs 700 N on Earth. What would be their weight on the Moon? (Gravity on the Moon is frac{1}{6} th of Earth’s gravity.)
Solution:
So, the person’s weight on the Moon is 116.67 N.
Q9: A satellite is orbiting the Earth at a distance of 20,000 km. If the mass of the Earth is $5.97 times 10^{24} kg$ , calculate the gravitational force acting on the satellite. (Take G = 6.674 times 10^{-11} Nm^2/kg^2 )
Solution:
Using the formula:
Since the satellite’s mass is not given, we cannot determine the exact force, but the formula applies to any satellite of known mass.
3. Application-Based Questions
Q10: How does the value of ‘g’ change as we move away from the Earth’s surface?
A: The value of acceleration due to gravity (g) decreases as we move away from the Earth’s surface because it is inversely proportional to the square of the distance from the Earth’s center.
Gravitation plays a crucial role in understanding the motion of celestial bodies, tides, and falling objects. By practicing theoretical questions and numerical problems, students can develop a solid grasp of Newton’s Law of Gravitation, free fall, weight, and orbital motion.
Key takeaways:
- Gravitation is the force of attraction between any two objects with mass.
- Acceleration due to gravity on Earth is ** $9.8 m/s^2$ ** but varies on other celestial bodies.
- Mass remains constant, but weight changes based on gravity.
- Objects in free fall experience acceleration due to gravity without any external force.
- Satellites orbit the Earth due to the balance between gravitational pull and their velocity.
By regularly solving these questions on gravitation for Class 9, students can improve their problem-solving skills and be well-prepared for exams.