Unit 5: Gravitation
Detailed Smart Study Guide - Physics Class 9
Welcome to the Class! (Introduction)
Hello students! Have you ever wondered why an apple falls down to the ground instead of going up? Or why the Moon stays in its orbit around the Earth? The answer to all these mysterious questions lies in one word: Gravitation. In this unit, we will study the invisible force that holds our entire universe together, from the smallest pebble on Earth to the giant planets in space.
Gravity is not just a force that pulls things down; it is a universal attraction between every two objects in the universe. We will learn about Sir Isaac Newton’s brilliant discovery—the Law of Gravitation. This law explains how the force of attraction depends on the mass of objects and the distance between them. We will also see how scientists used this law to calculate the actual mass of our huge planet Earth!
Finally, we will discuss the fascinating world of Artificial Satellites. We will see how GPS works and how satellites stay in the sky without falling. This unit is very important because it connects the physics we see on Earth with the physics of the stars. I will explain everything in simple English so you can master this chapter without any confusion. Let’s start our journey into space!
Detailed Class Discussion: Q&A
Q1: State Newton's Law of Gravitation.
Teacher's Answer: Every body in the universe attracts every other body with a force. This force is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
Formula: $F = G \frac{m_1 m_2}{r^2}$
Q2: What is the Gravitational Constant (G)? Write its value.
Teacher's Answer: 'G' is a universal constant. Its value is the same everywhere in the universe.
Value: $6.673 \times 10^{-11} Nm^2kg^{-2}$. This is a very small value, which is why we don't feel gravity between small objects like two books.
Q3: How does the value of 'g' change with altitude (height)?
Teacher's Answer: The value of gravitational acceleration 'g' decreases as we go higher above the Earth's surface. This is because the distance from the center of the Earth increases. On top of a high mountain, 'g' is slightly less than at sea level.
Q4: Why is Law of Gravitation important?
Teacher's Answer: It helps us to determine:
1. The force that binds us to the Earth.
2. The motion of the Moon around the Earth.
3. The motion of planets around the Sun.
Q5: What are Artificial Satellites?
Teacher's Answer: Objects sent into space by humans to orbit the Earth or other planets are called artificial satellites. They are used for communication, weather forecasting, and GPS.
Q6: What is a Geostationary Orbit?
Teacher's Answer: It is an orbit where the satellite takes exactly 24 hours to complete one circle. To an observer on Earth, such a satellite appears to be stationary (fixed) at one point in the sky.
📝 Unit 5: Knowledge Check (10 MCQs)
1. The value of 'G' was first determined by:
(A) Newton (B) Cavendish (C) Einstein (D) Galileo
▶ See Answer
(B) Cavendish
2. The mass of Earth is approximately:
(A) $6 \times 10^{24} kg$ (B) $6 \times 10^{22} kg$ (C) $6 \times 10^{20} kg$ (D) $6 \times 10^{26} kg$
▶ See Answer
(A) $6 \times 10^{24} kg$
3. As we go up from the Earth's surface, the value of 'g' becomes:
(A) Greater (B) Smaller (C) Zero (D) Remains same
▶ See Answer
(B) Smaller
4. The orbital velocity of a satellite depends on:
(A) Mass of satellite (B) Radius of orbit (C) Color of satellite (D) None
▶ See Answer
(B) Radius of orbit
5. The value of 'g' at the center of Earth is:
(A) 10 (B) 9.8 (C) Zero (D) Infinite
▶ See Answer
(C) Zero
6. Distance of geostationary satellite from the center of Earth is nearly:
(A) 42,300 km (B) 36,000 km (C) 6,400 km (D) 1000 km
▶ See Answer
(A) 42,300 km (Height from surface is 36,000 km)
7. Global Positioning System (GPS) consists of how many satellites?
(A) 12 (B) 24 (C) 36 (D) 48
▶ See Answer
(B) 24
8. If the distance between two masses is doubled, the force becomes:
(A) Half (B) Double (C) One-fourth (D) Four times
▶ See Answer
(C) One-fourth (Inverse square law)
9. Earth's gravitational force of attraction vanishes at:
(A) 6400 km (B) Infinity (C) 42300 km (D) 1000 km
▶ See Answer
(B) Infinity
10. The velocity required to keep a satellite in orbit is called:
(A) Escape velocity (B) Terminal velocity (C) Orbital velocity (D) Speed of light
▶ See Answer
(C) Orbital velocity
Conclusion
In this unit, we learned that gravitation is the silent force that manages the motion of everything in the universe. From Newton’s simple observation of a falling apple to the complex orbits of GPS satellites, gravity is the key. Understanding these concepts helps us explore space and improve our technology on Earth. I hope you found this detailed guide helpful. Keep looking at the stars!
UNIT 5 COMPLETE ✅
Fantastic! You have mastered the details of Gravitation.