NCERT Class 11 Physics Physics Part-I: Chapter 7 — PHYSICS
This chapter introduces Kepler's three laws of planetary motion, which describe how planets orbit the Sun. The first law states that planets move in elliptical orbits with the Sun at one focus, deviating from the earlier Copernican model of circular orbits. The chapter explains how to draw an ellipse and defines its key components like foci and semi-major axis. The second law, the law of areas, states that a line joining a planet to the Sun sweeps equal areas in equal time intervals, implying planets move slower when farther from the Sun. These laws, derived from Tycho Brahe's observations by Johannes Kepler, were crucial for Newton's development of the universal law of gravitation. This chapter provides foundational knowledge for understanding celestial mechanics and gravitational forces within the CBSE Physics curriculum.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 11 |
| Subject | Physics |
| Book | Physics Part-I |
| Chapter | Chapter 7 — PHYSICS |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 3 minutes |
| Word count | 505 |
Learning outcomes
- Understand and state Kepler's three laws of planetary motion.
- Define and identify the components of an elliptical orbit (foci, semi-major axis).
- Explain the concept of the law of areas and its implications for planetary speed.
- Recognize the historical context and significance of Kepler's laws for Newton's theory of gravitation.
Vocabulary
| Word | Meaning |
|---|---|
| Ellipse | A closed curve where the sum of the distances from any point on the curve to two fixed points (foci) is constant. |
| Foci | The two fixed points (F1 and F2) used to define an ellipse. |
| Semi-major axis | Half the length of the longest diameter of an ellipse, connecting the center to the ellipse along the major axis. |
| Perihelion | The point in a planet's orbit where it is closest to the Sun. |
| Aphelion | The point in a planet's orbit where it is farthest from the Sun. |
| Law of areas | Kepler's second law, stating that a line joining a planet to the Sun sweeps out equal areas in equal intervals of time. |
The complete chapter text is read in the official NCERT PDF viewer below (streamed from ncert.nic.in). This page provides NCERT Help study material — summary, vocabulary, practice questions, and FAQs — not a full reproduction of the textbook.
Read chapter online
This PDF is loaded from the official NCERT website (ncert.nic.in). Use the page buttons below to read — download is disabled on NCERT Help.
Read page by page below. PDF is streamed from the official NCERT website — no download button on this page.
Practice questions
- State Kepler's first law of planetary motion. Answer: All planets move in elliptical orbits with the Sun situated at one of the foci.
- What is the significance of the semi-major axis in an elliptical orbit? Answer: The semi-major axis is half the distance between the farthest and closest points of the elliptical orbit from the Sun, defining the size of the orbit.
- According to Kepler's second law, when does a planet move faster? Answer: A planet moves faster when it is nearer to the Sun.
- Who analyzed Tycho Brahe's data to formulate Kepler's laws? Answer: Johannes Kepler analyzed Tycho Brahe's data.
Practice MCQs
Q1. Kepler's first law states that planets move in orbits that are:
Explanation: Kepler's first law, the law of orbits, specifies that planetary orbits are elliptical with the Sun located at one of the two foci.
Q2. The point in an elliptical orbit where a planet is farthest from the Sun is called:
Explanation: Aphelion is the term used for the point in an orbit where a celestial body is farthest from the Sun.
Q3. Kepler's second law deals with the concept of:
Explanation: The law of areas states that the line joining a planet to the Sun sweeps out equal areas in equal intervals of time.
Q4. Who provided the observational data that Johannes Kepler used?
Explanation: Tycho Brahe, a Danish nobleman, spent his life recording astronomical observations, which were later analyzed by his assistant Johannes Kepler.
Q5. For a circle, which of the following is true regarding Kepler's first law?
Explanation: A circle is a special case of an ellipse where the two foci coincide at the center, and the semi-major axis is equivalent to the radius.
Frequently asked questions
What are Kepler's three laws of planetary motion?
Kepler's three laws are: 1. Law of Orbits (planets move in elliptical orbits with the Sun at one focus), 2. Law of Areas (a line joining a planet to the Sun sweeps equal areas in equal times), and 3. Law of Periods (the square of the orbital period is proportional to the cube of the semi-major axis - though not explicitly detailed in this text snippet).
How is an ellipse drawn according to the text?
An ellipse is drawn by fixing the ends of a string at two points (foci) and moving a pencil tip while keeping the string taut.
What does Kepler's law of areas imply about a planet's speed?
It implies that a planet moves faster when it is closer to the Sun (perihelion) and slower when it is farther away (aphelion).
What is the relationship between a circle and an ellipse in the context of Kepler's first law?
A circle is a special case of an ellipse where the two foci merge into one point (the center), and the semi-major axis becomes the radius.
Who provided the observational data for Kepler's laws?
Tycho Brahe, a Danish astronomer, provided the extensive observational data that Johannes Kepler analyzed.
Why are Kepler's laws important in physics?
Kepler's laws accurately described planetary motion and provided the foundation for Isaac Newton's universal law of gravitation.
Related resources
Important topics
Topics covered
NCERT Class 11 Physics — Physics Part-I — Chapter 7 — PHYSICS. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.