# CBSE Class 11 Physics Chapter 8 Revision Notes

Chapter 8: Gravitation Revision Notes

## Kepler’s laws of Planetary Motion

**Three laws of planetary motion** were proposed by Johannes Kepler. The following are the details:

(i) **Law of Orbits**: Each planet travels in an elliptical orbit around the sun, with the sun at one of the elliptical foci.

(ii) **Law of Areas**: The planet’s speed varies to the point where the radius, or vector, drawn from the sun to the planet sweeps out equal areas in equal time.

(iii) **Law of Periods**

## Newton’s Law of Gravitation

- According to
**Newton’s law of gravitation**, every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of their separation. The force is d__irected along the line__ that connects the particles.

- The force of attraction between two particles of equal mass separated by a unit distance is numerically equal to the
**universal constant of gravitation G**.

## Important Gravitational Force Characteristics

(i) The gravitational force between two bodies is **a central force**, acting along the line connecting the two interacting bodies’ centers.

(ii) The **nature of the intervening medium** has no bearing on the gravitational force between two bodies.

(iii) The **presence of other bodies** has no bearing on the gravitational force between two bodies.

(iv) It is **valid** for spherically symmetrical and point objects.

(v) The magnitude of force is **negligible**.

### Acceleration Due to Gravity

- Acceleration due to gravity is the acceleration produced in a body as a result of the
**force of gravity**. - The letter ‘g’ is usually used to represent it. It always points to the
**center of the Earth.** - The
**gravitational force**acting on a body of mass ‘m’ lying on the earth’s surface is given by

### Gravitational Acceleration Variation

- The value of
**gravity’s acceleration**varies with height (i.e., altitude), depth, earth’s shape, and rotation around its own axis.

### Gravitational Field

- The
**gravitational field**is the space around a body in which its gravitational force of attraction is felt by other bodies.

### Gravitational Field Intensity

- The force experienced by a body of unit mass placed at a point in the gravitational field is defined as the
**intensity of the gravitational field of that body**, assuming that the presence of unit mass does not disturb the original gravitational field.

## Gravitational Potential

- The amount of work required to bring a body of unit mass from infinity to a point in the gravitational field of a body is defined as the
**gravitational potential**at that point. **Gravitational Potential Energy**is the gravitational potential at a point r from a body or particle of mass M.- Gravitational potential energy is the
**work done**in transporting a mass ‘m’ from infinity to a point at a distance r.