CBSE Class 11 Physics Exemplar Chapter 7 Gravitation NCERT Solutions

Reviewed by NCERT Help Editorial Team · Updated on 29 Jul 2026

NCERT Solutions PDF Class 11 PDF

This chapter provides NCERT Solutions for Class 11 Physics Exemplar, focusing on Gravitation. It covers multiple-choice questions that delve into the nuances of gravitational force, acceleration due to gravity in non-uniform density spheres, the apparent motion of celestial bodies like the Sun and Mercury as observed from Earth, and the factors affecting satellite orbits. The solutions explain why gravity might not be uniform across Earth's surface, the reasons behind Mercury's observed orbit, the concept of torque in the Earth-Sun system, and the eventual decay of satellite orbits due to viscous forces. It also touches upon the complexity of the Moon's orbit as seen from the Sun, considering the combined gravitational influences of the Sun and Earth. These solutions are designed to help students grasp the fundamental principles of gravitation and prepare effectively for their examinations.

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Quick info

Board	CBSE
Class	Class 11
Subject	Physics Exemplar
Session	2026
Language	English
Type	NCERT Solutions
Chapter	Chapter 7

Chapter summary

Chapter 7 of the NCERT Exemplar for Class 11 Physics deals with Gravitation. The provided solutions focus on multiple-choice questions, clarifying concepts related to the acceleration due to gravity in non-uniform bodies, orbital mechanics of planets and satellites, and the nature of gravitational forces. Students will find detailed explanations for why gravity varies, how celestial bodies move relative to each other, and the reasons for satellite decay, aiding in a thorough understanding of gravitational principles.

Learning outcomes

Understand the variation of acceleration due to gravity in non-uniform density spheres.
Analyze the apparent motion of celestial bodies from different reference frames.
Explain the factors influencing the orbits of planets and satellites.
Apply the concept of gravitational force to understand torque in celestial systems.
Identify the reasons for the finite lifespan of orbiting satellites.

Topics covered

Paper topics

Gravitation
Acceleration due to Gravity
Non-uniform Density
Celestial Motion
Orbital Mechanics
Apparent Orbits
Torque in Gravitational Systems
Satellite Lifespan
Viscous Forces on Satellites
Central Force

Important topics

Variation of 'g' in non-uniform spheres
Apparent motion of celestial bodies
Factors affecting satellite orbits
Torque and its absence in Earth-Sun system
Reasons for satellite decay

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Questions and Solutions

Question 1

The earth is an approximate sphere. If the interior contained matter which is not of the same density everywhere, then on the surface of the earth, the acceleration due to gravity:

(a) will be directed towards the centre but not the same everywhere

(b) will have the same value everywhere but not directed towards the centre

(d) cannot be zero at any point

Solution: If we consider the Earth as a sphere of uniform density, it can be treated as a point mass at its center. In such an idealized case, the acceleration due to gravity (

g

) would be zero at the very center. However, if the Earth's interior has non-uniform density, the gravitational force at different points on the surface will vary. Consequently, the acceleration due to gravity (

g

) will also vary in magnitude and direction across the surface. It is important to note that for a non-uniform sphere, the acceleration due to gravity cannot be zero at any point on or within the sphere, as there will always be a net gravitational pull towards the center of mass, which is not necessarily at the geometric center.

Answer: (d) cannot be zero at any point

Question 2

As observed from the earth, the sun appears to move in an approximate circular orbit. For the motion of another planet like mercury as observed from the earth, this would:

(a) be similarly true

(b) not be true because the force between the earth and mercury is not inverse square law

(d) not be true because mercury is influenced by forces other than gravitational forces

Solution: From the Earth's perspective, the Sun appears to follow an approximate circular path. This apparent motion is due to the Earth's revolution around the Sun. The gravitational force between any two celestial bodies, like the Earth and the Sun, or the Earth and Mercury, strictly follows the inverse square law. However, when observing Mercury from Earth, we must consider the dominant gravitational influence. Mercury is much more strongly attracted by the Sun than by the Earth. Therefore, the orbit of Mercury, as seen from Earth, will not be a simple circular path similar to the Sun's apparent motion. Instead, it will be a more complex trajectory influenced primarily by the Sun's gravity.

Answer: (c) not be true because the major gravitational force on mercury is due to the sun

Question 3

Different points in the earth are at slightly different distances from the sun and hence experience different forces due to gravitation. For a rigid body, we know that if various forces act at various points in it, the resultant motion is as if a net force acts on the CM (centre of mass) causing translation and a net torque at the CM causing rotation around an axis through the CM. For the earth-sun system (approximating the earth as a uniform density sphere):

(a) the torque is zero

(b) the torque causes the earth to spin

(d) the torque causes the earth to move around the sun

Solution: The Earth orbits the Sun due to the gravitational attraction between them. This gravitational force acts radially, meaning it is directed along the line connecting the centers of the Earth and the Sun. Let

\mathbf{r}

be the position vector from the Sun to the Earth, and

\mathbf{F}

be the gravitational force exerted by the Sun on the Earth. The torque (

\tau

) is given by the cross product

\tau = \mathbf{r} \times \mathbf{F}

. Since the force

\mathbf{F}

is radial, it is parallel to the position vector

\mathbf{r}

. The angle (

\theta

) between

\mathbf{r}

and

\mathbf{F}

is therefore 0 degrees. The magnitude of the torque is

|\tau| = |r| |F| \sin \theta

. Substituting

\theta = 0^{\circ}

, we get

|\tau| = rF \sin 0^{\circ} = rF(0) = 0

. Thus, the net torque on the Earth due to the Sun's gravity is zero. This zero net torque is consistent with the conservation of angular momentum, which explains why the Earth maintains its orbital motion without changing its angular velocity significantly (in an idealized circular orbit scenario).

Answer: (a) the torque is zero

Question 4

Satellites orbiting the earth have finite life and sometimes debris of satellites fall to the earth. This is because:

(a) the solar cells and batteries in satellites run out

(b) the laws of gravitation predict a trajectory spiralling inwards

(d) of collisions with other satellites

Solution: Satellites orbiting the Earth are not in a perfect vacuum. They encounter the extremely thin, tenuous upper layers of the Earth's atmosphere. This results in a small but continuous drag force, often referred to as viscous force or atmospheric drag, acting on the satellite. This drag force opposes the satellite's motion, causing it to lose kinetic energy and, consequently, its orbital speed decreases. As the speed decreases, the satellite's orbit decays, meaning its altitude above the Earth gradually lowers. If this process continues unabated, the satellite will eventually re-enter the denser parts of the atmosphere and burn up or fall to the Earth's surface. While solar cells and batteries are essential for a satellite's operation, their depletion doesn't directly cause orbital decay. Gravitational laws themselves don't predict spiraling inwards without a dissipative force, and while collisions are possible, they are not the primary reason for the finite life of most satellites.

Answer: (c) of viscous forces causing the speed of satellite and hence height to gradually decrease

Question 5

Both the earth and the moon are subject to the gravitational force of the sun. As observed from the sun, the orbit of the moon:

(a) will be elliptical

(b) will not be strictly elliptical because the total gravitational force on it is not central

(d) deviates considerably from being elliptical due to influence of planets other than the earth

Solution: When observing the Moon from the Sun, we must consider all significant gravitational forces acting on it. The Moon is primarily influenced by the Sun's gravity, which tends to keep it in an orbit around the Sun. However, the Earth also exerts a significant gravitational force on the Moon, pulling it towards the Earth. Since the Moon is simultaneously acted upon by the gravitational forces from both the Sun and the Earth, the resultant force acting on the Moon is not directed solely towards the Sun (the center of the Sun's gravitational field). A central force is one that is always directed towards a fixed point. Because the Moon experiences forces from two different bodies (Sun and Earth), the total force is not central. Consequently, the Moon's orbit as observed from the Sun is not a perfect ellipse, although it is a closed curve and generally resembles an ellipse due to the Sun's dominant influence.

Answer: (b) will not be strictly elliptical because the total gravitational force on it is not central

Common mistakes

Assuming acceleration due to gravity is uniform everywhere on Earth, even with non-uniform density.
Overlooking the dominant gravitational influence of the Sun on planets like Mercury.
Misunderstanding the conditions under which torque is zero in a gravitational system.
Not accounting for dissipative forces like viscosity in satellite orbital decay.

Revision tips

Focus on understanding the conditions that lead to variations in 'g'.
Visualize the motion of celestial bodies from different perspectives (e.g., Earth vs. Sun).
Pay close attention to the explanations for why orbits are or are not perfectly circular/elliptical.
Review the factors that cause satellites to lose energy and eventually fall back to Earth.

Practice MCQs

Q1. If the Earth's interior has non-uniform density, what can be said about the acceleration due to gravity on its surface?

Q2. When observing Mercury from Earth, why is its orbit not approximately circular like the Sun's apparent orbit?

Q3. In the Earth-Sun system, approximating Earth as a uniform sphere, why is the torque zero?

Q4. What is the primary reason for satellites having a finite life and eventually falling to Earth?

Q5. As observed from the Sun, why is the Moon's orbit not strictly elliptical?

Frequently asked questions

What is the main topic covered in these NCERT Solutions for Class 11 Physics Exemplar?

These solutions cover Chapter 7: Gravitation, focusing on multiple-choice questions related to the principles of gravitational force, acceleration due to gravity, and celestial mechanics.

How do these solutions help in understanding acceleration due to gravity?

The solutions explain how acceleration due to gravity can vary across the Earth's surface if its internal density is not uniform, and clarify its direction.

What is explained about the motion of planets like Mercury?

The solutions clarify that while the Sun appears to orbit the Earth, Mercury's observed orbit from Earth is complex due to the Sun's dominant gravitational pull on Mercury.

Why do satellites eventually fall back to Earth according to these solutions?

The solutions state that viscous forces from the Earth's atmosphere gradually reduce a satellite's speed and orbital height, leading to its eventual descent.

Are the solutions useful for exam revision?

Yes, the detailed explanations for each MCQ help reinforce key concepts and common misconceptions in gravitation, making them valuable for exam revision.

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