# NCERT Solutions for Class 11 Physics Chapter 8 Gravitation

** NCERT Solutions for Class 11 Physics Chapter 8 Gravitation**: Let's assume that you are standing at the North pole. According to you, a person standing at the south pole is inverted, still, he is not falling back why? We all know the reason that it is due to gravity. The solutions of NCERT class 11 physics chapter 8 gravitation discuss questions based on the mathematical and physical significance of gravitation. CBSE NCERT solutions for class 11 physics chapter 8 gravitation have questions based on Kepler's Law which is important for exams like NEET and JEE Main. Kepler's Law was obtained from the observations of Galileo on planetary motions. Another two important topics are the escape velocity and satellites. Questions based on these concepts are also discussed in the NCERT solutions for class 11 physics chapter 8 gravitation. You will study the unit electrostatics in NCERT class 12 physics, so, while studying Electrostatistics chapter you can compare the formula with the formulas that you will study in chapter Gravitation. NCERT solutions can make your learning easy if you are want to prepare for other classes too. The main NCERT topics of the chapter are listed below

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8.1 Introduction

8.2 Kepler’s Laws

8.3 Universal Law Of Gravitation

8.4 The Gravitational Constant

8.5 Acceleration Due To Gravity Of The Earth

8.6 Acceleration Due To Gravity Below And Above The Surface Of Earth

8.7 Gravitational Potential Energy

8.8 Escape Speed

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8.9 Earth Satellites

8.10 Energy Of An Orbiting Satellite

8.11 Geostationary And Polar Satellites

8.12 Weightlessness

**NCERT solutions for class 11 physics chapter 8 gravitation exercise **

** Q: 8.1 (a) ** Answer the following questions:

** Answer: **

No, because gravitational force doesn't depend upon the material medium. It is independent of the presence of other materials.

** Q: 8.1 (b) ** Answer the following :

** Answer: **

Yes if the size of the space station is large then he can detect gravity.

** Q: 8.1 (c) ** Answer the following:

** Answer: **

Apart from the gravitational pull, the tidal effect also depends upon the cube of the distance between the two. Since the distance between earth and sun is much larger than the distance between the sun and moon so it not also balances but is more than the effect of gravitational force. Thus the tidal effect of the moon’s pull is greater than the tidal effect of the sun.

** Q: 8.2 ** Choose the correct alternative :

(a) Acceleration due to gravity increases/decreases with increasing altitude.

** Answer: **

(a) Acceleration due to gravity ** decreases ** with increasing altitude.

The relation between the two is given by :

(b) Acceleration due to gravity ** decreases ** with increasing depth.

The relation is given below:

(c) Acceleration due to gravity is independent of the ** mass ** of the body.

Here M is the mass of the earth.

(d) The formula is ** more ** accurate than the formula for the difference of potential energy between two points and distance away from the centre of the earth.

** Answer: **

Time taken by planet to complete a revolution around sun =

Using Kepler's law of planetary motion we can write :

or

or

Thus the planet is 0.63 times smaller than earth.

** Q: 8.4 ** Io, one of the satellites of Jupiter, has an orbital period of days and the radius of the orbit is . Show that the mass of Jupiter is about one-thousandth that of the sun.

** Answer: **

The orbital period in days is

Mass is given by :

Thus the ratio of the mass of Jupiter and mass of the sun is :

or

or

Thus the mass of Jupiter is nearly one-thousandth that of the sun.

We know that one light year is .

The time period of rotation is given by :

Putting all the value (in SI units) in the above equation we get :

or

In years :

** Q: 8.6 ** Choose the correct alternative:

** Answer: **

(a) The total energy will be negative of its ** kinetic energy ** . Since at infinity potential energy is zero and total energy is negative.

(b) The energy required will be ** less ** as the stationary object on earth has no energy initially whereas the satellite has gained energy due to rotational motion.

** Answer: **

The escape velocity from the earth is given by :

Since the escape velocity depends upon the reference (potential energy), so it only depends upon the height of location.

(i) No

(ii) No

(iii) No

(iv) Yes

** Answer: **

Among all only angular momentum and total energy of the comet will be constant other all factors given will vary from point to point.

** Answer: **

(a) In space we have state of weightlessness, so the swollen feet does not affect astronaut as there is no gravitational pull (so cannot stand).

(b) The swollen face will be affected as the sense organs such as eyes, ears etc. will be affected.

(c) It will affect the astronaut as it may cause mental strain.

(d) It will affect the astronaut as space has different orientations.

** Answer: **

We know that the gravitational potential (V) in a sphere is constant throughout. Also, the gravitational intensity will act in the downward direction as its upper half is cut (gravitational force will also act in a downward direction). Hence the required direction of gravitational intensity is shown by arrow ** C ** .

** Answer: **

As stated in the previous question, the gravitational potential in a spherical shell is constant throughout. So the gravitational intensity will act in downward direction (as upper half is cut). So the required direction is shown by arrow ** e ** .

** Answer: **

Let the distance where the gravitational force acting on satellite P becomes zero be x from the earth.

Thus we can write :

or

or

Hence :

(Since r = )

** Answer: **

Mass of sun can be calculated by using the following formula:-

Putting the known values in the above formula, we obtain :

or

Thus the mass of the sun is nearly _{} .

** Answer: **

Kepler's third law give us the following relation :

Thus we can write :

or

or

or

Thus the distance between the sun and Saturn is .

** Answer: **

Acceleration due to gravity at height h from the surface of eath is :

For we have :

or

Thus the weight of the body will be :

or

or

** Answer: **

Position of the body is (depth) :

Acceleration due to gravity at depth d is given by :

or

or

Thus the weight of the body is:-

or

or

Thus the weight of the body is 125 N.

** Answer: **

The total energy is given by :

Total energy = Potential energy + Kinetic energy

At the highest point velocity will be zero.

Thus the total energy of the rocket is :

Now we will use the conservation of energy :

Total energy initially (at earth's surface) = Total energy at height h

or

or

or

or

Hence the height achieved by the rocket from earth's centre = R + h

or

** Answer: **

Let us assume the speed of the body far away from the earth is .

Total energy on earth is :

And the total energy when the body is far from the earth is :

(Since the potential energy at far from the earth is zero.)

We will use conservation of energy : -

or

or

or

or

** Answer: **

The total energy of the satellite at height h is given by :

We know that the orbital speed of the satellite is :

Thus the total energy becomes :

or

Thus the required energy is negative of the total energy :

nbsp;

or

or

** Answer: **

The total energy of stars is given by :

or

or

Now when starts are just to collide the distance between them is 2R.

The total kinetic energy of both the stars is :

And the total energy of both the stars is :

Using conservation of energy we get :

or

or

or

Thus the velocity is : _{}

** Answer: **

Gravitational force at the midpoint will be zero. This is because both spheres are identical and their forces will be equal but opposite in direction.

The gravitational potential is given by :

or

At the midpoint, we have equal forces in the opposite direction so it is in equilibrium but if we move the body slightly then the particle will move in one direction (as one force will be greater). So it is an unstable equilibrium.

** NCERT solutions for class 11 physics chapter 8 gravitation additional exercise **

** Answer: **

Height of satellite from earth's surface :

Gravitational potential is given by :

or

or

Thus potential due to earth gravity is _{ . }

** Answer: **

A body will get stuck at the star's surface if the centrifugal force of star is less than the gravitational force.

The gravitational force is given by :

or

The centrifugal force is given by :

or

or

As we can see that the gravitational force is greater than the centrifugal force thus the body will remain at the star.

** Answer: **

Firstly, the potential energy of spaceship due to the sun is given by :

and the potential energy of spaceship due to mars is given by :

It is given that the spaceship is stationary so its kinetic energy is zero.

Thus the total energy of spaceship is :

Thus the energy needed to launch the spaceship is :

or

or

or

** Answer: **

The kinetic energy of the rocket is (initial):-

And the initial potential energy is :

Thus total initial energy is given by :

Further, it is given that 20 per cent of kinetic energy is lost.

So the net initial energy is :

The final energy is given by :

Using the law of energy conservation we get :

Solving the above equation we get :

or

or

or

or

Thus the required distance is 495 Km.

## **NCERT solutions for class 11 physics chapter wise **

## **NCERT solutions for class 11 ** **Subject wise**

## ** Benefits of NCERT solutions for class 11 physics chapter 8 gravitation: **

Numerical problems and derivations of the chapter are important. The solutions of NCERT class 11 physics chapter 8 gravitation will help in the final exam and also in competitive exams. For exams like NEET and JEE Main, one or two questions are expected from the chapter gravitation and the NCERT solutions for class 11 will help in solving the questions asked such exams.

## Frequently Asked Question (FAQs) - NCERT Solutions for Class 11 Physics Chapter 8 Gravitation

**Question: **What is the weightage of the chapter gravitation for NEET

**Answer: **

One question can be expected from the NCERT chapter gravitation for NEET exam

**Question: **What is the weightage of the chapter gravitation for JEE Main

**Answer: **

one question from the chapter gravitation can be expected for JEE Main exam

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