## NCERT Solutions for Class 11 Physics Chapter 8 - Gravitation

Gravitation Class 11 is an essential chapter for scoring the best grades in your examination. All topics preceding Ch 8 physics class 11 are based on the concepts you’re going to learn from this chapter.

Before starting with Gravitation Class 11, it is crucial to understand that gravity and gravitation are not similar. Gravity is a force that pulls a body to the earth’s centre whereas; gravitation is the force acting between two bodies.

This chapter discusses several important questions, for example, the strength of the gravitational force existing on the earth and moon, Kepler’s law, escape velocity etc.

NCERT Solutions for Class for Class 11 Physics Chapter 8 gives a detailed explanation of the universal law of gravitation and three vital laws proposed by Kepler. It provides you detailed solutions to questions based on escape velocity, acceleration due to gravity and more.

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## Access NCERT Solutions for Class 11 Physics Chapter 8 – Gravitation

1. Answer the following:

A. You Can Shield a Charge from Electrical Forces by Putting it Inside a Hollow Conductor. Can You Shield a Body from the Gravitational Influence of Nearby Matter by Putting it Inside a Hollow Sphere or by Some Other Means?

Ans: No.

The gravitational force of matter on nearby objects cannot be eliminated by any means. This is because gravitational force is independent of the nature of the medium's material.

B. An Astronaut Inside a Small Spaceship Orbiting Around the Earth Cannot Detect Gravity. If the Space Station Orbiting Around the Earth Has a Large Size, Can He Hope to Detect Gravity?

Ans: Yes.

If the space station's size is large enough, then the astronaut will identify the change in Earth's gravity.

C. If You Compare the Gravitational Force on the Earth Due to the Sun to That Due to the Moon, You Would Find That the Sun's Pull Is Greater Than the Moon's Pull. (You Can Check This Yourself Using the Data Available in the Succeeding Exercises). However, the Tidal Effect of the Moon's Pull Is Greater Than the Tidal Effect of the Sun. Why?

Ans: The tidal effect depends on the inverse of distance's cube while gravitational force depends on the inverse of distance's square.

Since the distance between the Moon and the Earth is shorter than the distance between the Sun and the Earth, the tidal influence of the Moon's pull is higher than the tidal effect of the Sun's influence.

2. Choose the Correct Alternative:

A. Acceleration Due to Gravity Increases/decreases With Increasing Altitude.

Ans: Acceleration due to gravity decreases with increasing altitude.

Acceleration due to gravity at height \[h\] is given by:

\[{{g}_{h}}=\left( 1-\frac{2h}{{{R}_{e}}} \right)g\]

Where, \[{{R}_{e}}=\]Radius of the Earth

\[g=\]Acceleration due to gravity on the surface of the Earth

It is clear from the relation that acceleration due to gravity lowers with a height increment.

B. Acceleration Due to Gravity Increases/decreases With Increasing Depth. (assume the Earth to Be a Sphere of Uniform Density).

Ans: Acceleration due to gravity decreases with increasing depth.

Acceleration due to gravity at depth \[d\] is given by:

\[{{g}_{d}}=\left( 1-\frac{d}{{{R}_{e}}} \right)g\]

It is clear from the relation that acceleration due to gravity lowers with a depth increment.

C. Acceleration due to gravity is independent of mass of the earth/mass of the body.

Ans: Acceleration due to gravity is independent of mass of the body.

Acceleration due to gravity of body having mass \[m\] is given by: \[g=\frac{GM}{{{R}^{2}}}\]

Where, \[G\]= Universal gravitational constant

\[M\]= Mass of the Earth

\[R\]= Radius of the Earth

Hence, it is clear that acceleration due to gravity is not dependent on the body's mass.

D. The Formula \[-GMm\left( \frac{1}{{{r}_{2}}}-\frac{1}{{{r}_{1}}} \right)\] is more/less accurate than the formula \[mg({{r}_{2}}-{{r}_{1}})\] for the difference of Potential Energy Between Two Points and Distance Away from the Centre of the Earth.

Ans: Gravitational potential energy of two points at \[{{r}_{2}}\] and \[{{r}_{1}}\] distance away from the Earth centre is given by:

\[V({{r}_{1}})=-\frac{GmM}{{{r}_{1}}}\] and

\[V({{r}_{2}})=-\frac{GmM}{{{r}_{2}}}\]

\[\therefore \] Difference in potential energy,

\[V=V({{r}_{2}})-V({{r}_{1}})\]

\[\Rightarrow \text{V}=-GMm\left( \frac{1}{{{r}_{2}}}-\frac{1}{{{r}_{1}}} \right)\]

Hence, this formula is more reliable than the formula \[mg({{r}_{2}}-{{r}_{1}})\].

3. Suppose there Existed a Planet That Went Around the Sun Twice as Fast as the Earth. What would Its Orbital Size be Compared to that of the Earth?

Ans: Time taken by the Earth to complete one revolution, \[{{T}_{e}}=1\text{ year}\].

Orbital radius of the Earth in its orbit, \[{{R}_{e}}=1AU\].

Time taken by the planet to complete one revolution, \[{{T}_{p}}=\frac{1}{2}{{T}_{e}}=\frac{1}{2}year\].

Orbital radius of the planet \[={{R}_{p}}\].

From Kepler's third law of planetary motion, we can write:

\[{{\left(\frac{{{R}_{p}}}{{{R}_{e}}} \right)}^{3}}={{\left( \frac{{{T}_{p}}}{{{T}_{e}}} \right)}^{2}}\]

\[\Rightarrow\frac{{{R}_{p}}}{{{R}_{e}}}={{\left(\frac{{{T}_{p}}}{{{T}_{e}}} \right)}^{\frac{2}{3}}}\]

\[\Rightarrow\frac{{{R}_{p}}}{{{R}_{e}}}={{\left(\frac{\frac{1}{2}}{1} \right)}^{\frac{2}{3}}}\]

\[\Rightarrow \frac{{{R}_{p}}}{{{R}_{e}}}={{(0.5)}^{\frac{2}{3}}}\]

\[\Rightarrow \frac{{{R}_{p}}}{{{R}_{e}}}=0.63\]

Hence, the orbital radius of the planet will be \[0.63\] times smaller than that of the Earth.

4. ${{I}_{O}}$, One of the Satellites of Jupiter, Has an Orbital Period of \[1.769\text{ }days\] and the radius of the orbit is \[4.22\times {{10}^{8}}m\]. Show That the Mass of Jupiter is About One-Thousandth That of the Sun.

Ans: Given that,

Orbital period of \[{{I}_{O}}={{T}_{{{I}_{O}}}}=1.769days=1.769\times 24\times 60\times 60s\]

Orbital radius of \[{{I}_{O}}={{R}_{{{I}_{O}}}}=4.22\times {{10}^{8}}m\]

Satellite \[{{I}_{O}}\] is revolving around Jupiter.

Mass of the Jupiter is given by:

\[{{M}_{J}}=\frac{4{{\pi }^{2}}R_{{{I}_{o}}}^{3}}{GT_{{{I}_{o}}}^{2}}\]......(i)

Where, \[{{M}_{J}}\]= Mass of Jupiter

\[G\]is the Universal gravitational constant

Orbital period of the Earth,

\[{{T}_{e}}=365.25days=365.25\times 25\times 60\times 60s\]

Orbital radius of the Earth,

\[{{R}_{e}}=1AU=1.496\times {{10}^{11}}m\]

Mass of sun is given as:

\[{{M}_{s}}=\frac{4{{\pi }^{2}}R_{e}^{3}}{GT_{e}^{2}}\]......(ii)

\[\Rightarrow\frac{{{M}_{S}}}{{{M}_{J}}}=\frac{4{{\pi}^{2}}R_{e}^{3}}{GT_{e}^{2}}\times\frac{GT_{Io}^{2}}{4{{\pi}^{2}}R_{Io}^{3}}=\frac{R_{e}^{3}}{R_{Io}^{3}}\times \frac{T_{Io}^{2}}{T_{e}^{2}}\]

\[\Rightarrow \frac{{{M}_{S}}}{{{M}_{J}}}={{\left( \frac{1.769\times 24\times 60\times 60}{365.25\times 24\times 60\times 60} \right)}^{2}}\times {{\left( \frac{1.496\times {{10}^{11}}}{4.22\times {{10}^{8}}} \right)}^{3}}\]

\[\Rightarrow \frac{{{M}_{S}}}{{{M}_{J}}}=1045.04\]

\[\therefore \frac{{{M}_{s}}}{{{M}_{J}}}\sim 1000\]

We get,

\[{{M}_{s}}\sim 1000\times {{M}_{J}}\]

Hence, it can be concluded that the mass of Jupiter is about one-thousandth that of the Sun.

5. Let Us Assume That Our Galaxy Consists of \[2.5\times {{10}^{11}}\] stars Each of One Solar Mass. How Long Will a Star be at a Distance of \[50,000ly\] From the Galactic Centre to Complete One Revolution? Take the Diameter of the Milky Way to be \[{{10}^{5}}ly\].

Ans: Mass of galaxy Milky way, \[M=2.5\times {{10}^{11}}\] solar mass

Solar mass = Mass of sun\[=2.0\times {{10}^{36}}kg\]

Mass of our galaxy, \[M=2.5\times {{10}^{11}}\times 2\times {{10}^{35}}\]

\[M=5\times {{10}^{41}}kg\]

Diameter of Milky Way, \[d={{10}^{5}}ly\]

Radius of Milky Way, \[r=5\times {{10}^{4}}ly\]

\[1\text{ }ly=9.46\times 1{{0}^{15}}m\]

\[r=5\times {{10}^{4}}\times 9.46\times 1{{0}^{15}}\]

\[\Rightarrow \text{r}=4.73\times {{10}^{20}}m\]

Since a star rotates around the galactic centre of the Milky Way, its time period is given by:

\[T={{\left( \frac{4{{\pi }^{2}}{{r}^{3}}}{GM} \right)}^{\frac{1}{2}}}=\left( \frac{4\times {{\left( 3.14 \right)}^{2}}\times {{\left( 4.73 \right)}^{3}}\times {{10}^{60}}}{6.67\times {{10}^{-11}}\times 5\times {{10}^{41}}} \right)\]

\[\Rightarrow T={{\left( 125.27\times {{10}^{30}} \right)}^{\frac{1}{2}}}\]

\[\Rightarrow \text{T}=1.12\times {{10}^{16}}s\]

As we know,

\[1\text{ }year=365\times 24\times 60\times 60s\]

We get,

\[1s=\frac{1}{365\times 24\times 60\times 60}years\]

\[\Rightarrow 1.12\times {{10}^{16}}s=\frac{1.12\times {{10}^{16}}}{365\times 24\times 60\times 60}\]

\[\therefore 1.12\times {{10}^{16}}s=3.55\times {{10}^{8}}years\]

Star will take \[3.55\times {{10}^{8}}years\] to complete one revolution.

6. Choose the Correct Alternative:

A. If the Zero of Potential Energy is at Infinity, the Total Energy of an Orbiting Satellite Is Negative of Its Kinetic/Potential Energy.

Ans: Kinetic energy.

The sum of kinetic energy and potential energy is the total mechanical energy. The satellite's gravitational potential energy is zero at infinity. As we know, the Earth-satellite system is a bound system; then, the satellite's total energy is negative.

Thus, at infinity, the orbiting satellite's total energy is equal to the negative of its kinetic energy.

B. the Energy Required to Launch an Orbiting Satellite Out of Earth's Gravitational Influence is More/less Than the Energy Required to Project a Stationary Object at the Same Height (as the Satellite) Out of Earth's Influence.

Ans: Less.

An orbiting satellite gets a certain amount of energy that permits it to rotate around the Earth. Its orbit gives this energy. It requires comparatively lesser energy to go out of the influence of the Earth's gravitational field than a fixed object on the Earth's surface that initially holds no energy.

7. Does the Escape Speed of a Body from the Earth Depend on

A. The Mass of the Body?

Ans: No.

Escape velocity of a body from the Earth is given by the relation:

\[v=\sqrt{2gR}\]

The escape speed of a body does not depend on the mass of the body.

B. The Location from Where it is Projected?

Ans: No.

Escape velocity of a body from the Earth is given by the relation:

\[v=\sqrt{2gR}\]

The escape speed of a body does not depend on the location from where it is projected.

C. The Direction of Projection?

Ans: No.

Escape velocity of a body from the Earth is given by the relation:

\[v=\sqrt{2gR}\]

The escape speed of a body does not depend on the direction of projection.

D. The Height of the Location from Where the Body is Launched?

Ans: Yes.

Escape velocity of a body from the Earth is given by the relation:

\[v=\sqrt{2gR}\]

The escape speed of a body does slightly depend on the height of the location from where the body is launched. This is because escape velocity is slightly dependent on the gravitational potential at a certain height.

8. A Comet Orbits the Sun in a Highly Elliptical Orbit. Does the Comet have a Constant

A. Linear Speed?

Ans: No.

The expression for angular momentum is:

$L=\frac{m{{v}^{2}}}{r}$

As angular momentum is constant, linear speed varies as $r$ varies.

B. Angular Speed?

Ans: No.

The expression for angular momentum is:

$L=mw{}^{2}r$

As angular momentum is constant, angular speed varies as $r$ varies.

C. Angular Momentum?

Ans: Yes.

There is no external torque and thus, the angular momentum is constant.

D. Kinetic Energy?

Ans: No.

The expression for kinetic energy is:

$K=\frac{1}{2}m{{v}^{2}}$

Kinetic energy is not constant because linear speed varies.

E. Potential Energy?

Ans: No.

By law of conservation of energy, total energy is constant and it is the sum of potential energy and kinetic energy.

As kinetic energy is not constant, potential energy is also not constant.

F. Total Energy Throughout Its Orbit? Neglect Any Mass Loss of the Comet When it Comes Very Close to the Sun.

Ans: Yes.

By law of conservation of energy, the total energy is constant.

9. Which of the Following Symptoms is Likely to Afflict an Astronaut in Space

A. Swollen Feet

Ans: Legs hold the whole mass in a standing place due to gravitational force.

In space, an astronaut appears weightless because of the absence of gravity. Therefore, swollen feet of a spaceman do not affect him/her in space.

B. Swollen Face

Ans: A swollen face is usually caused because of seeming weightlessness in space.

Sense organs such as the nose, eyes, ears, and mouth establish a person's face. This symptom can affect a spacewalker in space.

C. Headache

Ans: Headaches are because of mental stress.

It can influence the working of an astronaut in space.

D. Orientational Problem

Ans: Space has diverse orientations.

Therefore, orientational difficulty can affect an astronaut in space.

10. Choose the Correct Answer from Among the Given Ones:

The gravitational intensity at the centre of a hemispherical shell of uniform mass density has the direction indicated by the arrow (see Fig 8.12).

a,

b,

c,

O

Ans: Option (iii) is correct.

Gravitational potential (\[V\]) is fixed at all points in a spherical shell. Hence, the potential gravitational gradient \[\left( \frac{dV}{dr} \right)\] is zero everywhere inside the spherical surface.

The potential gravitational gradient is equivalent to the negative of gravitational intensity. Hence, intensity is also zero at all locations inside the spherical shell. This shows that gravitational forces operating at a point in a spherical shell are symmetric.

If the top half of a spherical shell is cut out, then the net gravitational force working on a particle located at centre O will be downward.

Since gravitational intensity is described as the gravitational force per unit mass at that location, it will also act downward. Thus, the gravitational intensity at the centre of the given hemispherical shell has the direction indicated by the arrow \[c\].

11. Choose the Correct Answer from Among the Given Ones:

For the problem 8.10, the direction of the gravitational intensity at an arbitrary point \[P\] is indicated by the arrow

d,

e,

f,

g.

Ans: Option (ii) is correct.

Gravitational potential (\[V\]) is fixed at all points in a spherical shell. Hence, the potential gravitational gradient \[\left( \frac{dV}{dr} \right)\] is zero everywhere inside the spherical surface.

The potential gravitational gradient is equivalent to the negative of gravitational intensity. Hence, intensity is also zero at all locations inside the spherical shell. This shows that gravitational forces operating at a point in a spherical shell are symmetric.

If the top half of a spherical shell is cut out, then the net gravitational force working on a particle located at centre P will be downward.

Since gravitational intensity is described as the gravitational force per unit mass at that location, it will also act downward.

Thus, the gravitational intensity at the arbitrary point P of the given hemispherical shell has the direction indicated by the arrow \[e\].

12. A Rocket is Fired from the Earth Towards the Sun. At What Distance from the Earth's Centre is the Gravitational Force on the Rocket Zero?

Mass of the sun\[=2\times {{10}^{30}}kg\], mass of the earth\[=6\times {{10}^{24}}kg\]. Neglect the effect of other planets etc. (orbital radius\[=1.5\times {{10}^{11}}m\]).

Ans: Mass of the Sun, \[{{M}_{s}}=2\times {{10}^{30}}kg\]

Mass of the Earth, \[{{M}_{e}}=6\times {{10}^{24}}kg\]

Orbital radius, \[r=1.5\times {{10}^{11}}m\]

Mass of the rocket=\[m\]

Let \[x\] be the distance from the Earth centre where the gravitational force working on the satellite \[P\] becomes zero.

From Newton's law of gravitation, we can equalize gravitational forces acting on satellite \[P\] under the effect of the Sun and the Earth as:

\[\frac{Gm{{M}_{s}}}{{{\left( r-x \right)}^{2}}}=Gm\frac{{{M}_{e}}}{{{x}^{2}}}\]

We get,

\[{{\left( \frac{r-x}{x} \right)}^{2}}=\frac{{{M}_{s}}}{{{M}_{e}}}\]

\[\Rightarrow \frac{r-x}{x}={{\left( \frac{2\times 1{{0}^{30}}}{60\times {{10}^{24}}} \right)}^{\frac{1}{2}}}=577.35\]

\[\Rightarrow 1.5\times {{10}^{11}}-x=577.35x\]

\[\Rightarrow 578.35x=1.5\times {{10}^{11}}\]

\[\Rightarrow x=\frac{1.5\times {{10}^{11}}}{578.35}=2.59\times {{10}^{8}}m\]

Therefore, \[2.59\times {{10}^{8}}m\] is the distance from the earth's centre at which the gravitational force on the rocket is zero.

13. How will you 'weigh the sun', that is, estimate its mass? The mean orbital radius of the earth around the sun is \[1.5\times {{10}^{8}}km\].

Ans: Orbital radius of the Earth around the Sun, \[r=1.5\times {{10}^{11}}m\].

Time taken by the Earth to cover one revolution around the Sun,

\[T=1\text{ year}\]

\[\Rightarrow \text{T=}365.25\text{ days}\]

We get,

\[\text{T=365}\text{.25}\times \text{24}\times \text{60}\times \text{60}s\]

Universal gravitational constant,

\[G=6.67\times {{10}^{-11}}N{{m}^{2}}k{{g}^{-2}}\].

Thus, we can use the relation given below for calculating the mass of the Sun,

\[M=\frac{4{{\pi }^{2}}{{r}^{3}}}{G{{T}^{2}}}\]

\[\Rightarrow \text{M}=\frac{4\times {{(3.14)}^{2}}\times {{(1.5\times {{10}^{11}})}^{3}}}{6.67\times {{10}^{-11}}\times {{(365.25\times 24\times 60\times 60)}^{2}}}\]

\[\Rightarrow \text{M}=\frac{133.24\times {{10}^{33}}}{6.64\times {{10}^{4}}}\]

\[\Rightarrow \text{M}=2.0\times {{10}^{30}}kg\]

Hence, the mass of the Sun is \[2\times {{10}^{30}}kg\].

14. A Saturn year is \[\mathbf{29}.\mathbf{5}\] times the earth year. How far is Saturn from the sun if the earth is \[1.50\times {{10}^{8}}km\] away from the sun?

Ans: Distance of the Earth from the Sun, \[{{r}_{e}}=1.5\times 1{{0}^{8}}km=1.5\times {{10}^{11}}m\]

Time period of the Earth \[={{T}_{e}}\]

Time period of Saturn, \[{{T}_{s}}=29.5{{T}_{e}}\]

Distance of Saturn from the Sun \[={{r}_{s}}\]

From Kepler's third law of planetary motion, we have,

\[T={{\left( \frac{4{{\pi }^{2}}{{r}^{3}}}{GM} \right)}^{\frac{1}{2}}}\]

For Saturn and Sun, we can write,

\[\frac{{{r}_{s}}^{3}}{{{r}_{e}}^{3}}=\frac{{{T}_{s}}^{2}}{{{T}_{e}}^{2}}\]

\[\Rightarrow {{r}_{s}}={{r}_{e}}{{\left( \frac{{{T}_{s}}}{{{T}_{e}}} \right)}^{\frac{2}{3}}}\]

\[\Rightarrow {{r}_{s}}=1.5\times {{10}^{11}}{{\left( \frac{29.5{{T}_{e}}}{{{T}_{e}}} \right)}^{\frac{2}{3}}}\]

\[\Rightarrow {{r}_{s}}=1.5\times {{10}^{11}}{{(29.5)}^{\frac{2}{3}}}\]

\[\Rightarrow {{r}_{s}}=1.5\times {{10}^{11}}\times 9.55\]

\[\Rightarrow {{r}_{s}}=14.32\times {{10}^{11}}m\]

Hence, the distance between the Sun and Saturn is \[1.43\times {{10}^{12}}m\].

15. A Body weighs \[63N\] on the surface of the earth. What is the gravitational force on it due to the earth at a height equal to half the radius of the Earth?

Ans: Weight of the body, \[W=63N\]

Acceleration due to gravity at height \[h\] from the Earth’s surface

\[{{R}_{e}}=\]Radius of the Earth

For \[h=\frac{{{R}_{e}}}{2}\], gravity at $h$ is given by:

\[{{g}_{h}}=\frac{g}{{{\left( 1+\frac{h}{{{R}_{e}}} \right)}^{2}}}\]

\[\Rightarrow {{g}_{h}}=\frac{g}{{{\left( 1+\frac{1}{2} \right)}^{2}}}\]

\[\Rightarrow {{g}_{h}}=\frac{4}{9}g\]

Weight of a body of mass \[m\] at height \[h\] is given as:

\[W'=m{{g}_{h}}\]

\[\Rightarrow W'=m\times \frac{4}{9}g\]

\[\Rightarrow W'=\frac{4}{9}\times mg\]

\[\Rightarrow \text{W }\!\!'\!\!\text{ }=\frac{4}{9}W\]

\[\Rightarrow \text{W }\!\!'\!\!\text{ }=\frac{4}{9}\times 63\]

\[\Rightarrow W'=28N\]

Thus, the gravitational force on the body due to the earth at a height equal to half the radius of the earth is $28N$.

16. Assuming the Earth to Be a Sphere of Uniform Mass Density, How Much Would a Body Weigh Half Way Down to the Centre of the Earth If it Weighed \[250N\] on the Surface?

Ans: Weight of a body of mass \[m\] at the Earth's surface is given by,

\[W=mg=250N\]

Body of mass \[m\] is located at depth, \[d=\frac{1}{2}{{R}_{e}}\]

Where, \[{{R}_{e}}\] is the radius of the Earth.

Acceleration due to gravity at depth \[{{g}_{d}}\] is given by the relation:

\[{{g}_{d}}=\left( 1-\frac{d}{{{R}_{e}}} \right)g\]

\[\Rightarrow \left( 1-\frac{{{R}_{e}}}{2\times {{R}_{e}}} \right)g=\frac{1}{2}g\]

Weight of the body at depth \[d\],

\[W'=mg\]

\[\Rightarrow W'=m\times \frac{1}{2}g=\frac{1}{2}mg=\frac{1}{2}W\]

\[\Rightarrow W'=\frac{1}{2}\times 250=125N\]

The weight of body half way down to the centre of the earth is $125N.$

17. A Rocket is fired vertically with a speed of \[5km{{s}^{-1}}\] from the earth's surface. How far from the earth does the rocket go before returning to the earth? Mass of the earth\[=6.0\times {{10}^{24}}kg\]; mean radius of the earth\[=6.4\times {{10}^{6}}m;G=6.67\times {{10}^{-11}}N{{m}^{2}}k{{g}^{-2}}\].

Ans: Distance from the centre of the Earth \[=8\times {{10}^{6}}m\]

Velocity of the rocket, \[v=5km{{s}^{-1}}=5\times {{10}^{3}}m{{s}^{-1}}\]

Mass of the Earth, \[{{M}_{e}}=6.0\times {{10}^{24}}kg\]

Radius of the Earth, \[{{R}_{e}}=6.4\times {{10}^{6}}m\]

Height reached by rocket mass $m$, is \[h\].

At the surface of the Earth,

Total energy of the rocket = Kinetic energy + Potential energy

\[=\frac{1}{2}m{{v}^{2}}+\left( \frac{-G{{M}_{e}}m}{{{R}_{e}}} \right)\]

At highest point $h,$ \[v=0\]

And, Potential energy \[=-\frac{G{{M}_{e}}m}{{{R}_{e}}+h}\]

Total energy of the rocket\[=0+\left( \frac{G{{M}_{e}}m}{{{R}_{e}}+h} \right)=-\frac{G{{M}_{e}}m}{{{R}_{e}}+h}\]

From the law of energy conservation, we have

Total energy of the rocket at the Earth's surface = Total energy of rocket at height \[h\].

We have,

\[\frac{1}{2}m{{v}^{2}}+\left( -\frac{G{{M}_{e}}m}{{{R}_{e}}} \right)=-\frac{G{{M}_{e}}m}{{{R}_{e}}+h}\]

\[\Rightarrow \frac{1}{2}{{v}^{2}}=G{{M}_{e}}\left( \frac{1}{{{R}_{e}}}-\frac{1}{{{R}_{e}}+h} \right)\]

\[\Rightarrow \frac{1}{2}{{v}^{2}}=G{{M}_{e}}\left( \frac{{{R}_{e}}+h-{{R}_{e}}}{{{R}_{e}}({{R}_{e}}+h)} \right)\]

\[\Rightarrow \frac{1}{2}{{v}^{2}}=\frac{G{{M}_{e}}h}{{{R}_{e}}({{R}_{e}}+h)}\times \frac{{{R}_{e}}}{{{R}_{e}}}\]

\[\Rightarrow \frac{1}{2}{{v}^{2}}=\frac{g{{R}_{e}}h}{{{R}_{e}}+h}\]

Where, \[g=\frac{GM}{R_{e}^{2}}=9.8m{{s}^{-2}}\] , is the acceleration due to gravity on the Earth’s surface.

Clearly,

\[{{v}^{2}}({{R}_{e}}+h)=2g{{R}_{e}}h\]

\[\Rightarrow {{v}^{2}}{{R}_{e}}=h(2g{{R}_{e}}-{{v}^{2}})\]

\[\Rightarrow h=\frac{{{R}_{e}}{{v}^{2}}}{2g{{R}_{e}}-{{v}^{2}}}\]

\[\Rightarrow \text{h}=\frac{6.4\times 25\times {{10}^{12}}}{100.44\times 1{{0}^{6}}}\]

\[\Rightarrow \text{h}=1.6\times {{10}^{6}}m\]

Height achieved by the rocket with respect to the centre of the Earth, $H$ is given by: \[H={{R}_{e}}+h\]

\[\Rightarrow H=6.4\times {{10}^{6}}+1.6\times {{10}^{6}}\]

We get,

\[H=8.0\times {{10}^{6}}m\]

The distance from the earth is \[8.0\times {{10}^{6}}m\] where the rocket goes before returning to the earth.

18. The escape speed of a projectile on the earth's surface is \[11.2km{{s}^{-1}}\]. A body is projected out with thrice this speed. What is the speed of the body far away from the earth? Ignore the presence of the sun and other planets.

Ans: Projectile’s escape velocity from the Earth, \[{{v}_{esc}}=11.2km{{s}^{-1}}\]

Projection velocity of the projectile, \[{{v}_{p}}={{3}_{{{v}_{esc}}}}\]

Mass of the projectile \[=m\]

Projectile’s velocity far away from the Earth \[={{v}_{f}}\]

Projectile’s total energy on the Earth \[=\frac{1}{2}m{{v}_{p}}^{2}-\frac{1}{2}m{{v}_{esc}}^{2}\]

The projectile's gravitational potential energy far away from the Earth is zero.

Total energy of the projectile far away from the Earth \[=\frac{1}{2}m{{v}_{f}}^{2}\]

From the law of energy conservation, we have

\[\frac{1}{2}m{{v}_{p}}^{2}-\frac{1}{2}m{{v}_{esc}}^{2}=\frac{1}{2}m{{v}_{f}}^{2}\]

We get,

\[{{v}_{f}}=\sqrt{{{v}_{p}}^{2}-{{v}_{esc}}^{2}}\]

\[\Rightarrow {{v}_{f}}=\sqrt{{{(3{{v}_{esc}})}^{2}}-{{({{v}_{esc}})}^{2}}}\]

\[\Rightarrow {{v}_{f}}=\sqrt{8}{{v}_{esc}}\]

\[\Rightarrow {{v}_{f}}=\sqrt{8}\times 11.2\]

\[\Rightarrow {{v}_{f}}=31.68km{{s}^{-1}}\]

Thus, the speed of the body far away from the earth is \[31.68km{{s}^{-1}}\].

19. A Satellite Orbits the Earth at a height of \[400km\] above the surface. How much Energy must be expended to Rocket the Satellite out of the earth's gravitational influence?

Mass of the satellite\[=200kg\];

mass of the earth\[=6.0\times {{10}^{24}}kg\];

radius of the earth\[=6.4\times {{10}^{6}}\]; \[G=6.67\times {{10}^{-11}}N{{m}^{2}}k{{g}^{-2}}\].

Ans: Given that,

Mass of the Earth, \[{{M}_{e}}=6.0\times {{10}^{24}}kg\]

Mass of the satellite, \[m=200kg\]

Radius of the Earth, \[{{R}_{e}}=6.4\times {{10}^{6}}m\]

Universal gravitational constant, \[G=6.67\times {{10}^{-11}}N{{m}^{2}}k{{g}^{-2}}\]

Height of the satellite, \[h=400km=4\times {{10}^{5}}m=0.4\times {{10}^{6}}m\]

Total energy of the satellite at height $h$,

\[TE=\frac{1}{2}m{{v}^{2}}+\left( \frac{-G{{M}_{e}}m}{{{R}_{e}}+h} \right)\]

Orbital velocity of the satellite, \[v=\sqrt{\frac{G{{M}_{e}}}{{{R}_{e}}+h}}\]

Total energy at height $h$, \[TE=\frac{1}{2}m\left( \frac{G{{M}_{e}}}{{{R}_{e}}+h} \right)-\frac{G{{M}_{e}}m}{{{R}_{e}}+h}=-\frac{1}{2}\left( \frac{G{{M}_{e}}m}{{{R}_{e}}+h} \right)\]

The negative sign explains that the satellite is attached to the Earth. This is called the bound energy of the satellite.

The energy needed to send the satellite out of its orbit is equal to the negative of Bound energy.

Bound energy is, \[BE=\frac{1}{2}\left( \frac{G{{M}_{e}}m}{{{R}_{e}}+h} \right)\]

Where, ${{M}_{e}}$ is the mass of the Earth.

${{R}_{e}}$ is the radius of the Earth.

h is the height.

m is the mass of the satellite.

\[\Rightarrow BE=\frac{1}{2}\times \frac{6.67\times {{10}^{-11}}\times 6.0\times {{10}^{24}}\times 200}{\left( 6.4\times {{10}^{6}}+0.4\times {{10}^{6}} \right)}\]

\[\Rightarrow BE=\frac{1}{2}\times \frac{6.67\times 6\times 2\times 10}{6.4\times {{10}^{6}}}\]

\[\Rightarrow BE=5.9\times {{10}^{9}}J\]

Clearly, \[5.9\times {{10}^{9}}J\] energy must be expended to rocket the satellite out of the earth's gravitational influence.

20. Two stars each of one solar mass (\[=2\times {{10}^{30}}kg\]) are approaching each other for a head on collision. When they are a distance \[{{10}^{9}}km\], their speeds are negligible. What is the speed with which they collide? The radius of each star is \[{{10}^{4}}km\]. Assume the stars to remain undistorted until they collide. (Use the known value of \[G\]).

Ans: Mass of each star, \[M=2\times {{10}^{30}}kg\]

Radius of each star, \[R={{10}^{4}}km={{10}^{7}}m\]

Distance between the stars, \[r={{10}^{9}}km={{10}^{12}}m\]

For negligible speeds, \[v=0\]

Total energy of two stars separated at distance \[r\] is given by

\[TE=\frac{-GMM}{r}+\frac{1}{2}m{{v}^{2}}\]

\[\Rightarrow TE=\frac{GMM}{r}+0.\]……(i)

Now, consider the case when the stars are about to collide:

Velocity of the stars \[=v\]

Distance between the centres of the stars \[=2R\]

Total kinetic energy of both stars \[=\frac{1}{2}M{{v}^{2}}+\frac{1}{2}M{{v}^{2}}=M{{v}^{2}}\]

Total potential energy of both stars\[=\frac{-GMM}{2R}\]

Total energy of the two stars \[=M{{v}^{2}}-\frac{GMM}{2R}\]........(ii)

Using the law of conservation of energy, we can write:

\[M{{v}^{2}}-\frac{GMM}{2R}=\frac{-GMM}{R}\]

\[\Rightarrow {{v}^{2}}=\frac{-GM}{r}+\frac{GM}{2r}=GM\left( -\frac{1}{r}+\frac{1}{2R} \right)\]

\[\Rightarrow {{v}^{2}}=6.67\times 1{{0}^{-10}}\times 2\times {{10}^{30}}\left[ -\frac{1}{{{10}^{12}}}+\frac{1}{2\times {{10}^{7}}} \right]\]

\[\Rightarrow {{v}^{2}}=13.34\times {{10}^{19}}\left[ -{{10}^{-12}}+5\times {{10}^{-8}} \right]\]

\[\Rightarrow {{v}^{2}}\sim 13.34\times {{10}^{19}}\times 5\times {{10}^{-8}}\]

\[\Rightarrow {{v}^{2}}\sim 6.67\times {{10}^{12}}\]

\[\Rightarrow v=\sqrt{6.67\times {{10}^{12}}}\]

\[\Rightarrow v=2.58\times {{10}^{6}}m/s\]

The speed with which the two stars collide is \[2.58\times {{10}^{6}}m/s\].

21. Two heavy spheres each of mass \[100kg\] and radius \[0.10m\] are placed \[1.0m\] apart on a horizontal table. What is the gravitational force and potential at the midpoint of the line joining the centres of the spheres? Is an object placed at that point in equilibrium? If so, is the equilibrium stable or unstable?

Ans: The situation is represented in the given figure:

Mass of each sphere, \[M=100kg\]

Separation between the spheres, \[r=1m\]

\[X\] is the midpoint between the spheres. Gravitational force at point \[X\] will be zero. This is because the gravitational force applied by each sphere will act in opposite directions.

Gravitational potential at point \[X\]\[=\frac{-GM}{\left( \frac{r}{2} \right)}-\frac{GM}{\left( \frac{r}{2} \right)}\]

\[\Rightarrow PE=-4\frac{GM}{r}\]

\[\Rightarrow PE=\frac{4\times 6.67\times {{10}^{-11}}\times 100}{r}\]

\[\Rightarrow PE=-2.67\times {{10}^{-8}}J/kg\]

Any object placed at point \[X\] will be in equilibrium state, but the equilibrium is unstable. This is because any change in the position of the object will vary the effective force in that direction.

22. As you have learnt in the text, a geostationary satellite orbits the earth at a height of nearly \[36,000km\] from the surface of the earth. What is the potential due to earth's gravity at the site of this satellite? (Take the potential energy at infinity to be zero).

Mass of the Earth = \[6.0\times {{10}^{24}}kg\],

Radius = \[6400km.\]

Ans: Mass of the Earth, \[M=6.0\times {{10}^{24}}kg\]

Radius of the Earth, \[R=6400km=6.4\times {{10}^{6}}m\]

Height of a geostationary satellite from the surface of the Earth,

\[h=36000km=3.6\times {{10}^{7}}m\]

Gravitational potential energy due to Earth’s gravity at height \[h\],

\[PE=\frac{-GM}{(R+h)}\]

\[\Rightarrow PE=-\frac{6.67\times 1{{0}^{-11}}\times 6.0\times {{10}^{24}}}{3.6\times {{10}^{7}}+0.64\times {{10}^{7}}}\]

\[\Rightarrow PE=-\frac{6.67\times 6}{4.24}\times {{10}^{13-7}}\]

\[\Rightarrow PE=-9.4\times {{10}^{6}}Jk{{g}^{-1}}\]

The potential energy due to earth's gravity at the site of this satellite is \[-9.4\times {{10}^{6}}Jk{{g}^{-1}}\].

23. A star 2.5 times the mass of the sun and collapsed to a size of \[12km\] rotates with a speed of \[1.2\] evolutions per second. (Extremely compact stars of this kind are known as neutron stars. Certain stellar objects called pulsars belong to this category). Will an object placed on its equator remain stuck to its surface due to gravity? (Mass of the sun\[=2\times {{10}^{30}}kg\]).

Ans: Yes.

A body gets held to the star's surface if the inward gravitational force is larger than the outward centrifugal force generated by the star's rotation.

Gravitational force,

\[{{f}_{g}}=\frac{GMm}{{{R}^{2}}}\]

Where, M is mass of the star, \[M=2.5\times 2\times {{10}^{30}}=5\times {{10}^{30}}kg\]

m is the mass of the body

R is the radius of star, \[\text{R}=12km=1.2\times {{10}^{4}}m\]

\[\therefore {{f}_{g}}=\frac{6.67\times {{10}^{-11}}\times 5\times {{10}^{30}}\times m}{{{(1.2\times {{10}^{4}})}^{2}}}\]

\[{{f}_{g}}=2.31\times {{10}^{11}}mN\]

Centrifugal force, \[{{f}_{c}}=mr{{\omega }^{2}}\]

\[\omega =\text{Angular speed}=2\pi v\]

\[v=\text{Angular frequency}=1.2rev\text{ }{{s}^{-1}}\]

\[{{f}_{c}}=mR{{(2\pi v)}^{2}}\]

\[\Rightarrow {{f}_{c}}=m\times (1.2\times {{10}^{4}})\times 4\times {{(3.14)}^{2}}\times {{(1.2)}^{2}}\]

\[\Rightarrow {{f}_{c}}=1.7\times {{10}^{5}}mN\]

Since \[{{f}_{g}}>{{f}_{c}}\], the body will remain held to the surface of the star.

24. A Spaceship is Stationed on Mars. How Much Energy Must Be Expended on the Spaceship to Launch it Out of the Solar System?

Mass of the spaceship\[=1000kg\]; mass of the Sun\[=2\times {{10}^{30}}kg\];

mass of mars\[=6.4\times {{10}^{23}}kg\]; radius of mars\[=3395km\];

radius of the orbit of mars\[=2.28\times {{10}^{8}}kg;G=6.67\times {{10}^{-11}}{{m}^{2}}k{{g}^{-2}}\].

Ans: Mass of the spaceship, \[{{m}_{s}}=1000kg\]

Mass of the Sun, \[M=2\times 1{{0}^{30}}kg\]

Mass of Mars, \[{{M}_{m}}=6.4\times {{10}^{23}}kg\]

Orbital radius of Mars, \[R=2.28\times {{10}^{8}}kg=2.28\times {{10}^{11}}m\]

Radius of Mars, \[r=3395km=3.395\times {{10}^{6}}m\]

Universal gravitational constant, \[G=6.67\times {{10}^{-11}}{{m}^{2}}k{{g}^{-2}}\]

Potential energy of the spaceship due to the gravitational attraction of the Sun\[=\frac{-GM{{m}_{s}}}{R}\]

Potential energy of the spaceship due to the gravitational attraction of Mars\[=\frac{-G{{M}_{m}}{{m}_{s}}}{r}\]

Since the spaceship is stationed on Mars, its velocity and hence, its kinetic energy will be zero.

Total energy of the spaceship \[=\frac{-GM{{m}_{s}}}{R}-\frac{G{{M}_{m}}{{m}_{s}}}{r}\]

\[TE=-G{{m}_{s}}\left( \frac{M}{R}+\frac{{{M}_{m}}}{r} \right)\]

The negative sign indicates that the system is in a bound state.

Energy required for launching the spaceship out of the solar system = – (Total energy of the spaceship)

We have,

\[E=G{{m}_{s}}\left( \frac{M}{R}+\frac{{{m}_{m}}}{r} \right)\]

\[\Rightarrow E=6.67\times 1{{0}^{-11}}\times {{10}^{3}}\times \left( \frac{2\times {{10}^{30}}}{2.28\times {{10}^{11}}}+\frac{6.4\times {{10}^{23}}}{3.395\times {{10}^{5}}} \right)\]

\[\Rightarrow E=6.67\times {{10}^{-8}}\left( 87.72\times {{10}^{17}}+1.88\times {{10}^{17}} \right)\]

\[\Rightarrow E=6.67\times {{10}^{-8}}\times 1.88\times {{10}^{17}}\]

\[\Rightarrow E=596.97\times {{10}^{9}}\]

\[\Rightarrow E=6\times {{10}^{11}}J\]

The required energy for the spaceship to launch it out of the solar system is \[6\times {{10}^{11}}J\].

25. A rocket is fired 'vertically' from the surface of mars with a speed of \[2km/s\]. If \[20%\] of its initial energy is lost due to Martian atmospheric resistance, how far will the rocket go from the surface of mars before returning to it? Mass of mars\[=6.4\times {{10}^{23}}kg\]; radius of mars\[=3395km\];\[G=6.67\times {{10}^{-11}}N{{m}^{2}}k{{g}^{-2}}\].

Ans: Initial velocity of the rocket, \[v=2km/s=2\times {{10}^{3}}m/s\]

Mass of Mars, \[M=6.4\times {{10}^{23}}kg\]

Radius of Mars, \[R=3395km=3.395\times {{10}^{6}}m\]

Universal gravitational constant, \[G=6.67\times {{10}^{-11}}N{{m}^{2}}k{{g}^{-2}}\]

Mass of the rocket\[=m\]

Initial kinetic energy of the rocket \[=\frac{1}{2}m{{v}^{2}}\]

Initial potential energy of the rocket \[=\frac{-GMm}{R}\]

Total initial energy \[=\frac{1}{2}m{{v}^{2}}-\frac{GMm}{R}\]

If \[20%\] of initial kinetic energy is lost due to Martian atmospheric resistance, then it shows only \[80%\] of its kinetic energy helps in reaching a height.

Total initial energy available, \[TE=\frac{80}{100}\times \frac{1}{2}m{{v}^{2}}-\frac{GMm}{R}\]

\[TE=0.4m{{v}^{2}}-\frac{GMm}{R}\]

Maximum height reached by the rocket\[=h\]

At this height, the velocity and hence, the kinetic energy of the rocket will become zero.

Total energy of the rocket at height $h,$ \[=-\frac{GMm}{\left( R+h \right)}\]

Applying the law of energy conservation for the rocket, we can write:

\[0.4m{{v}^{2}}-\frac{GMm}{R}=\frac{-GMm}{\left( R+h \right)}\]

\[\Rightarrow 0.4{{v}^{2}}=\frac{GM}{R}-\frac{GM}{R+h}\]

\[\Rightarrow 0.4{{v}^{2}}=GM\left( \frac{1}{R}-\frac{1}{R+h} \right)\]

\[\Rightarrow 0.4{{v}^{2}}=GM\left( \frac{R+h-R}{R\left( R+h \right)} \right)\]

After solving further, we get

\[0.4{{v}^{2}}=\frac{GMh}{R\left( R+h \right)}\]

\[\Rightarrow \frac{R+h}{h}=\frac{GM}{0.4{{v}^{2}}R}\]

\[\Rightarrow \frac{R}{h}+1=\frac{GM}{0.4{{v}^{2}}R}\]

\[\Rightarrow \frac{R}{h}=\frac{GM}{0.4{{v}^{2}}R}-1\]

\[\Rightarrow h=\frac{R}{\frac{GM}{0.4{{v}^{2}}R}-1}\]

\[\Rightarrow h=\frac{0.4{{R}^{2}}{{v}^{2}}}{GM-0.4{{v}^{2}}R}\]

Putting all the values, we get,

\[h=\frac{0.4\times {{\left( 3.395\times {{10}^{6}} \right)}^{2}}\times {{\left( 2\times {{10}^{3}} \right)}^{2}}}{6.67\times 1{{0}^{-11}}\times 6.4\times {{10}^{23}}-0.4\times {{\left( 2\times {{10}^{3}} \right)}^{2}}\times \left( 3.395\times {{10}^{6}} \right)}\]

\[\Rightarrow h=\frac{18.442\times {{10}^{18}}}{42.688\times {{10}^{12}}-53432\times {{10}^{12}}}\]

\[\Rightarrow h=\frac{18.442}{37.256}\times {{10}^{6}}\]

\[\Rightarrow h=495\times {{10}^{3}}m\]

\[\Rightarrow h=495km\]

So, the height attained by the rocket from the surface will be $495km$.

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## FAQs on NCERT Solutions for Class 11 Physics Chapter 8

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From NCERT Solutions for Class 11 Physics Chapter 8, you can learn that the phrase “weight of the earth” does not mean anything when it’s used in relation to the gravitational field of the earth. It will only be possible to elucidate the earth’s weight by comparing it to the weight of another body. The gravitational force acting between the earth and the body will be able to determine the earth’s weight.

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**2. What are the Three Laws of Kepler?**

The three laws of Kepler are as follows:

Law of Orbits

Law of Areas

Law of Periods

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**3. What is the Universal Law of Gravitation?**

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**4. What are some of the important points about the gravitational force?**

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It is a central and conservative force.

It is known to be the weakest force in the nature

It is applicable to every form and object irrespective of its size, shape or position

The third law of Newton is applicable to the gravitational force.

It is usually independent of the medium between the particles, unlike the electrostatic force.

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**5. What is a satellite?**

A satellite is a heavenly object that rotates around the planet. There are two types of satellites- natural satellites and artificial satellites.Natural satellites are those that are not man-made and revolve around the earth. Artificial on the other hand are those that are man-made and are launched with some objective and purpose.Some artificial satellites are

Geostationary or Parking satellites

Polar satellites.

**6. What is weightlessness?**

This is the situation when the effective weight of the body comes down to zero.

It can take place while falling under gravity

It can also take place inside the spacecraft or the satellite.

It is also observed at the centre of the earth

It can be experienced when the body is lying in a freely falling lift.

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