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Edited By Safeer PP | Updated on Aug 08, 2022 12:03 PM IST

The chapter consists of Newton’s most astonishing discovery about the apple falling on his head. It was very profoundly named ‘Gravitation.’ NCERT Exemplar Class 11 Physics solutions chapter 8, deals with the chapter where the law of attraction comes into the picture. A group of professionals that the students would enjoy their learning has prepared these solutions. NCERT Exemplar Class 11 Physics solutions chapter 8 has got all the topics covered, where the students will not feel burdened about the high amount of learning. It will easily help students grasp. The chapter not only covers gravitation but other topics like Kepler’s law. Here in the chapter, the effect on the earth’s centre regarding the attraction flow. NCERT Exemplar Class 11 Physics solutions chapter 8, has experts developed it, and thus the students will enjoy the learning.

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This Story also Contains

- NCERT Exemplar Class 11 Physics Solutions Chapter 8: MCQI
- NCERT Exemplar Class 11 Physics Solutions Chapter 8: MCQII
- NCERT Exemplar Class 11 Physics Solutions Chapter 8: Very Short Answer
- NCERT Exemplar Class 11 Physics Solutions Chapter 8: Short Answer
- NCERT Exemplar Class 11 Physics Solutions Chapter 8: Long Answer
- Main Subtopics of NCERT Exemplar Class 11 Physics Solutions Chapter 8
- NCERT Exemplar Class 11 Physics Solutions Chapter-Wise
- NCERT Exemplar Class 11 Solutions
- Important Topics To Cover For Exams From NCERT Exemplar Class 11 Physics Solutions Chapter 8

Also, check NCERT Solutions for Class 11 other subjects

Question:8.1

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

c) will be same everywhere in magnitude directed towards the centre

d) cannot be zero at any point

Answer:

The answer is the option (d) According to the formula of gravityG can never be equal to 0 because G is constant and R and can never be 0.

Question:8.2

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

a) be similarly true

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

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

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

Answer:

The answer is the option (c) The universal law of force of attraction is inversely dependent on the distance between them. For mercury, this force due to the sun is exceptionally large than that due to earth. Due to relative motion of sun and earth with mercury, this would not be true.Question:8.3

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 causing translation and a net torque at the cm causing rotation around an axis through the cm. For the earth-sun system

a) the torque is zero

b) the torque causes the earth to spin

c) the rigid body result is not applicable since the earth is not even approximately a rigid body

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

Answer:

The answer is the option (a)The direction of gravitational force F and the line joining the center of mass of the earth (point of application of force) both lie in the same direction. So, the angle between them is 0.In the formula :

Hence, torque is zero.

Question:8.4

Satellites orbiting the earth have a 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

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

d) of collisions with other satellites

Answer:

The answer is the option (c)P.E is given by

Negative sign shows the force of attraction between satellite and earth.

P.E gets gradually reduced due to atmospheric friction, and the radius/ height of satellite keeps decreasing until it comes back to earth with increasing speed and burns in the atmosphere.

Question:8.5

Both earth and 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

c) is not elliptical but will necessarily be a closed curve

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

Answer:

The answer is the option (b) Moon is not always on the line joining the sun and earth. Therefore, the gravitational force of attraction due to earth and sun have different lines of action or let us say the forces are not central. So, the orbit of the moon will not be strictly elliptical.Question:8.6

In our solar system, the inter-planetary region has chunks of matter called asteroids. They

a) will not move around the sun since they have very small masses compared to the sun

b) will move in an irregular way because of their small masses and will drift away outer space

c) will move around the sun in closed orbits but not obey Kepler’s laws

d) will move in orbits like planets and obey Kepler’s laws

Answer:

The answer is the option (d) Central gravitational forces act upon asteroids. Hence, they will move in orbits and obey Kepler's law.Question:8.7

Choose the wrong option.

a) inertial mass is a measure of the difficulty of accelerating a body by an external force whereas the gravitational mass is relevant in determining the gravitational force on it by an external mass

b) that the gravitational mass and inertial mass are equal is an experimental result

c) that the acceleration due to gravity on earth is the same for all bodies is due to the equality of gravitational mass and inertial mass

d) gravitational mass of a particle-like proton can depend on the presence of neighbouring heavy objects, but the inertial mass cannot

Answer:

The answer is the option (d) The gravitational mass of a proton is equivalent to its inertial mass & is independent of the presence of neighbouring heavy objects.Question:8.8

Particles of masses 2M, m and M are respectively at points A, B, and C with . M is much-much smaller than M and at time , they are all at rest. At subsequent times before any collision takes place.

a) m will remain at rest

b) m will move towards M

c) m will move towards 2M

d) m will have oscillatory motion

Answer:

The answer is the option (c) m will move towards 2M.Let be the force experienced by m at point B due to M at point C and be force experienced by m at point due to 2 M at point A

If then

Thus, m will move towards point A.

Question:8.9

Which of the following options is correct?

a) acceleration due to gravity decreases with increasing altitude

b) acceleration due to gravity increases with increasing depth

c) acceleration due to gravity increases with increasing latitude

d) acceleration due to gravity is independent of the mass of the earth

Answer:

The answer is the option (a) and (c)g at an altitude

Acceleration due to gravity decreases with increasing altitude. Hence a is correct

As cos is decreasing function, the value of gravity increases with increase in latitude, i.e. from the equator to pole. Hence,c is correct

Where M is the mass of earth. Hence, d is incorrect

Question:8.10

If the law of gravitation, instead of being inverse-square law, becomes an inverse-cube-law

a) planets will not have elliptic orbits

b) circular orbits of planets is not possible

c) projectile motion of a stone thrown by hand on the surface of the earth will be approximately parabolic

d) there will be no gravitational force inside a spherical shell of uniform density

Answer:

The correct answer is the option (a), (b), and (c).We know that Force of gravitation = Centrifugal force for a body in a circular motion

If the law of gravitation becomes inverse cube law then

where m = mass of the planet, M = mass of the sun and r = radius of the orbit

Thus, v is inversely proportionate to r

The time period of a planet around the sun

So the path is not elliptical. (By Keplers law )

So, a is correct

When

Hence, b is incorrect

Since, ; the path of the projectile will be parabolic. Thus, (c) is correct

Gravitational force is a universal force as it acts everywhere. Hence, (d) is incorrect

Question:8.11

If the mass of sun were ten times smaller and gravitational constant G were ten times larger in magnitudes

a) walking on ground would become more difficult

b) the acceleration due to gravity on earth will not change

c) raindrops will fall much faster

d) aeroplanes will have to travel much faster

Answer:

The correct answer is the option (a), (c), and (d).If

And

Then

Weight of a person =

Hence a is correct, and b is incorrect

F on man due to sun

The terminal velocity

Hence raindrops fall 10 times faster. Option c is correct

In order to maintain the speed after the increase in gravitational force, the aeroplane will have to travel faster.

Question:8.12

If the sun and the planets carried huge amounts of opposite charges,

a) all three of Kepler’s laws would still be valid

b) only the third law will be valid

c) the second law will not change

d) the first law will still be valid

Answer:

The correct answer is the option (a), (c), and (d). Due to opposite charges, large electrostatic forces of attraction will be produced in addition to the gravitational forces. Both forces, when added, will be radial in nature. Since they are central forces and obey inverse square law, all 3 Kepler's laws will be valid.Question:8.13

There have been suggestions that the value of the gravitational constant G becomes smaller when considered over an exceptionally large time period in the future. If that happens for our earth,

a) nothing will change

b) we will become hotter after billions of years

c) we will be going around but not strictly in closed orbits

d) after a sufficiently long time we will leave the solar system

Answer:

The correct answer is the option (c) and (d)Force of gravitation = Centrifugal force for a body in a circular motion

If G becomes smaller, the centripetal force also decreases

We know that , Thus, the centrifugal force becomes larger than the centripetal force

Distance from sun increases and earth will move in larger orbit in time.

C, D are correct.

Question:8.14

Supposing Newton’s law of gravitation for gravitation forces **F1** and **F2** between two masses m1 and m2 at positions **r1** and **r2** read

where Mo is a constant of the dimension of mass, and n is a number. In such a case,

a) the acceleration due to gravity on earth will be different for different object

b) none of the three laws of Kepler will be valid

c) only the third law will become invalid

d) for n negative, an object lighter than water will sink in water

Answer:

The correct answer is the option (a) and (c)given,

Acceleration due to gravity (g) =

tells the direction of g from 1 to 2

Since g is dependent on position vector

*, mass of body m. So, g on eath will be different for different bodies of different mass and their*

Position. Hence a and c are correct

As the force is central in nature, Kepler's first and second law is valid rejecting option b.

So

The lighter object can sink in water. Hence, d is correct

Question:8.15

Which of the following are true?

a) a polar satellite goes around the earth’s pole in a north-south direction

b) a geostationary satellite goes around the earth in an east-west direction

c) a geostationary satellite goes around the earth in a west-east direction

d) a polar satellite goes around the earth in an east-west direction

Answer:

The correct answer is the option (a) and (c) A geostationary satellite appears stationary relative to the earth and revolves around the earth from west to east with same angular velocity as earth's rotation about its axis from west to east. Polar satellite revolves around the earth from north to south independent of the earth's rotation.Question:8.16

The centre of mass of an extended body on the surface of the earth and its centre of gravity

a) are always at the same point for any size of the body

b) are always at the same point only for spherical bodies

c) can never be at the same point

d) is close to each other for objects, say of sizes less than 100 m

e) both can change if the object is taken deep inside the earth

Answer:

The correct answer is the option (d). For smaller bodies, the COM (Center of mass) and COG (Center of gravity) usually coincide. For extended objects like mountains and lake, they are far away. COG is the point in a body where net torque due to gravity is 0. The COM is where the whole mass of a body is supposed to be concentrated.Question:8.17

Answer:

Air molecules possess thermal energy. The resultant velocity due to all the forces acting on them is not directed downwards, so they do not fall. In case of an apple only gravitational force s acting downward, that is why they fall into the earth.Question:8.18

Give one example each of central force and non-central force.

Answer:

Gravitational force, electrostatic force and spring force are examples of central force Magnetic forces between 2 current carrying loops is an example of non-central forceQuestion:8.19

Draw areal velocity versus time graph for mars.

Answer:

According to Kepler's second law, a planet sweeps an equal area in equal times. So the areal velocity is constant with respect to time.Question:8.20

What is the direction of areal velocity of the earth around the sun?

Answer:

Real velocity of the earth around the sun is given byWhere L is angular momentum and m is mass of earth

Since

Thus, the direction of areal velocity is perpendicular to the plane of and (Maxwell's right-hand rule)

Question:8.22

Is it possible for a body to have inertia but no weight?

Answer:

Everybody has some mass or inertia. Weight of a body is equal to mg. It is possible for a body to have no weight when g=0 and hence mg=0, which is possible in space where there is no gravity.Question:8.27

What is the angle between the equatorial plane and the orbital plane of

a) polar satellite?

b) geostationary satellite?

Answer:

(a) The orbital plane of polar satellite makes with equatorial plane.(b) Geostationary satellite makes with the equatorial plane as it revolves from west to east.

Question:8.26

Out of aphelion and perihelion, where is the speed of the earth more and why?

Answer:

According to Kepler's second lawSince “r” at aphelion is more; the velocity decreases and “r” at perihelion are smaller; the velocity increases.

Question:8.30

Show the nature of the following graph for a satellite orbiting the earth.

a) KE vs orbital radius R

b) PE vs orbital radius R

c) TE vs orbital radius R

Answer:

Mass of Earth =

Radius of orbit of satellite=R

mass of satellite=m

Question:8.32

Answer:

Let be the initial P.E when mass is at the surface of earthLet be the final P.E when mass is lifted to height equal to R

Gain in

Gain in

Question:8.33

Answer:

Let us consider a small element of a ring of mass dM at point A and distance etween dM and m is x.The gravitational force between dM and m,

dF has 2 components: one along PO and another perpendicular to PO

(due to symmetry of ring)

When the mass is displaced to a distance of 2 h

Where h =r

Question:8.34

Answer:

In equilibrium condition, the gravitational pull results in a centripetal force when a body moves around a star. Let us assume that a body of mass m is revolving around a star of mass M in a circle with radius r. Let us assume that a body of mass m is revolving around a star of mass M in a circle with radius r.Orbital velocity

On the increasing radius of the circular path, orbital velocity decreases.

(by Kepler's third law)

On increasing the radius, angular velocity decreases.

On increasing the radius, the kinetic energy decreases.

Gravitation potential energy

On increasing the radius, P.E. increases.

Total Energy

Om increasing radius of the circular path the total energy also increases.

Angular momentum

The increasing radius of the circular orbit increases angular momentum

Question:8.35

Answer:

Force on A due to B along B to A

Force on A due to C along C to A

Force on A due to D along D to A

Force on A due to E along E to A

Question:8.36

Answer:

Orbital Radius = R+h

Orbital velocity is

Let a satellite S is at h m above the earth surface. Let the angle subtended by it at the centre of the earth be 2

(height of geostationary satellite)

Number of satellites required to cover

Question:8.37

Answer:

Perihelion distance for an ellipse of eccentricity e and semi-major axis a,Aphelion distance

(Kepler's law)

where is the meanangular velocity of earth

If is it corresponds to 1 day i.e., avg angular speed per day

and

Since = 24 hrs mean solar day. We get which corresponds to 24 hrs 8.14'

This does not explain the actual variation of length of day during the year.

Class 11 Physics NCERT Exemplar solutions Chapter 8 includes the following topics:

- 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 the earth
- 8.7 Gravitational potential energy
- 8.8 Escape speed
- 8.9 Earth satellites
- 8.10 Energy of an orbiting satellite
- 8.11 Geostationary and polar satellites
- 8.12 Weightlessness

What will students learn from NCERT Exemplar Class 11 Physics solutions chapter 8? Here, the students will learn about the gravitational force and how it proves its existence on the inner and outer surface of the earth. Acceleration is the potential energy difference between the distances of two points on the earth’s surface from the centre. Acceleration increase or decrease is all-dependent upon the increase or decrease in altitude. NCERT Exemplar Class 11 Physics solutions chapter 8 focuses on many practical explanations when it comes to a clear understanding. The effect of gravity in the space limiting to the gravitational force of the earth is also discussed in the chapter. Astronauts, stars and the moon are all affected by the earth’s gravitational force.

Read NCERT Class 11 Physics Solutions

Some of the important points are:

· Constant acceleration, about Galileo Galilei, explaining that the body being irrespective of their masses, is accelerated toward the earth’s surface with a constant acceleration.

**Kepler’s law:**

- All planets move in elliptical orbits with the Sun at one of the focal points
- The radius vector drawn from the sun to a planet sweeps out equal areas in equal time intervals. This follows from the fact that the force of gravitation on the planet is central and hence angular momentum is conserved.
- (c) The square of the orbital period of a planet is proportional to the cube of the semi-major axis of the elliptical orbit of the planet.

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**Check NCERT Notes subject wise -**

- NCERT Notes Class 11 Maths
- NCERT Notes for Class 11 Physics
- NCERT Notes for Class 11 Chemistry
- NCERT Notes for Class 11 Biology

Chapter 1 | Physical world |

Chapter 2 | Units and Measurement |

Chapter 3 | Motion in a straight line |

Chapter 4 | Motion in a Plane |

Chapter 5 | Laws of Motion |

Chapter 6 | Work, Energy and Power |

Chapter 7 | System of Particles and Rotational motion |

Chapter 8 | Gravitation |

Chapter 9 | Mechanical Properties of Solids |

Chapter 10 | Mechanical Properties of Fluids |

Chapter 11 | Thermal Properties of Matter |

Chapter 12 | Thermodynamics |

Chapter 13 | Kinetic Theory |

Chapter 14 | Oscillations |

Chapter 15 | Waves |

1. Why should one prefer using Class 11 Physics NCERT Exemplar solutions Chapter 8 to solve their doubts?

NCERT Exemplar Class 11 Physics Solutions Chapter 8, do the experts solve a set of the solution; thus, being always reliable.

2. How helpful are these solutions?

The solutions are proven to be helpful for students to clear doubts and grasp knowledge easily. Based on the individual syllabus subject and our Subject Matter Expert and provide clear instructions throughout the learning process.

3. What are the topics covered in this chapter?

Here are the topics covered in this chapter

- 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 the earth
- 8.7 Gravitational potential energy
- 8.8 Escape speed
- 8.9 Earth satellites
- 8.10 Energy of an orbiting satellite
- 8.11 Geostationary and polar satellites
- 8.12 Weightlessness

Jul 13, 2024

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