# Class 11 Physics Chapter 8 Important Questions Gravitation

Q.1. What is gravitational force? Ans. The force between two bodies by virtue of their masses is called gravitational force. Q.2. Which is greater,

It is important for the students that all the concepts should be very clear for better marks in future. Here, we are providing important conceptual questions and answers for class 11 physics chapter 8 Gravitation. In this lesson, students will learn about Gravitation. This will not only help the students to know the important questions but will also help them during revision.

Q.1. What is gravitational force?
Ans. The force between two bodies by virtue of their masses is called gravitational force.

Q.2. Which is greater, the attraction of the earth for 1 kg of iron or attraction of 1 kg iron for the earth? Why?
Ans. In accordance with Newton's law of gravitation, the force is equal in the two cases. It is because, when the two bodies interact due to their masses (gravitational interaction) they accept equal forces on each other but an opposite directions.

Q.3. Define universal gravitational constant.
Ans. It is numerically equal to the force of attraction between two unit masses placed at unit distance apart.

Q.4. Why is G called universal constant of gravitation?
Ans. It is because the value of G is independent of the two objects taking part in gravitational interaction.

Q.5. What is the nature of motion of an object falling freely under the action of gravity?
Ans. Uniformly accelerated motion in one dimension.

Q.6. What is the acceleration of a particle projected upward at its highest point of motion?
Ans. 9.8 ms-2 (vertically downwards).

Q.7. Is the value of g same or different for different objects at a given place?
Ans. The value of g is same for all the objects.

Q.8. Explain, why a body weighs more at poles and less at equator?
Ans. As the value of g is more at the poles than at the equator, the body weighs more at the poles.

Q.9. What is the weight of a body at the centre of earth?
Ans. The weight of a body becomes zero at the centre of the earth.

Q.10. Why weight of a body becomes zero at the centre of earth?
Ans. The weight of a body is the force with which the body is attracted towards the centre of earth. Obviously, when the body reaches the centre of the earth, it will no longer be attracted and hance its weight becomes zero at the centre of the earth.

Q.11. Where does the body weigh more - at the surface of the earth or in a mine?
Ans. At the surface of the earth.

Q.12. What is the effect of rotation of earth on the value of g?
Ans. Due to rotation of the earth, the value of the acceleration due to gravity decreases.

Q.13. At what place on Earth, the value of g does not change due to its rotational motion?
Ans. At the poles, the value of g does not change due to the rotational motion on earth.

Q.14. At what place on Earth, the centripetal force is maximum?
Ans. At the equator, the centripetal force is maximum on earth.

Q.15. The moon revolves around the earth and the earth-moon system revolves around the Sun. If the earth could be removed suddenly without disturbing the motion of the moon, what would be the subsequent path of the moon?
Ans. From kepler's third law, we know that for all the heavenly objects moving in a stable orbit around the sun,

\frac{T^2}{r^3}= constant (independent of mass of the object)

Since r and T are same for both the moon and earth (and also for the earth-moon system), the present orbit would be an equilibrium orbit for either the moon or the earth alone as well as the two together. Therefore, if the earth could be removed, the moon will revolve around the sun in the present orbit without any change.

Q.16. According to Newton's universal law of gravitation, everybody in this universe attracts every other body. But we do not see bodies on the surface of the earth moving towards one another one account of this force of attraction. Why?
Ans. Due to very large mass of the earth, all the bodies lying on the surface of earth are attracted towards the centre of the earth. However, the two bodies lying on the surface of the earth also attract each other. But owing to their small masses, they exert a force on each other, which is too small to cause any acceleration (due to the frictional force between the surface of earth and the bodies).

Q.17. Imagine a spacecraft going from the earth to the moon. How the does its weight vary as it goes from the Earth to the moon?
Ans. As the spacecraft moves away from the surface of the earth towards moon, there will be no change in the mass of the spacecraft. However, its weight will keep on changing as described below:

• (1) Its weight will decrease in the beginning.
• (2) It will become zero at some point, where the force of attraction on the spacecraft due to the earth and that due to the moon becomes just equal and opposite.
• (3) It will again start increasing as the spacecraft for the moves towards the moon.

Q.18. Among the known types of forces in nature, the gravitational force is the weakest. Why then does it play a dominant rule for motion of bodies on the terrestrial astronomical and cosmological scale?
Ans. The nuclear forces are short range forces. Such forces are effective only over a small distance of the order of 1015 to 1014 m. On the other hand, electrical forces are long range forces but such forces can be both attractive as well as repulsive. Therefore, nuclear forces and electrical forces do not play any role for the motion of the massive neutral bodies on the terrestrial, astronomical and cosmological scale. It is due to this reason that the gravitational force (which is always attractive though weakest) plays dominant role for the motion of such bodies.

Q.19. Do the forces of friction and other contact forces arise due to gravitational attraction? If not, what is the origin of these forces?
Ans. The forces of friction and other contact forces do not arise due to gravitational attraction. The contact forces (and thus including forces of friction) have electrical origin.

Q.20. A bullet is dropped from the same height and at exactly the same moment, another bullet is fired horizontally discuss the motion of each of the two bullets.
Ans. The vertical motion of the bullet fired horizontally will be exactly identical to that of the bullet dropped vertically. In other words, the position of the two bullets along the vertical will be the same at any instant. The motion of the two bullets will differ only as regards the distance travelled by them along the horizontal. The bullet, which is fired, will traverse a large horizontal distance, while the distance covered by the other bullet will be zero.

Q.21. Explain, why one can jump higher on the surface of moon than that on the earth?
Ans. Acceleration due to gravity on the moon is 1/6th of that on the surface of Earth. Therefore, one can jump higher on the surface of the moon then on the earth.

Q.22. If the earth suddenly stops rotating about its axis, what would be the effect on g? Would this effect be same at all places?
Ans. The effect of rotation of the earth on acceleration due to gravity is to decrease its value. Therefore, if the earth stops rotating the value of g will increase.

The effect will not be same at all places. It will be maximum at the equator.

Q.23. State kepler's law of planetary motion.
Ans.

(1) Law of orbit:- Each planet revolves around the sun in an elliptical orbit, with the sun at one focus of the elliptical path.

(2) Law of area:- The position vector of the planet from the sun i.e. the line joining the planet to the sun sweeps out equal areas in equal times i.e. the areal velocity of a planet around the sun always remains constant.

(3) Law of periods:- The square of the period of any planet about the sun is proportional to the cube of the semi-major axis of the elliptical path.

Q.24. Does acceleration due to gravity depends upon the mass of a planet?
Ans. Yes, the acceleration due to gravity depends upon the mass of the planet. It is because,

g=\frac{GM}{R^2}

i.e. acceleration due to gravity is directly proportional to the mass of the planet.

Q.25. Define gravitational field.
Ans. The space surrounding a material body in which its gravitational force of attraction can be measured is called its gravitational field.

Q.26. What is meant by intensity of the gravitational field?
Ans. The intensity of the gravitational field of a material body at any point in its field is defined as the force experienced by a unit mass (may be called test mass) placed at that point, provided the unit mass (test mass) itself does not produce any change in the field of the body.

Q.27. What is the value of gravitational field on the surface of Earth?
Ans. Gravitational field (strength) on the surface of earth

=\frac{GM}{R^2}=g

Q.28. What is the gravitational potential of two point masses infinite distance away from each other?
Ans. The gravitational potential of two point masses and finite distance away from each other is zero.

Q.29. Where is the value of gravitational potential energy maximum?
Ans. The value of gravitational potential energy is maximum at infinity.

Q.30. Is the potential energy of a galaxy positive or negative. Give reason in support of your answer.
Ans. The force between different galaxies are attractive in nature has the potential energy of a galaxy is negative.

Q.31. Can gravitational potential have positive value?
Ans. No, it is always negative.

Q.32. Does the escape velocity depend upon the mass of the object to be projected?
Ans. No, the escape velocity is independent of the mass of the object to be projected.

Q.33. An elephant and an ant are to be projected out of the gravitational pull of the earth. Do we need different velocity is to achieve so?
Ans. No, the same velocity is needed in the two cases. It is because, the escape velocity does not depend upon the mass of the object to be projected.

Q.34. Why do different planets have different escape velocities?
Ans. The values of mass and radius for different planets are different and hands the value of acceleration due to gravity is different for different planets.

Since v_e = \sqrt{2gR} ,

the different planets have different escape velocities.

Q.35. Name two factors which determine whether a planet has an atmosphere or not.
Ans. (1) Acceleration due to gravity.

(2) The surface temperature of the planet.

Q.36. Four equal masses are placed at the four corners of a square. What is the intensity of gravitational field at the point of intersection of the diagonals of the square?
Ans. The gravitational fields produced by the four masses at the point of intersection of the diagonals of the square just cancel each other. It follows from the symmetry of the arrangement of the four masses. Hence, the intensity of gravitational field at that point will be zero.

Q.37. The earth's gravitational field at a certain point out in space accelerates a 1 kg mass at 5 ms-2. How much will it accelerate a 3 kg mass?
Ans. The gravitational force on a body is proportional to its mass. Hence, if the mass increases by a factor of 3 (from 1 kg to 3 kg), the gravitational force in the second case will also become three times. However, the acceleration due to gravity will remain the same. Thus, at a point in space where the earth's gravitational field accelerates of 1 kg mass at 5 ms-2, a 3 kg mass will also be accelerated at 5 ms-2.

Q.38. Why do the stars appear displaces away from the Sun?
Ans. Due to gravitational field of the sun, the light rays from the stars band towards the sun. Due to this the stars appear displaced away from the Sun.

Q.39. Does the change in gravitational potential energy of a body between two points depend upon the nature of path followed? Explain.
Ans. The gravitational force is a conservative force. The work done to move a body between two points under the action of a conservative force is independent of the path followed. Therefore change in gravitational potential energy (work done per unit mass) between two points is independent of the path followed.

Q.40. Briefly explain, why there is practically no atmosphere on the surface of the moon.
Ans. Due to low value of acceleration due to gravity the escape velocity on the moon is quite small. It can be shown that the values of r.m.s. velocity of the molecules of different gases is much above the value of the escape velocity on the moon. As a result, all the gases have escaped the surface of moon with the passage of time.

Q.41. Why does the atmosphere of Jupiter contain light gases (mostly hydrogen), whereas the earth atmosphere has little of hydrogen gas?
Ans. The escape velocity on Jupiter is so large that even light gases cannot escape.

Q.42. What is escape velocity?
Ans. The minimum velocity with which a body must be projected up so as to enable it to just overcome the gravitational pull, is known as escape velocity.

Q.43. Define gravitational potential energy.
Ans. The gravitational potential energy of a body at a point is defined as the amount of work done in bringing the body from infinity to that point against the gravitational force.

Q.44. Define gravitational potential.
Ans. The gravitational potential at a point in the gravitational field of the Earth is defined as the amount of work done in bringing a body of unit mass from infinity to that point.

Q.45. What provides the centripetal force to a satellite revolving round the earth?
Ans. The weight of the satellite provides the necessary centripetal force to it, so as to enable it to revolve around the planet.

Q.46. Does the orbital velocity of a satellite depend upon its mass?
Ans. No, it is independent of the mass of the satellite.

Q.47. What is the time period and radius of the moons orbit around the earth?
Ans. Time period = 27.3 days

Radius of the orbit = 3.84 × 105 km.

Q.48. If suddenly the gravitational force of attraction between earth and a satellite revolving around it becomes zero, what will happen to the satellite?
Ans. If the gravitational force suddenly becomes zero, the satellite will move tangentially to the original orbit with the velocity it was revolving around the earth.

Q.49. What are the signs of kinetic energy, potential energy and the total energy of a satellite revolving around the earth?
Ans. Kinetic energy : positive,

potential energy : negative,

total energy : negative.

Q.50. What is the sense of rotation of a geostationary satellite?
Ans. Its sense of rotation should be same as that of the earth about its own access i.e. in anticlockwise direction (from west to east).

Q.51. What is the time period of a geostationary satellite?
Ans. Time period of a geostationary satellite = 24 h.

Q.52. What is the height of a geostationary satellite above the surface of the earth?
Ans. Height of a geostationary satellite = 35,930 km.

Q.53. What is a parking orbit?
Ans. The orbit of the geostationary satellite is called parking orbit.

Q.54. What is the full form of geostationary satellite 'APPLE' ?
Ans. Ariana Passenger Pay Load Experiment.

Q.55. Name India's first cosmonaut.
Ans. Rakesh Sharma.

Q.56. Give two uses of a geostationary satellite.
Ans. (1) The geostationary satellites are widely used for communication purposes i.e. for transmission of radio and T.V. programmes and for sending telephone signals across the oceans.

(2) They are used for weather forecasting.

Q.57. Give two uses of polar satellites.
Ans. (1) The polar satellites are used for weather forecasting. The weather forecasts made by Apollo satellite are more reliable than those made by a geostationary satellite used for this purpose.

(2) they are also used for various military purposes, such as to keep an eye on the moment of ships, troops and vehicles. They are also employed for spying and surveillance.

Q.58. What is weightlessness?
Ans. The apparent loss in weight of a body, when it falls freely under gravity is called weightlessness.

Q.59. Why an astronaut in an orbiting spacecraft is not in zero gravity although weightless?
Ans. At any point on the orbit of the spacecraft, there is some value of the acceleration due to gravity. However, its weight is used up in providing the necessary centripetal force for the orbital motion.

Q.60. What is the frequency of oscillation of a simple pendulum mounted in a cabin that is freely falling under gravity?
Ans. A simple pendulum in a cabin falling freely will not oscillate. It is because, the bob of the pendulum becomes weightless during the free fall of the cabin. Hence, the frequency of oscillation of the simple pendulum is zero.

Q.61. The earth is acted upon by the gravitational attraction of the sun. Why does not the earth fall into the sun?
Ans. The earth does not fall into the sun due to its stable orbit around the Sun. The gravitational pull of the sun provides the necessary centripetal force to the earth, so as to make it revolve in a stable orbit. We may say that the Earth is falling freely towards the sun but always missing it.

Q.62. The artificial satellite does not have any fuel, but even then it remains orbiting around the earth. why? Explain.
Ans. The gravitational pull due to Earth provides the necessary centripetal force to the satellite to make it go around the earth. As such no fuel is required.

Q.63. If a spoon is dropped from an artificial satellite orbiting around the earth, will it reach the surface of earth? If not, then explain why?
Ans. The spoon, when dropped from the orbiting satellite will not reach the surface of earth. When the spoon is dropped from the satellite, it also starts moving with the speed of the satellite. As the orbit of a satellite does not depend upon the mass, the spoon continues to move along with the satellite in the same orbit.

Q.64. Is moon a planet? What is the speed of the moon around the sun compared to that of the earth around the sun?
Ans. The moon is not a planet. It is a natural satellite of the earth. The earth and moon revolve about their common centre of mass, which in turns revolve around the sun. Thus, both the earth and the moon complete one revolution around the sun in one year and hence their speeds of motion are equal.

Q.65. The moon revolves around the earth and the earth-moon system revolves around the sun. When the moon is towards the sun from the earth, why does not the sun steal the moon?
Ans. The sun exerts a force on both the moon and the earth. If we could maintain the sun's attraction for the earth and moon, but turn off the attraction of the earth and moon from each other, they would both continue to orbit around the sun at the same radius and at the same speed. When we add the attraction between the earth and the moon, unless they revolve about the centre of mass, they would move towards each other. Thus, the sun cannot capture (steal) the moon even without an attraction between the moon and the earth.

Q.66. What are the necessary conditions for a satellite to appear stationary?
Ans. (1) It should revolve in an orbit coplanar and concentric with the equatorial plane.

(2) the sense of its orbital motion should be same as that of the rotational motion of earth i.e. in anticlockwise direction.

(3) its time period should be exactly 24 hours.

Q.67. What is the difference between ordinary and geostationary satellite?
Ans. A satellite, put in a circular orbit concentric and coplanar with equatorial plane of the earth and at a height, such that its period of revolution is just 24 hours is called a stationary satellite. The calculations shows that the height of the orbit of such a satellite should be about 36,000 km. Such a satellite always stays over the same place on the surface of earth and hence appears stationary. As ordinary satellite has time period other than 24 hours and hence appears to be moving one, place it in a desired orbit and allow it to make a soft landing.

Q.68. What is retorocket and its function?
Ans. A retorocket is a rocket engine, which works on the reaction principle. On being fired, it produces thrust in a direction opposite to the direction of firing. A spacecraft is always provided with retorockets. A retorocket is used to change the speed of the spacecraft, separate it from another one, place it in a desired orbit or allow it to make a soft landing.

Q.69. Imagine yourself in a spacecraft in circular orbit, well behind a space station in the same orbit. You intend to dock with the space station. How can it be done? Explain.
Ans. In order to catch up the space station, a forward thrust is applied so as to increase the speed of the spacecraft, it will not prove helpful. It is because, as the speed (or energy) of the spacecraft would increase, the radius of its orbit would also increase. In turn, spacecraft would travel more slowly falling father behind the space station. Contrary to it, if backward trust is applied on the spacecraft, it would move to lower orbit and speed up. Once the space station has been overtaken, now an appropriate forward thrust should be applied to put the spacecraft back into the proper orbit for docking.

Q.70. It is usually said that inside an artificial satellite, a simple pendulum does not oscillate. Do you agree with the statement? Justify your answer.
Ans. For the pendulum to oscillate, the required restoring force is provided by component of its weight. Inside the satellite, the pendulum is in the state of weightlessness. In the absence of weight and hence the restoring force, the pendulum does not oscillate inside the satellite.

Q.71. A body has a sense of weightlessness in a satellite revolving around the earth. Why?
Ans. The astronaut and the satellite to require the centripetal force to revolve around the earth. Their weight is used up in providing the necessary centripetal force. Hence, the astronaut feels weightlessness in the space.

Q.72. The astronaut in the satellite orbiting the earth feel weightlessness. Does the weightlessness depend upon the distance of the satellite from the earth? If so, how? Explain your answer.
Ans. A satellite revolving in an orbit of any radius experience is weightlessness. It is because, when a satellite revolves in an orbit, its weight is used up in providing the necessary centripetal force. Thus, the weightlessness does not depend upon the distance of the satellite from the earth.

Q.73. A person sitting in an artificial satellite of earth feels weightlessness, but a person standing on the moon has weight, do the moon is also a satellite of Earth?
Ans. It is the gravitational pull of the Earth on a person in the satellite, which provides him the necessary centripetal force to revolve around the earth and hence he experiences weightlessness. Had the size (weight) of the artificial satellite been very large (as in case of the moon), a person will be conscious of his weight in the satellite. Therefore, a person standing on the moon, has weight due to the additional gravitational pull of the moon on him.

Q.74. What is a natural satellite?
Ans. A heavenly body revolving around a planet in a stable orbit is called a natural satellite.

Q.75. What is an artificial satellite?
Ans. A satellite put to in its orbit around a planet by the man is called an artificial satellite.

Q.76. Define orbital velocity.
Ans. Orbital velocity of a satellite is the velocity required to put the satellite into its orbit around the earth.

Q.77. What is a polar satellite?
Ans. The satellite that revolves in polar orbit around the earth is called a polar satellite.

Q.78. What is a communication satellite?
Ans. A satellite used for communication purposes is known as communication satellite.

Q.79. What is weightlessness?
Ans. The apparent loss in weight of a body when it falls freely under gravity, is called weightlessness.

Q.80. Define time period of satellite.
Ans. The time period of a satellite is the time taken by it to go once around the earth.