Class 11 Physics Chapter 8 Important Questions Gravitation
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.
Read also: Gravitation Class 11 Physics Notes Chapter 8
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).
Read also: Class 11 Physics Chapter 8 MCQs with Answer Gravitation
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.