Gravitation–9th Standard Science Notes and Question Bank

Gravitation
Choose the correct answer.
1.Gravitational force is always -----
a) attractive b)repulsive c)neither attractive nor repulsive d)zero
2. Unit of acceleration due to gravity is --------
a) ms-1 b)ms-2 c)ms-3 d)ms-4
3. As the altitude increases, the acceleration due to gravity -------
a) remains constant b)becomes zero c)decreases d)increases
4. The relations connecting acceleration due to gravity and gravitational constant is --------
a) N2m2kg2 b) Nmkg -2 c) N2mkg-2 d) Nm2kg-2
5.SI unit of gravitational constant is ---------
a) g = GM/R2 b) g= GM/R c) g = GMR2 d) g = GMR
6. At the center of the earth acceleration due to gravity is ---------
a)zero b)infinity c)9.8 d)98
Fill in the blanks with suitable words.
1. Value of gravitational constant (G)=--------- (6.67X10-11 Nm2kg-2)
2. Acceleration due to gravity is independent of ----------( Nature , chemical combination,
temperature)
3. According to Newton’s law of gravitation F = ---------( Gm1m2/d2 )
4. In vacuum if a coin and a feather are released from the same height at the same time, then they will reach the ground -------- ( simultaneously )
Match the followings.
1.Force between sun & moon         a. Gm1m2/d2 ( 4)
2.Gravitational constant                 b.used to determine acceleration due to gravity (3)
3. simple pendulum                        c.depends on masses
4. Newton’s law of gravitation        d. action at a distance force (1)
                                                      e. Nm2kg-2 (2 )
Answer the following questions :
1.Name the types of force between objects are categorized ?
force between objects are categories into two types . They are Contact forces and action-at-a-distance.
2.Define contact force. give examples.
The force that result by the two interacting and physically containing objects.
For eg : normal force, frictional force and tension force.
3. Define action-at-a-distance force. give examples.
The force that result even when two interacting objects are not in physical contact with each other.
for eg: electric force, magnetic force, gravitational force etc.
4.Define gravitational force. write its nature.
Force of attraction between any two bodies is called gravitational force. gravitational force is independent of nature, chemical composition, temperature and medium of substance.
5. Define Newton’s universal law of gravitation.
Force of attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them – this is called Newton’s universal law of gravitation.
6.What is acceleration due to gravity ? write its unit & value.
Acceleration caused by the earths gravitational force is called acceleration due to gravity. Unit of acceleration due to gravity is ms-2. its average value is about 9.8 ms-2 ( it is maximum at poles 9.83 ms-2 & minimum at equator 9.78 ms-2.)
7. How acceleration due to gravity can be determined ?
Using simple pendulum we can calculate acceleration due to gravity with help of the formula
g = 42L
        T2
8. How acceleration due to gravity varies on earth ?
a) acceleration due to gravity is maximum at poles & minimum at equator .
b) As we go up from surface of earth and as we go inside the earth acceleration due to gravity decreases.

Additional Questions and answers
1. What is the value of ‘G’?
G = 6.67 x 10-11 Nm2/kg2
2. Why is law of gravitation termed as universal?
Because it is unaffected by temperature, pressure or any physical condition.
3. Who forwarded the laws of planetary motion?
J. Kepler forwarded laws of planetary motion
4. Which law is useful in finding the mass of the sun?
Kepler’s third law.
5. Why does a child descending from a giant wheel feel strange?
Because the child feels weightlessness.
6. What is weightlessness?
The condition of zero reaction is known as weightlessness.
7. What happens to weight when g = 0?
When g = 0, w = m(0) = 0, this condition is known as weightlessness.
8. What is the weight of an object in space which weighs 10kg on earth?
Since g = 0, the weight of the object in space is zero
9. What is the relation between ‘h’ and ‘g’?
g is inversely proportional to ‘h’
10. Write the laws of Kepler
First law : The planets move in elliptical orbits around the sun with the sun at one focus.
Second law - An imaginary line drawn from the sun to a planet sweeps equal areas in equal intervals of time.
Third law-The cube of the average distance (r) of a planet from the sun is proportional to the square of its period (T) of revolution r3 α T2
11. List down the features of gravitational force existing between two bodies
a) varies directly on the product of the masses of the two bodies.
b) varies inversely as the square of the distance between the two bodies.
c) Is not affected by any change in temperature, pressure, does not depend on the intervening medium.
d) acts along the line joining the two bodies
e) Is one of the basic force in nature and is the weakest between two bodies.
f) constitute an action reaction pair of forces.

12. What is the weight of a body? Give an expression for the weight of a body of mass m.
Weight of a body is the force with which a body is attracted towards the centre of the earth.
Weight of the body (W) = mg Where
m = mass of the body
g = acceleration due to gravity at a place
13. Mention situations where weightlessness can be experienced.
a) inside a freely falling lift
b) inside a space craft orbiting the earth
c) inside a descending giant wheel
d) for free fall of a body
14. Where is the value of acceleration due to gravity g greater at the poles or at the equator? Why is it so?
The value of acceleration due to gravity ‘g’ is greater at the poles than at the equator. This is due to the geoid shape of the earth.
15. What is weightlessness? Explain with an example
A body of mass m inside the lift experiences its weight which is given by mg. This is the force with which the body is attracted towards the centre of the earth.
In normal case, the floor of the lift exerts an equal but opposite force on the body (reaction force).
If the cable of the lift breaks, then the lift and the body accelerates towards the centre of the earth with the value g. The resultant force acting on the body inside the freely falling lift is:
mg = mg
mg - mg = 0
The condition where the reaction force acting on the body is zero is called weightlessness.

Fill in the blanks:
16. If the distance between two objects is doubled, then the gravitational force between them decreases by 4 times.
17. The gravitational force between two objects is directly proportional to the product of their . masses
18. The tides on ocean can be explained on the basis of Gravitational law.
19. Kepler’s 3rd law can be used to find the mass of the sun.
20. The average value of acceleration due to gravity on earth is 9.8m/s
21. When an object is taken from equator to the poles, its mass will remain the same but it weights changes.
22. A stone of 2kg weight is dropped from a height of 100m to fall freely under gravity. Its weight at a height of 50m while freely falling will be zero.
23. A spacecraft of mass 1000 kg is orbiting the earth. Its weight will be zero .
24. The value of ‘acceleration due to gravity’ varies on earth due to geoid shape of the earth.
25. The planets move in elliptical orbits around the Sun.
26. According to the Universal law of Gravitation, the gravitational force between two objects is inversely proportional to square of the distance between them.
27. The acceleration due to gravity is independent of mass of the object
28. The presence of Neptune and Pluto was predicted on the basis of Gravitational law.
29. In the law of magnetic poles, the force is either attractive or repulsive. But in Universal law of gravitation there is only attractive force

Multiple choice questions
30. If the distance between two heavenly bodies increases by two times, then the gravitational force between them
A] decreases by 4 times B] increases by 4 times C] decreases by 1/2-time D] increases by 2 times.
31. Which one of the following is true if an object is falling freely under gravity?
A] Its mass decreases B] It experiences weightlessness C] Its weight increases D] Its mass becomes zero.
32. For a satellite orbiting the earth, centripetal force is provided by
A] the place of launch B] gravitational force C] the size of the satellite D] mass of the satellite
33. The value of ‘g’
A] does not depend on the mass of the object
B] is proportional to the mass of the object
C] does not depend on the mass of the earth
D] does not depend upon the radius of the earth
34. When the distance between the two objects is doubled, the forces between the two objects before and after doubling are in the ratio
A] 1:1 B] 4:1 C] 1:2 D] 2:1
35. The tides due to the sun and moon can be explained by
A] Kepler’s first law B] Kepler’s second law C] Newton's third law D] law of gravitation
36. When the distance between two celestial bodies is increased by two times the original distance, the gravitational force between them
A] increases by 2 times B] decreases by 2 times C] increases by 4 times D] decreases by 4 times.
37. The weight of an object is slightly higher near the poles than at the equator, due to earth's
A] atmosphere B] spherical shape C] geoid shape D] magnetic field
38. Minimum velocity required for a body to escape from gravitational field of the earth is called
A] orbital velocity B] escape velocity C] initial velocity D] final velocity
39. The force of attraction between any two objects is not related to the
A] physical nature of objects
B] mass of the objects
C] distance between the objects
D] force acting along the line joining two objects
40. Weightlessness is experienced in which of the following situations?
A] Travelling in aircraft
B] Moving in upward direction in a lift
C] Moving up in a giant wheel
D] In a spacecraft orbiting around the earth
41. "An imaginary line drawn from the sun to planet sweeps equal areas in equal intervals of time." This law is
A] First law of planetary motion
B] Second law of planetary motion
C] Third law of planetary motion
D] Universal law of gravitation
42. "The planets move in elliptical orbits around the sun, with sun at one focus." This is
A] First law of Newton B] Hubble’s law C] Kepler's law D] Third law of Newton
43. Weightlessness is felt when you are
A] moving up in a giant wheel
B] in a rotating merry-go-round
C] moving up in a lift
D] coming down in a giant wheel
44. In which of the cases, weight of an object is halved?
A] Mass is halved
B] Gravity is doubled
C] Both mass and gravity are halved
D] Both mass and gravity are doubled
45. The planets discovered, based on the law of gravitation are
A] Jupiter- Saturn B] Saturn - Uranus C] Neptune – Pluto D] Uranus - Neptune
46. One of the laws that can be suggested to find the distance between the sun and the earth is
A] Newton’s first law B] Newton’s third law C] Kepler’s second law D] Kepler’s third law
47. Every particle in this universe attracts every other particle with a force that is:
A] directly proportional to the product of their masses and inversely proportional to the distance between them.
B] inversely proportional to the product of their masses and directly proportional to the square of the distance between them.
C] directly proportional to the product of their masses and the square of the distance between them.
D] directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
48. Every particle in this universe attracts every other particle with a force. This force:
A] acts along the line joining the centres of the two particles.
B] acts at the centre of the particle with heavier mass.
C] acts along the line joining the surfaces of the two particles.
D] acts from the centre of one particle to the surface of the other particle.

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