Chapter 10 Gravitation

NCERT Solutions of Gravitation Class 9

  1. What is the source of centripetal force that a planet requires to revolve around the sun? On i what factors does that force depend?
    Solution: A planet requires centripetal force to revolve around the sun which is provided by the gravitational force of sun on the planet.
    As   the force depends on the mass of sun (Ms), mass of planet (MP) and the distance between the two (r).
  2. Suppose gravity of earth suddenly becomes zero, then in which direction will the moon begin to move if no other celestial body affects it? Solution: The circular motion of the moon around earth is due to the centripetal force provided by gravitational force of earth. Therefore, when gravity of earth suddenly becomes zero, the moon will begin to move in a straight line in the direction in which it was moving at that instant. That is the moon will move along the tangent to the circular orbit at that instant.
  3. Identical packets are dropped from two aeroplanes, one above the equator and the other above the north pole, both at height h. Assuming all conditions are identical, will those packets take same time to reach the surface of earth. Justify your answer. Solution: The value of acceleration due to gravity is greater at the poles than at the equator. So, the packet dropped at North Pole from a height h, will accelerate more than the packet dropped at equator from the same height and hence will reach the surface of earth earlier.
  4. The weight of any person on the moon is about 1 /6 times that on the earth. He can lift a mass of 15 kg on the earth. What will be the maximum mass, which can be lifted by the same force applied by the person on the moon?

Answer 90kg

5. The earth is acted upon by gravitation of sun, even though it does not fall into the sun. Why?
Solution: if The gravitational force of sun on earth is utilised in providing it centripetal force which is required for revolution around the sun. Hence, the earth does not fall in to the sun.

6. (a) A cube of side 5 cm is immersed in water and then in saturated salt solution. In which case will it experience a greater buoyant force. If each side of the cube is reduced to 4 cm and then immersed in water, what will be the effect on the buoyant force experienced by the cube as compared to the first case for water. Give reason for each case.
(b) A ball weighing 4 kg of density 4000 kg m s-3 is completely immersed in water of density 103 kg nrr3. Find the force of buoyancy on it. (Given g = 10 m s-2 .)
Solution:
(a) The cube will experience a greater buoyant force in saturated salt solution than in water as the density of saturated salt solution is greater than that of water.
As buoyant force = weight of liquid displaced by the object =Vag (a is density of liquid) As each side of the cube is reduced to 4 cm from 5 cm, so volume of cube decreases and hence the buoyant force also decreases.
(b) Here, Mass of ball, m = 4 kg Density of ball, ρ= 4000 kg m-3 Density of water σ=103 kg m-3

10 N

MCQ Practice Questions of Gravitation Class 9

  1. Two objects of different masses falling freely near the surface of moon would
    (a) have same velocities at any instant
    (b) have different accelerations
    (c) experience forces of same magnitude
    (d) undergo a change in their inertia
  2. An apple falls from a tree because of gravitational attraction between the earth and apple. If F, is the magnitude of force exerted by the earth on the apple and F2 is the magnitude of force exerted by apple on earth, then
    (a) F1 is very much greater than F2
    (b)F2 is very much greater than F1
    (c)F1 is only a little greater than F2
    (d)F1 and F2 are equal
  3. A girl stands on a box having 60 cm length, 40 cm breadth and 20 cm width in three ways. In which of the following cases, pressure exerted by the brick will be
    (a) maximum when length and breadth form the base
    (b) maximum when breadth and width form the base
    (c) maximum when width and length form the base
    (d) the same in all the above three cases
  4. An object weighs 10 N in air. When immeped fully in water, it weighs only 8 N. The weight of the liquid displaced by the object will be
    (a) 2N
    (b) 8 N
    (c) 10N
    (d) 12 N
  5. The weight of an object at the centre of the earth of radius R is
    (a) zero
    (b) infinite
    (c) R times the weight at the surface of the earth
    (d) 1/R2 times the weight at surface of the earth
  6. The force of attraction between two unit point masses separated by a unit distance is called
    (a) gravitational potential
    (b) acceleration due to gravity
    (c) gravitational field
    (d) universal gravitational constant
  7. The atmosphere is held to the earth by
    (a) gravity
    (b) wind
    (c) clouds
    (d) earth’s magnetic field
  8. Two particles are placed at some distance. If the mass of each of the two particles is doubled, keeping the distance between them unchanged, the value of gravitational force between them will be
    (a) 1/4 times
    (b) 4 times
    (c) 1/2 times
    (d) unchanged
  9. The value of quantity G in the law of gravitation
    (a) depends on mass of earth only
    (b) depends on radius of earth only
    (c) depends on both mass and radius of earth
    (d) is independent of mass and radius of the earth
  10. Law of gravitation gives the gravitational force between
    (a) the earth and a point mass only
    (b) the earth and sun only
    (c) any two bodies having some mass
    (d) two charged bodies only
  11. In the relation  , the quantity G
    (a) depends on the value of g at the place of observation
    (b) is used only when the earth is one of the two masses
    (c) is greatest at the surface of the earth
    (d) is universal constant of nature
  12. An object is put one by one in three liquids having different densities. The object floats with 1/9, 2/11 and 3/7 parts of their volumes outside
    the liquid surface in liquids of densities d1 d2 and d3 respectively. Which of the following statement is correct?
    (a) d1 > d3 > d2
    (b) d1>d2< d3
    (c) d1<d3>d2
    (d) d1<d2<d3
  13. A boy is whirling a stone tied with a string in an horizontal circular path. If the string breaks, the stone
    (a) will continue to move in the circular path
    (b) will move along a straight line towards the centre of the circular path
    (c) will move along a straight line tangential to the circular path
    (d) will move along a straight line perpendicular to the circular path away from the boy
  14. The gravitational force between two objects is F. If masses of both objects are halved without changing distance between them, then the
    gravitational force would become
    (a) F/4
    (b) F/2
    (c) F
    (d) 2 F
  15. The value of acceleration due to gravity
    (a) is same on equator and pole?
    (b) is least on poles
    (c) is least on equator
    (d) increases from pole to equator