CBSE Class 11 Physics Gravitation Assignment

Comprehension-1

During the formation of stars, from clouds of hydrogen gas in space, due to gravitational force of attraction, volume of gas decreases, which in turn heats the gas. Specific heat capacity of gas is S and universal gravitational constant is G. and mass in a hydrogen cloud is M.

1. If radius of gas cloud decreases from R to R/2, the increment in temperature of gas is (assume No loss of energy out side due to radiations, and clouds are spherical in shape)
(a) GM/ RS (b) 3GM/5 RS (c) 3GS/5MR (d)  -3GM/ RS

2. Assume the initial temperature of gas is 0 K and thermonuclear reactions will start at T₀ K temperature, the minimum mass of gas required so that thermonuclear reactions starts when radius of cloud becomes half of initial radius (R). Assume uniform temperature in entire volume of gas.
(a) 3SRT₀/5 G  b)  3SRT₀/5G  (c) SRT₀/G  (d) none radius.

Comprehension -2

In some parts of universe it is found that acceleration produced in a body is inversely proportional to the square of its mass and directly proportional to the net force (F) according to equation a =c F/m². Where c is constant, whose magnitude is 1, if m is measured in kg, a is measured in m/s² and F Bose. Also action and reaction force are equal and opposite and on different interacting bodies.

3. In the shown figure two block of mass m₁ = 2kg and m₂ = 4 kg are attached via ideal massless string over frictionless mass less pully as shown in the figure. If acceleration due to gravity g = 5 m/s². The tension in the string is

(a) 6 Bose (b) 1.67 Bose m1 m(c) 3 Bose (d) 32 Bose 2

4. In the shown figure a block of mass m = 2 kg is placed on smooth inclined plane. The minimum value of force F needed to support the block is (g = 5 m/s²)

(a) zero, Newton (b) 10 Bose θ = 30° F (c) 20 Bose (d) 10 Newton

Each of the questions given below consists of two statements, an assertion (A) and reason (R).
Select the number corresponding to the appropriate alternative as follows
(a) If both A and R are true and R is the correct explanation of A
(b) If both A and R are true but R is not the correct explanation of A
(c) If A is true but R is false
(d) If A is false but R is true

5. A: If the atmospheric air suddenly condenses and forms a thin layer on the surface of the earth, the duration of the day will increase.
R: The moment of inertia of the earth will increase and the speed of rotation will decrease.

6. A: The orbit of the earth’s satellite always lies in a plane containing the centre of the earth.
R: Work done to keep the satellite in orbit is zero.

7. A: A space ship entering the earth’s atmosphere is likely to catch fire.
R: The temperature of upper atmosphere is very high.

8. A. A satellite, moving around the earth in a circular path, is in equilibrium.
R. The centripetal force and the gravitational pull by the earth on the satellite are balanced and so resultant force on the satellite is zero.

9. A. If the polar ice caps melt and spread uniformly, then the duration of the day will decrease.
R. Due to melting of ice and spreading of water uniformly, the moment of inertia of the earth will decrease.

10. A. When a heavy body and a light body are released from same position, at a height above the surface of earth, forces acting on them are different but both fall down with same acceleration.
R. Force of attraction on body (by the earth) is directly proportional to the mass of the body.

11. A. With time, if the density of mass, due to concentration of heavy metals, increases near the centre of earth but decreases above, then acceleration due to gravity at the surface of earth will remain unaffected.
R. Mass of the earth, and distance between surface and centre of mass of earth will remain same

12. A. Weight of a body is more at the poles than the equator.
R. The only reason is that radius of earth at poles is smaller than that at the equator.

13. A. To determine time in a geostationary satellite, a spring watch is used instead of a pendulum clock.
R. If any object is gently released from a geostationary satellite, then it starts moving with the velocity of the satellite and itself becomes a satellite.

14. A. There will not be any change in the value of acceleration due to gravity at the poles, if earth stops rotating.
R. There will be increase in the value of acceleration due to gravity at equator, if earth stops rotating.

Assignment
1. (b) 2. (a) 3. (d) 4. (b) 5. (d) 6. (c) 7. (c) 8. (d) 9. (d) 10. (a) 11. (a) 12. (c) 13. (b) 14. (b)