## 02 July 2011

### Class-ix-Science-physics-Gravitation Solved Questions :Numerical problems

 Gravitation Solved Questions :Numerical problems 1. How does the force of gravitation between two objects change when the distance between them is reduced to half? Answer:  when the distance between the objects is reduced to half the gravitational force increases by four times the original force. 2. The gravitational force acts on all objects in proportion to their masses. Why, then, a heavy object does not fall faster than a light object? Answer: Acceleration due to gravity does not depend on mass of object . Hence, all bodies fall with the same acceleration provided there is no air or other resistance 3. The earth and the moon are attracted to each other by gravitational force. Does the earth attract the moon with a force that is greater or smaller or the same as the force with which the moon attracts the earth? Why? Answer: According to Newton’s 3rd law of motion Every action has equal reaction in opposite direction. Since, The earth surface attracts the moon with the same force with which the moon attracts the earth and cancel them 4.  If the moon attracts the earth, why does the earth not move towards the moon? Answer: The earth is much larger than the moon so, the acceleration produced on the earth surface cannot be noticed. 5. What is the importance of Universal Law of Gravitation? Ans: There are many importance of Universal Law of Gravitation 1. The force of attraction that binds us to the earth, 2. The motion of planets moving around the sun, 3. the motion of moon around the earth 4. The occurring of tides due to sun and moon. 6 What is Gravitation? Answer: Gravitation is the force of attraction between two objects in the universe. i) Gravitation may be the attraction of objects by the earth. Eg :- If a body is dropped from a certain height, it falls downwards due to earth’s gravity. If a body is thrown upwards, it reaches a certain height and then falls downwards due to the earth’s gravity. ii) Gravitation may be the attraction between objects in outer space.Eg :- Attraction between the earth and moon. Attraction between the sun and planets 7. What is Centripetal force? Answer: When a body moves in a circular path, it changes its direction at every point. The force which keeps the body in the circular path acts towards the centre of the circle. This force is called centripetal force.  If there is no centripetal force, the body will move in a straight line tangent to the circular path. 8. State Universal law of gravitation? Answer: The universal law of gravitation states that, ‘Every object in the universe attracts every other object with a force which is directly proportional to product of the masses and inversely proportional to the square of the distance between them.’ 9. In what direction does the buoyant force on an object immersed in a liquid act? Ans: The buoyant force acts on an object in the vertically upward direction through the center of gravity of the displaced liquid. 10. A stone is released from the top of a tower of height 19.6 m. calculate its final velocity just before touching the ground. Ans: Given that,       u = 0, g = 9.8 ms–2, s = 19.6 m Now, v2 - u2 = 2gs or,     v2 - 0 = 2 x 9.8 x 19.6 = (19.6)2