The document NCERT Exemplar - Gravitation (Part - A) Class 9 Notes | EduRev is a part of the Class 9 Course Science Class 9.

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**SHORT ANSWER TYPE QUESTIONS**

**Q.1. What is the source of centripetal force that a planet requires to revolve around the Sun? On what factors does that force depend?****Ans: **The gravitational force of attraction of the Sun provides centripetal acceleration to the planet to revolve around the Sun.

F ∝ Mass of the Sun

F ∝ Mass of the planet**Q.2. On the earth, a stone is thrown from a height in a direction parallel to the earth’s surface while another stone is simultaneously dropped from the same height. Which stone would reach the ground first and why?****Ans:** Both reach the ground together because time to reach the ground depends only on vertical height.

Initial velocity in downward direction is zero and the value of 'g' is same in both cases.**Q.3. Suppose gravity of the earth suddenly becomes zero, then in which direction will the moon begin to move if no other celestial body affects it?****Ans:** Moon will move along a straight line in tangential direction at the point where gravitational force ceases to exist.**Q.4. 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 the earth? Justify your answer:Ans:**

**LONG ANSWER TYPE QUESTIONS**

**Q.8. How does the weight of an object vary with respect to mass and radius of the earth? In a hypothetical case, if the diameter of the earth becomes half of its present value and its mass becomes four times of its present value, then how would the weight of any object on the surface of the earth be affected?****Ans: **W = mg and g = GM/R^{2}

Hence W ∝ Mass of the earth (M)

As M becomes 4 times and R is halved, W becomes

Thus the weight becomes 16 times of earlier weight.**Q.9. How does the force of attraction between the two bodies depend upon their masses and distance between them? A student thought that two bricks tied together would fall faster than a single one under the action of gravity. Do you agree with this hypothesis or not? Comment.****Ans:** According to Newton’s law of gravitation,

F ∝ m_{1}m_{2} (i.e. product of masses)

F ∝ (i.e. square of distance between them)

Time taken during free fall,

Thus the hypothesis of student is incorrect as t is independent of mass of bricks.**Q.10. Two objects of masses m _{1} and m_{2} having the same size are dropped simultaneously from heights h_{1} and h_{2} respectively. Find out the ratio of time they would take in reaching the ground. Will this ratio remain the same if (i) one of the objects is hollow and the other one is solid and (ii) both of them are hollow, size remaining the same in each case. Give reason.**

Ans:

Thus,

(i) If one object is hollow and the other is solid, the ratio of time to reach the ground remains the same, as.t is independent of masses.

(ii) The result is still unchanged as t depends only on height of fall.

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