Class 9 Science · 26 questions · 25 with answers
Two objects of different masses falling freely near the surface of moon would
(a) have same velocities at any instant
The value of acceleration due to gravity
(c) is least on equator
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
A boy is whirling a stone tied with a string in an horizontal circular path. If the string breaks, the stone
(c) will move along a straight line tangential to the circular path
An object is put one by one in three liquids having different densities. The 1 2 3 object floats with , and parts of their volumes outside the liquid 9 11 7 surface in liquids of densities d1, d2 and d3 respectively. Which of the following statement is correct?
(d) d1< d2< d3 16-04-2018
In the relation F = G M m/d2, the quantity G
(d) is universal constant of nature
Law of gravitation gives the gravitational force between
(c) any two bodies having some mass
The value of quantity G in the law of gravitation
(d) is independent of mass and radius of the earth
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 1 1
(b) 4 times
The atmosphere is held to the earth by
(a) gravity
The force of attraction between two unit point masses separated by a unit distance is called
(d) universal gravitational constant
The weight of an object at the centre of the earth of radius R is
(a) zero
An object weighs 10 N in air. When immersed fully in water, it weighs only 8 N. The weight of the liquid displaced by the object will be
(a) 2 N
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
(b) maximum when breadth and width form the base
An apple falls from a tree because of gravitational attraction between the earth and apple. If F1 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
(d) F1 and F2 are equal
What is the source of centripetal force that a planet requires to revolve around the Sun? On what factors does that force depend?
Gravitational force. This force depends on the product of the masses of the planet and sun and the distance between them.
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?
Both stones will take the same time to reach the ground because the two stones fall from the same height.
Suppose gravity of earth suddenly becomes zero, then in which direction will the moon begin to move if no other celestial body affects it?
The moon will begin to move in a straight line in the direction in which it was moving at that instant because the circular motion of moon is due to centripetal force provided by the gravitational force of earth.
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.
The value of ‘g’ at the equator of the earth is less than that at poles. Therefore, the packet falls slowly at equator in comparison to the poles. Thus, the packet will remain in air for longer time interval, when it is dropped at the equator.
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?
ge = g and gm = g6 Force applied to lift a mass of 15 kg at the earth, F = m ge = 15 ge N Therefore, the mass lifted by the same force on the moon, 15 g m =F gm = g = 90 kg g × R2 mass g × R2
Calculate the average density of the earth in terms of g, G and R.
g = G M or M = ⇒ Density D = = R2 G volume G × Ve (Where Ve is the volume of the earth ) g × R2 3g or D= = 4 π GR G × π R3
The earth is acted upon by gravitation of Sun, even though it does not fall into the Sun. Why? 16-04-2018
The gravitational force is responsible for providing the necessary centripetal force. Long Answer Questions
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?
Weight of an object is directly proportional to the mass of the earth and inversely proportional to the square of the radius of the earth. i.e., Weight of a body ∝ R2 Original weight Wo = m g = m G M R2 When hypothetically M becomes 4 M and R becomes 4M M then weight becomes W n= m G = (16 m G) 2 = 16 × Wo R 2 R ( ) The weight will become 16 times.
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 his hypothesis or not? Comment.
F ∝ m1m2 and F ∝ d2 This hypothesis is not correct. The two bricks, like a single body, fall with the same speed to reach the ground at the same time in case of free-fall. This is because acceleration due to gravity is independent of the mass of the falling body. 1 1
Two objects of masses m1 and m2 having the same size are dropped simultaneously from heights h1 and h2 respectively. Find out the ratio of time they would take in reaching the ground. Will this ratio remain the same if
h1 = g 2 h2 = g 2 , as x = o 2 t1 2 t2 t1 h1 t2 = h2 . Ratio will not change in either case because acceleration remains the same. In case of free-fall acceleration does not depend upon mass and size.