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ScienceClass 9CBSE

Which one of the following sets of phenomena would increase on raising the temperature? (a) Diffusion, evaporation, compression of gases (b) Evaporation, compression of gases, solubility (c) Evaporation, diffusion, expansion of gases Evaporation, solubility, diffusion, compression of gases

Soln: Answer is c) Evaporation, diffusion and expansion of gases Explanation: Particles of matter are continuously moving as they possess kinetic energy. When temperature us raised particles of matter intermix with each other which is called diffusion. Evaporation means conversion of liquid state into vapour state. When we increase temperature molecules move and vibrate so quickly that they escape into the atmosphere in the form of vapours. Increase in temperature causes the molecules move faster this results in expansion of gases.

ScienceClass 9CBSE

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–3 is completely immersed in water of density 103 kg m–3 Find the force of buoyancy on it. (Given g = 10 m s–2.)

Buoyant force, F = Vpg p= Density of water, V = Volume of water displaced by the body Volume and density of an object decides its Buoyancy. Cube will experience a greater byoyancy in saturated solution. If cube is reduces to 4 cm on each side, volume of cube becomes less as the buoyancy will be reduced as buoyant force is directly proportional to volume. (ii) The magnitude of the buoyant force given by F = Vpg where V = Volume of body immersed in water or volume of water displaced, p = Density of liquid. [∴ Given, mass of a ball = 4 kg, density = 4000 kgm-3]. Hence Volume of solid = 𝑀𝑎𝑠𝑠 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 4 4000 = 1 1000 Buoyancy F= Vpg == 1 1000 10N x 1000x10

ScienceClass 9CBSE

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 (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.

h1 = 1 gt21 2 h2 = 1 gt22 2 as x = o 𝑡1= √ℎ1 𝑡2 ℎ2 As the acceleration remain same ratio between two objects remain same. In this case acceleration does not depend upon mass and size.

ScienceClass 9CBSE

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.

Force of attraction between two masses separated by distance r is given by Newton law of gravitation where F= Gm1m2 Where G is gravitational force and it is an universal constant. R2 Gravitational force is directly proportional to the product of the masses of two bodies and inversely proportional to the square of the distance between them. Bodies thrown from same height fall with same speed irrespective of their mass. This is due to acceleration due to gravity. g= GM Where M is mass R is radius of the earth R2 This equation shows that acceleration due to gravity depends on mass of the earth and radius of the earth. Henec tow brick tied will not fall faster than single one.

ScienceClass 9CBSE

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?

Soln: Let R and M be the radius and mass of the earth Then Weight of the object= M R2 Original weight W0= mg = mg M R2 Hypothetically 4m and R becomes R 2 Then, weight = mG 4M (𝑅) 2 2 =(16m g) M R2 = 10xW0 Weight will be 16 times.

ScienceClass 9CBSE

The earth is acted upon by gravitation of Sun, even though it does not fall into the Sun. Why?

Soln: Sun provides enough centripetal force to keep earth in its orbit and the earth provides centrifugal force due to its motion. These two force balance each other which prevents earth from falling into sun.

ScienceClass 9CBSE

Calculate the average density of the earth in terms of g, G and R.

Soln: g= GM R2 M=gR2 G Density of earth D= 𝑀𝑎𝑠𝑠 𝑉𝑜𝑙𝑢𝑚𝑒 = Gr2 GxVe = gR2 G4πR3 3 = 3g 4µGR

ScienceClass 9CBSE

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?

Soln: Weight of person is = 1/6th of weight on earth This proves acceleration due to gravity on moon is 1/6th of acceleration due to gravity on earth. Hence the person can lift a mass 6 times heavier on moon than on earth.

ScienceClass 9CBSE

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.

Soln: Value of g is greater than value of g in poles . Since packets are thrown from same height packet thrown froom poles reaches the earth first.

ScienceClass 9CBSE

Suppose gravity of earth suddenly becomes zero, then in which direction will the moon begin to move if no other celestial body affects it?

Soln: If there is no gravitational pull from the earth moon start to move in a straight line tangent to its circular path.

ScienceClass 9CBSE

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?

Soln: Both the stones reach the ground simultaneously as they are dropped from same height and their intial velocity will be same .

ScienceClass 9CBSE

What is the source of centripetal force that a planet requires to revolve around the Sun? On what factors does that force depend?

Soln: Gravitational force is the source of centripetal force required to revolve around the sun. This force depends on distance between the planet and sun along with their masses.

ScienceClass 9CBSE

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 (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 F1 and F2 are equal

Soln: Answer is (d) F1 and F2 are equal Explanation: Netwon’s third law of motion states that for every action there is equal and opposite reaction. Hence F1 and F2 are equal.

ScienceClass 9CBSE

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 the same in all the above three cases

Soln: Answer is (b) maximum when breadth and width form the base Explanation: Surface area and pressure are inversely proportional. Pressure will be maximum when surface area is minimum hence the answer is maximum when breadth and width form the base

ScienceClass 9CBSE

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 (b) 8 N (c) 10 N (d) 12 N

Soln: Answer is (a) 2 N Explanation: Object of weight displace d by liquid = weight in air- weight in liquid =10N-8N = 2N

ScienceClass 9CBSE

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 1/R2 times the weight at surface of the earth

Soln: Answer is (a) zero Explanation: At the centre of the earth acceleration due to gravity is zero . Since weight is the product of mass and gravity. Weight of object at the centre of the earth will be zero.

ScienceClass 9CBSE

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 universal gravitational constant

Soln: Answer is (d) universal gravitational constant Explanation: F= Gx𝑚1𝑥𝑚2 𝑟2 Here point masses are separated by unit distance Hence m1, m2 and r = 1 Hence F=G which is a universal constant hence answer is universal gravitational constant

ScienceClass 9CBSE

The atmosphere is held to the earth by (a) gravity (b) wind (c) clouds earth’s magnetic field

Soln: Answer is (a) gravity

ScienceClass 9CBSE

1. 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 unchanged

<ul><li><strong>Soln:</strong></li></ul><p>Answer is (b) 4 times</p><p>Explanation:</p><p>F= Gx𝑚1𝑥𝑚2</p><p>𝑟2</p><p>F=Gx2𝑚1𝑥2𝑚2</p><p>𝑟2</p><p>F= 4xGx<sup>𝑚1𝑥𝑚2</sup></p><p>𝑟2</p>

ScienceClass 9CBSE

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 is independent of mass and radius of the earth

Soln: Answer is (d) is independent of mass and radius of the earth Explanation: G is an universal constant hence it is independent of mass and radius of the earth.

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