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GRAVITATION CLASS 9
POINTS TO REMEMBER
•The law of gravitation states that the force of attraction between any two objects is proportional to the product of their masses and inversely proportional to the square of the distance between them.
F=Gm1m2/r2
This law applies to objects anywhere in the universe. That is why, it is said to be universal.
•Gravitation is a weak force unless large masses are involved.
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•Kepler’s laws:
First law: Each planet revolves around the sun in an elliptical orbit, with the sun at one of the two foci of the ellipse.
Second law: The straight line joining the sun and the planet sweeps out equal areas in equal time intervals
Third law: The square of the orbital period of a planet is proportional to the cube of mean distance from the sun.
T2 r3 or T2 = kr3 where k is a constant
•Force of gravitation due to the earth is called gravity.
•The constant acceleration produced in a freely falling body due to earth’s gravitational pull is known as we acceleration due to gravity.
•g = GMe /R2e
The acceleration due to gravity decreases with altitude. It also varies on the surface of the earth, decreasing from poles to the equator.
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•Equations of motion for freely falling bodies
v = u + gt, h = ut + ½ gt2, v2 = u2 +2gh
•The weight of a body is the force with which the earth attracts it.
•The weight is equal to the product of mass and acceleration due to gravity (W = mg).
•The weight may vary from place to place. Weight of a body on the surface of moon is about 1/6 of the weight at the surface of earth. W = 0 at the centre of the earth.
•Mass of the body remains constant at every place.
•Mass is measured by beam balance and the weight of the body is measured by spring balance.
•The total force acting perpendicular on a given surface is called thrust.
•Thrust acting upon a unit area is called pressure and denoted by ‘P’.
Thus,
Pressure = Thrust / Area of contact
S.I. unit of pressure is N/m² which is known as Pascal (Pa).
•Density of a substance is the mass of the substance contained in a unit volume of that substance.
Density of an object is given as mass/volume.
Or
d = Mass/ Volume
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•Relative density of a substance is the ratio of density of substance to the density of water. It is also called specific gravity.
Thus,
Relative density= Density of substance/ Density of water
•Buoyancy can be defined as the tendency of the fluid to exert an upward force on an object, which is wholly or partially immersed in a fluid.
•The upward force that acts on an object when immersed in a fluid is called buoyant force.
•The S.I. unit of buoyant force is Newton.
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Two main factors on which the buoyant force depends are
(i) Density of the fluid. (ii) Value of acceleration due to gravity at that place.
•hen an iron nail is placed on the surface of water it sinks whereas ship made up of iron floats. This is because size or volume of the ship is more.
•If the density of the liquid is more than the density of the material of the body then the body floats due to the buoyant force exerted by it and vice-versa.
•All objects experience a force of buoyancy when they are immersed in a fluid. Buoyant force experienced by a body when submerged in a liquid depends on the volume of the body and the density of the liquid.
•Floating and sinking depends on density of water and the density of an object. If the density of an object is more than the density of water it sinks. This means that the upthrust of water on an object is less than the weight of the object. Therefore, objects of density less than that of a liquid float on the liquid. The objects of density greater than that of a liquid sink in the liquid.
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•Two forces act on an object immersed in water. One is the gravitational force (weight), which pulls the object downwards, and the other is the buoyant force, which pushes the object upwards. If the upward buoyant force is greater than the downward gravitational force (weight), then the object comes up to the surface of the water.
•Archimedes’ principle states that when a body is immersed partially or fully in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.
•Archimedes’ principle is used in designing ships and submarines. The lactometers and hydrometers used for measuring the purity of a sample of milk and for determining the density of the liquids are based on this principle.
•Relative density of a substance helps us to determine the purity of a substance by measuring its relative density. Examples: We use lactometer to differentiate between pure and impure (measure the impurity) milk. We can measure the impurity of water using hydrometer.
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Table of Contents
GRAVITATION CLASS 9 PDF CBSE TERM II PREPARATION
1.Mass of an object is 10 kg. What is its weight on the earth?
a)97N
b)99N
c)95N
d)98N
Ans. D
2.An object weighs 10N when measured on the surface of the earth. What would be its weight when measured on the surface of the moon?
a)1.67N
b)1.68N
c)1.69N
d)1.70N
Ans. A
3.A stone is released from the top of a tower of height 19.6m. calculate its final velocity just before touching the ground.
a)20.5m/s
b)19.7m/s
c)19.6m/s
d)21.7m/s
Ans. C
4.Two objects of different masses falling freely near the surface of moon would:
a)Have same velocities at any instant
b)Have different accelerations
c)Experience forces of same magnitude
d)Undergo a change in their inertia
Ans. A
5.The value of acceleration due to gravity:
a)Is same on equator and poles
b)Is least on poles
c)Is least on equator
d)Increases from pole to equator
Ans. C
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6.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
b)F/2
c)F
d)2F
Ans. A
7.A boy is whirling a stone tied with a string in an horizontal circular path. If the string breaks, the stone:
a)Will continue to move in the circular path
b)Will move along a straight line towards the centre of the circular path
c)Will move along a straight line tangential to the circular path
d)Will move along a straight line perpendicular to the circular path
Ans. C
8.An object is put one by one in three liquids having different densities. The object floats with 1/9, 2/11 and 3/7 parts of their volumes outside the liquid surface in liquids of densities d2, d3 and d3 respectively. Which of the following statement is correct?
a)d1 > d2 > d3
b)d1 > d2 < d3 c)d1 < d2 > d3
d)d1 < d2 < d3
Ans. D
9.In the relation F = GMm / d2, the quantity G:
a)Depends on the value of g at the place of observation
b)Is used only when the earth is one of the two masses
c)Is greatest at the surface of the earth
d)Is universal constant of nature
Ans. D
10.Law of gravitation gives the gravitational force between:
a)The earth and a point mass only
b)The earth and sun only
c)Any two bodies having some mass
d)Two charged bodies only
Ans. C
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11.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
d)Is independent of mass and radius of the earth
Ans. D
12.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)¼ times
b)4 times
c)½ times
d)Unchanged
Ans. B
13.The atmosphere is held to the earth by:
a)Gravity
b)Wind
c)Clouds
d)Earth’s magnetic field
Ans. A
14.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
d)Universal gravitational constant
Ans. D
15.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
d)½ R times the weight at surface of the earth
Ans. A
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16.An object weighs 10N in air. When immersed fully in water, it weighs only 8N. The weight of the liquid displaced by the object will be:
a)2N
b)8N
c)10N
d)12N
Ans. A
17.A girl stands on a box having 60cm length, 40cm breadth and 20cm width in three ways. In which of the following cases, pressure exerted by the brick will be:
a)Maximum when length and breadth from the base
b)Maximum when breadth and width from the base
c)Maximum when width and length from the base
d)The same in all these above three cases
Ans.B
18.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 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
d)F1 and F2 are equal
Ans. D
19.The weight of any person on the moon is about 1/6 times that on the earth. He can lift a mass of 15kg on the earth. What will be the maximum mass, which can be lifted by the same force applied by the person on the moon?
a)60kg
b)70kg
c)80kg
d)90kg
Ans. D
20.A stone is thrown vertically upward with an initial velocity of 40m/s. Taking g = 10m/s2, find the maximum height reached by the stone. What is the net displacement and the total distance covered by the stone?
a)80m
b)90m
c)70m
d)60m
Ans. A
ALSO VISIT : ARCHIMEDES PRINCIPLE CLASS 9TH
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