Q.1. 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)
Explanation:
Q.2. 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)
Explanation:
Q.3. 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) 2 F
Ans: (a)
Explanation:
Q.4. 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 away from the boy
Ans: (c)
Explanation: At any instance of time object in circular motion tend to be in rectilinear motion. Object keeps on moving due to centripetal force and it moves along a straight line tangential to the circular path when strings breaks.
Q.5. 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 d_{1}, d_{2} and d_{3} 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)
Explanation:
Q.6. In the relation F = G M m/d^{2}, 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)
Explanation:
Q.7. 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)
Explanation: Newton's law of gravitation can be used to calculate the gravitational force that exists between any two bodies of a certain mass. The force of attraction between two bodies is inversely proportional to the square of the distance between them and directly proportional to the product of their masses.
Q.8. 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)
Explanation: G is an universal constant hence it is independent of mass and radius of the earth.
Q.9. 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) 14 times
(b) 4 times
(c) 12 times
(d) unchanged
Ans: (b)
Explanation: The gravitational force between two bodies is inversely proportional to the square of the distance between them and directly proportional to the product of their masses. Therefore, the gravitational force between the two objects would increase to four times its initial value when their masses are both doubled while remaining at the same distance.
Q.10. The atmosphere is held to the earth by
(a) gravity
(b) wind
(c) clouds
(d) earth’s magnetic field
Ans: (a)
Explanation: Due to gravity, everything is drawn toward the earth. In fact, the air encircling our planet is kept in place by gravity, and this protective layer of air is referred to as the atmosphere.
Q.11. 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)
Explanation:
Here point masses are separated by unit distance
Hence m_{1}, m_{2 }and r = 1
Hence F = G which is a universal constant. Hence answer is universal gravitational constant
Q.12. 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) 1/R2 times the weight at surface of the earth
Ans: (a)
Explanation: At the centre of the earth acceleration due to gravity is zero. Since weight is the product of mass and acceleration due to gravity. Weight of the object at the centre of the earth will be zero.
Q.13. 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
Ans: (a)
Explanation:According to Archimedes principle,
Weight of displaced liquid = weight of object in air weight of object in liquid
= 10N  8N
= 2N
Q.14. 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
(d) the same in all the above three cases
Ans: (b)
Explanation: Surface area and pressure are inversely proportional to each other. Pressure will be maximum when surface area is minimum. Hence, the answer is maximum when breadth and width form the base because surface area is going to be minimum when breadth and width form the base.
Area of the base formed by length and breadth = 60 × 40 = 2400 cm²
Area of the base formed by breadth and width = 40 × 20 = 800 cm²
Area of the base formed by width and length = 20 × 60 = 1200cm²
Q.15. An apple falls from a tree because of gravitational attraction between the earth and apple. If F_{1} 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) F_{1} is very much greater than F_{2}
(b) F_{2} is very much greater than F_{1}
(c) F_{1} is only a little greater than F_{2}
(d) F_{1} and F_{2} are equal
Ans: (d)
Explanation: Netwon’s third law of motion states that for every action there is equal and opposite reaction. Hence F_{1 }and F_{2} are equal.
Q.16. What is the source of centripetal force that a planet requires to revolve around the Sun? On what factors does that force depend?
Ans: Gravitational force is the source of centripetal force required to revolve around the sun. This force depends on the distance between the planet and sun along with their masses. If this force becomes zero as a result of the absence of centripetal force, the planet would shift to moving tangentially outwards to the circular route.
Q.17. 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 the stones reach the ground simultaneously as they are dropped from the same height and their initial velocity will be the same.
Q.18. Suppose gravity of earth suddenly becomes zero, then in which direction will the moon begin to move if no other celestial body affects it?
Ans: If there is no gravitational pull from the earth, the moon starts to move in a straight line tangent to its circular path.
Q.19. 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.
Ans:
Q.20. 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?
Ans:
Weight of person on moon = 1/6^{th} of weight on earth
Therefore, ‘g’ on moon = 1/6th ‘g’ on earth
The force that is applied by the man to lift mass ‘m’ is
F = mg = 15g (on earth)
If he can lift a certain mass ‘m’ by applying the same force on moon, then
F = 15*6 = 90kg
This proves acceleration due to gravity on the moon is 1/6^{th} of acceleration due to gravity on earth. Hence the person can lift a mass 6 times heavier on moon than on earth.
Q.21. Calculate the average density of the earth in terms of g, G and R.
Ans:
Acceleration due to gravity, g = GM /R^{2}, where R is the radius of the Earth.
Mass of the Earth, M = gR^{2}/G
Density of earth D= mass/volume
= gR^{2}/G x Ve
= gR^{2}/G x 4πR^{3}/3
= 3g/4πGR
Q.22. The earth is acted upon by gravitation of Sun, even though it does not fall into the Sun. Why?
Ans: Sun provides enough centripetal force to keep the earth in its orbit and the earth provides centrifugal force due to its motion. These two forces balance each other which prevents the earth from falling into the sun.
Q.23. 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: Let R and M be the radius and mass of the earth
Then, the 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 the object ∝ M/R^{2}
Original weight W_{0} = mg = mG M/R^{2}
Hypothetically M becomes 4M and R becomes R/2
Then, weight = mG 4M /(R/2) ^{2}
= (16m G) M/R^{2}
= 10xW_{0}
Weight will be 16 times.
Q.24. 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.
Ans:
Q.25. 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:
We know that
v=u+at
Here v=0 and a=g
so 0=u+gt
u=−gt  (i)
we also know
v^{2} u^{2}=2as (ii)
Here v=0,u=−gt from eqn (i) and S=h_{1} and taking t=t_{1}
putting all above values in eq (ii)
g^{2}t_{1}^{2}=2gh_{1}
or
h_{1}=1/2gt_{1}^{2} (iii)
Similarly for S=h_{2} and t=t_{2}
h_{2}=1/2gt_{2}^{2} (iv)
Dividing eqn (iii) & (iv) we get,
t_{1}/t_{2}=√h_{1}/h_{2}
The ratio will not change in either case because acceleration remains the same. In the case of freefall acceleration does not depend upon mass and size of the body.
26. (a) 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})
Ans: (i) 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 buoyancy in a saturated solution. If the cube is reduced to 4 cm on each side, the 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
66 videos352 docs97 tests

1. What is gravitation? 
2. How does the force of gravity work? 
3. What is the difference between mass and weight? 
4. How does the force of gravity affect objects on Earth? 
5. What is the universal law of gravitation? 
66 videos352 docs97 tests


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