SUBJECT: PHYSICS(
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Read the following questions carefully and answer:
(A) If both Assertion and reason are true and the reason is correct explanation of the Assertion.
(B) If both assertion and Reason are true, but Reason is not correct explanation of the Assertion.
(C) If Assertion is true, but the Reason is false.
(D) If Assertion is false, but the Reason is true.
Question:1
Assertion : The shape of a liquid drop is spherical.
Reason : The pressure inside the drop is greater than that outside.
Solution:1
(B)The shape of liquid drop is spherical due to surface tension of liquid.
Therefore both Assertion and Reason are true, but Reason is not correct explanation of the Assertion.
Question:2
Assertion : All the raindrops hit the surface of the earth with the same constant velocity.
Reason : An object falling through a viscous medium eventually attains a terminal velocity.
Solution:2
(D) Here Assertion is wrong; hence Reason though correct cannot explain assertion.
Question:3
Assertion : Machine parts are jammed in winter.
Reason : The viscosity of libricant used in machine parts increase at low temperature.
Solution:3
(A) With the decrease in temperature the viscosity of lubricant increase. Due to which the machine parts are jammed in winter.
Question:4
Assertion : A needle placed carefully on the surface of water may float, whereas a ball of the same material will always sink.
Reason : The buoyancy of an object depends both on the material and shape of the object.
Solution:4
( C ) A needle placed carefully on the surface of water may float due to surface tension, as upward forces due to surface tension balances the weight of the needle. But these upward forced due to surface tension are very small as compared to weight of ball, also the weight of liquid displaced by the ball immersed in liquid is less than the weight of the ball, hence ball sinks into the liquid.
Question:5
Assertion : A rain drop after falling through some height attains a constant velocity.
Reason : At constant velocity, the viscous drag is just equal to its weight.
Solution: 5
(A) When a body falls through a viscous medium, finally, it attains terminal velocity. At this velocity, viscous force on raindrop balances the weight of the body.
Question:6
Assertion : Aeroplanes are made to run on the runway before take off, so that they acquire the necessary lift.
Reason : This is as per Bernoulli?s theorem.
Solution:6
(A) According to Bernoulli?s theorem, when wind vel. over the wings is larger than the wind velocity under the wings, pressure of wind over the wings becomes less than the pressure of wind under the wings. This provides the necessary lift to the aeroplane.
Question:7
Assertion : Mean free path of gas molecules varies inversely as density of the gas.
Reason : Mean free path varies inversely as pressure of the gas.
Solution:7
(B) As density increases, chances of molecular collision increases. The mean free path decreases. The reason, though true, is not a correct explanation of the assertion.
Question:8
Assertion : All molecular motion ceases at ? 2730C.
Reason : Temperature below ? 2730C cannot be attained.
Solution:8
(B) When T = -2730C = -273 + 273 = 00K,
C = 0 i.e. all molecular motion stops. The reason is correct, but it has no relevance with the assertion.
Question:9
Assertion : For the flow to be streamline, value of critical velocity should be as low as possible.
Reason : Once the actual velocity of flow of a liquid becomes greater than the critical velocity, the flow becomes turbulent.
Solution:9
(D) The moment actual velocity of flow of liquid exceeds critical velocity, the flow becomes turbulent. Hence of the flow to be streamline, the limiting value of critical velocity should be as large as possible.
Question:10
Assertion : When height of a tube is less than calculated height of liquid in the tube, the liquid does not overflow.
Reason : The meniscus of liquid at the top of the tube becomes flat.
Solution:10
(A) It can be shown that R ´ h = constant, where R is radius of curvature of the meniscus of liquid in the tube. When height of tube is less the meaniscus becomes flat i.e. R = ¥. That is why liquid does not overflow.
Question:11
Assertion : Hot soap solution is preferred for washing dirty clothes.
Reason : Surface tension of soap solution decreases on heating.
Solution:11
(A) On heating, surface tension of soap solution decreases. It can spread much faster and take out the dirt.
Question:12
Assertion : To protect any instrument from external magnetic field, it is put inside an iron box.
Reason : Iron is a magnetic substance.
Solution:12
(B) Both the assertion and reason are true, but reason is not a correct explanation of the assertion. Infact, iron is a ferromagnetic material.
