Physics · Properties of bulk matter · chapter 9 of 14

Class 11 Physics Chapter 9: Mechanical Properties of Fluids Assertion-reason questions with answers

2 assertion-reason questions on Mechanical Properties of Fluids, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter. Each question gives an Assertion (A) and a Reason (R); choose how they stand to each other.

Question 1
Assertion (A): A submarine designed to dive 1 km below the sea surface must have a far stronger hull than one that only goes 10 m down. Reason (R): The pressure in a liquid at rest increases with depth below its surface.
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Answer: (a) Both A and R are true, and R is the correct explanation of A.

Both are true. P = Pa + ρgh grows with depth h: at 10 m the pressure is about 2 atm, at 1 km the increase alone is about 100 atm, so the deep-diving hull must withstand a far larger pressure. R explains A.

Question 2
Assertion (A): The viscosity of a gas increases as its temperature rises. Reason (R): The surface tension of a liquid usually falls as its temperature rises.
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Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Both statements are true, but the surface tension of a liquid has nothing to do with the viscosity of a gas. A gas's viscosity rises because heating increases the random motion of its molecules. R does not explain A.

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