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#	Question	Answer
1	Is pressure a scalar or a vector quantity?	Scalar.
2	What is the density of water at 4°C?	\( 1.0 \times 10^3 \) kg m⁻³.
3	What is the principle behind hydraulic machines?	Pascal's law: pressure applied on a fluid is transmitted undiminished and equally in all directions.
4	What is steady flow?	A flow in which the velocity of each fluid particle at any given point remains constant in time.
5	What is Torricelli's law?	The speed of efflux from an open tank is \( v = \sqrt{2gh} \).
6	What is turbulent flow?	The flow that loses steadiness beyond a critical speed and becomes irregular.
7	How does the viscosity of gases change with temperature?	It increases.
8	What is laminar flow?	A flow in which the layers of the fluid slide smoothly over one another.
9	What is the angle of contact?	The angle between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid.
10	Why does a liquid rise in a capillary tube?	Due to surface tension.
11	Why does water rise in a capillary tube?	Because the meniscus is concave, leading to a pressure difference that supports the liquid column.
12	How does the surface tension of liquids change with temperature?	It decreases.
13	What is the surface tension of a soap film that supports a weight of \( 1.5 \times 10^{-2} \) N with a slider of length 30 cm?	\( 2.5 \times 10^{-2} \) N m⁻¹.
14	Three differently-shaped vessels A, B, C, connected at the bottom, are filled with water to the same height. Do they hold the same amount of water, and is the pressure the same at the bottom of each?	They hold different amounts of water, but the pressure at the bottom is the same in all three — this is the 'hydrostatic paradox', since pressure depends only on height, not on the vessel's shape or base area.
15	Why do hydraulic brakes apply equal braking force to all four wheels of a car?	Because the pressure set up by pressing the brake pedal is transmitted equally and undiminished to the cylinders at all four wheels (Pascal's law).
16	How does the viscosity of water at 100°C compare to 20°C, from Table 9.2?	Much lower — about 0.3 mPl at 100°C vs 1.0 mPl at 20°C, illustrating that liquid viscosity falls sharply with temperature.
17	What is the density of mercury, from Table 9.1?	\(13.6\times10^3\) kg m⁻³.
18	Is surface tension a property of a single fluid alone?	No, it is a property of the interface between two substances (at least one a fluid), and depends on both materials.
19	A small sphere falls through a viscous liquid and reaches terminal velocity. How does the rate of heat production at terminal velocity scale with the sphere's radius r?	\(\propto r^5\): the viscous force at terminal velocity equals the net weight (\(\propto r^3\)), and \(v_t\propto r^2\), so power \(=F\times v_t\propto r^5\). (NEET/AIPMT 2018)
20	Two metal balls of equal mass have densities \(\rho_1=8\rho_2\) and radii 1 mm and 2 mm respectively, and fall through a medium of density \(0.1\rho_2\). What is the ratio of their terminal velocities \(v_1/v_2\)?	79/36, using \(v\propto r^2(\rho_{ball}-\sigma)\): ball 1 gives \(1^2(8-0.1)=7.9\), ball 2 gives \(2^2(1-0.1)=3.6\). (NEET 2019, Odisha)
21	A thin circular disc of radius 4.5 cm rests on a water surface (S=0.07 N/m). What is the excess force needed to lift it off the surface?	≈ 19.8 mN, from \(F=S\times2\pi r\) (surface tension acts along the disc's perimeter). (NEET 2024)