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Class 10 Class 12

Mechanical Properties of Fluids

Two beakers containing water and honey are stirred rapidly and kept on a table. Which liquid will come to rest earlier?

The coefficient of viscosity of honey is very large as compared to that of water. Therefore, honey kept in the beaker after stirring will come to rest earlier than water.

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Explain why a parachute is invariably used, while jumping from an aeroplane.

The parachute during descending, through air experiences very large viscous force due to large structure. The large structure of the parachute leads to a greater air resistance and hence a tendency to reach a slower terminal velocity. Therefore a person landing on ground will get very little hurt.

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What are the limitations of Bernoulli's equation?

Limitations of Bernoulli's equations are as follows:

(i) Bernoulli's equation is derived by assuming that there is no loss of mechanical energy i.e. sum of pressure energy, potential energy and kinetic energy is constant. But when fluid is in motion, a part of mechanical energy is converted into heat energy.

(ii) Bernoulli's equation is derived by assuming that liquid is non-viscous. But actually none of the liquids is non-viscous. All liquids have a tendency to flow, less or more.

(iii) Bernoulli's equation is derived by assuming that velocity of every fluid particle across any cross-section of pipe is same. But the particles of central layer have maximum velocity and velocity decreases towards the walls of pipe.

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Dust generally settles down in a closed room. Explain.

Dust particles being small spherical particles remain suspended in air during dusting.

Terminal velocity is given by v ∝ r^2.Since the dust particles are minutely small, these particles will acquire a small terminal velocity. Hence, the particles will settled down in a closed room after some time.

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A soap bubble of radius 10mm is blown from soap solution of surface tension 0.06 N/m. Find the work done in blowing the bubble. What addition work will be done in further blowing to double the radius?

The soap bubble is formed from the soap solution.

Therefore, increase in the surface area of soap bubble is equal to total surface area of soap bubble.

Since the soap bubble has two free surfaces, therefore increase in area of free surface of bubble is,