# NCERT Physics Class 11 Exemplar Ch 10 Mechanical Properties of Fluids Part 4

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# Short Answer

Q. 16 The sap in trees, which consists mainly of water in summer, rises in a system of capillaries of radius The surface tension of sap is and the angle of contact is Does surface tension alone account for the supply of water to the top of all trees?

Answer: The height to which the sap will rise is

This is the maximum height to which the sap can rise due to surface tension. Since many trees have heights much more than this, capillary action alone cannot account for the rise of water in all trees.

Q.17 The free surface of oil in a tanker, at rest, is horizontal. If the tanker starts accelerating the free surface will be titled by an angle . If the acceleration is a , what will be the slope of the free surface?

Answer: If the tanker accelerates in the positive x direction, then the water will bulge at the back of the tanker. The free surface will be such that the tangential force on any fluid parcel is zero.

Consider a parcel at the surface, of unit volume. The forces on the fluid are

and

The component of the weight along the surface is

The component of the acceleration force along the surface is

Hence, . 18 Two mercury droplets of radii . and . collapse into one single drop. What amount of energy is released? The surface tension of mercury

Answer: Let and be the volume of the droplets and v of the resulting drop.

Then

Q. 19 If a drop of liquid breaks into smaller droplets, it results in lowering of temperature of the droplets. Let a drop of radius R, break into N small droplets each of radius r. Estimate the drop-in temperature.

Answer:

Suppose all this energy is released at the cost of lowering the temperature. If s is the specific heat then the change in temperature would be,