# Mechanical Properties of Fluids

## Class 11 NCERT Physics

### NCERT

1   Explain why$\\$ (a)The blood pressure in human s is greater at the feet than at the brain$\\$ (b)Atmospheric pressure at a height of about $6 km$ decreases to nearly half of its value at the sea level, though the height of the atmosphere is more than $100 km .$$\\ (c)Hydrostatic pressure is a scalar quantity even though pressure is force divided by area. ##### Solution : (a)The pressure of a liquid is given by the relation:\\ P = h \rho g$$\\$ Where,$\\$ $P =$ Pressure$\\$ $h =$Height of the liquid column$\\$ $\rho=$ Density of the liquid$\\$ $g =$ Acceleration due to the gravity$\\$ It can be inferred that pressure is directly proportional to height. Hence, the blood pressure in human vessels depends on the height of the blood column in the body. The height of the blood column is more at the feet than it is at the brain. Hence, the blood pressure at the feet is more than it is at the brain.$\\$ (b)Density of air is the maximum near the sea level. Density of air decreases with increase in height from the surface. At a height of about $6 km,$ density decreases to nearly half of its value at the sea level. Atmospheric pressure is proportional to density. Hence, at a height of $6 km$ from the surface, it decreases to nearly half of its value at the sea level.$\\$ (c)When force is applied on a liquid, the pressure in the liquid is transmitted in all directions. Hence, hydrostatic pressure does not have a fixed direction and it is a scalar physical quantity.

2   Explain why$\\$ (a)The angle of contact of mercury with glass is obtuse, while that of water with glass is acute.$\\$ (b)Water on a clean glass surface tends to spread out while mercury on the same surface tends to form drops. (Put differently, water wets glass while mercury does not.)$\\$ (c)Surface tension of a liquid is independent of the area of the surface$\\$ (d)Water with detergent dissolved in it should have small angles of contact.$\\$ (e)A drop of liquid under no external forces is always spherical in shape