# NCERT Solutions for Class 11 Physics Chapter 10 Mechanical Properties of Fluids

## NCERT Solutions Class 11 Physics Chapter 10 Mechanical Properties of Fluids

NCERT Solutions for Class 11 Physics Chapter 10 Mechanical Properties of Fluids is designed and prepared by the best teachers across India. All the important topics are covered in the exercises and each answer comes with a detailed explanation to help students understand concepts better. These NCERT solutions play a crucial role in your preparation for all exams conducted by the CBSE, including the JEE.

### NCERT TEXTBOOK QUESTIONS SOLVED

**1. Explain why**

(a) The blood pressure in humans 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.

(a) The blood pressure in humans 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.

**Ans.** (a) The height of the blood column is more for the feet as compared to that for the brain.

Consequently, the blood pressure in humans is greater at the feet than at the brain.

(b) The variation of air-density with height is not linear. So, pressure also does not reduce linearly with height. The air pressure at a height h is given by P = P0eâ€“Î±h where P0 represents the pressure of air at sea-level and Î± is a constant.

(c) Due to applied force on liquid, the pressure is transmitted equally in all directions inside the liquid. That is why there is no fixed direction for the pressure due to liquid. Hence hydrostatic pressure is a scalar quantity.

**2. Explain why**

(a) To keep a piece of paper horizontal, you should blow over, not under, it.

(b) When we try to close a water tap with our fingers, fast jets of water gush through the openings between our fingers.

(c) The size of a needle of a syringe controls flow rate better than the thumb pressure exerted by a doctor while administering an injection.

(d) A fluid flowing out of a small hole in a vessel results in a backward thurst on the vessel.

(e) A spinning cricket ball in air does not follow a parabolic trajectory.

(a) To keep a piece of paper horizontal, you should blow over, not under, it.

(b) When we try to close a water tap with our fingers, fast jets of water gush through the openings between our fingers.

(c) The size of a needle of a syringe controls flow rate better than the thumb pressure exerted by a doctor while administering an injection.

(d) A fluid flowing out of a small hole in a vessel results in a backward thurst on the vessel.

(e) A spinning cricket ball in air does not follow a parabolic trajectory.

**Ans.** (a) When we blow over the piece of paper, the velocity of air increases. As a result, the pressure on it decreases in accordance with the Bernoulliâ€™s theorem whereas the pressure below remains the same (atmospheric pressure). Thus, the paper remains horizontal.

(b) By doing so the area of outlet of water jet is reduced, so velocity of water increases according to equation of continuity av = constant.

(c) For a constant height, the Bernoulliâ€™s theorem is expressed as P +1/2 Ï v2 = Constant

In this equation, the pressure P occurs with a single power whereas the velocity occurs with a square power. Therefore, the velocity has more effect compared to the pressure. It is for this reason that needle of the syringe controls flow rate better than the thumb pressure exerted by the doctor.

(d) This is because of principle of conservation of momentum. While the flowing fluid carries forward momentum, the vessel gets a backward momentum.

(e) A spinning cricket ball would have followed a parabolic trajectory has there been no air. But because of air the Magnus effect takes place. Due to the Magnus effect the spinning cricket ball deviates from its parabolic trajectory.

**3. A vertical off-shore structure is built to withstand a maximum stress of 109 Pa. Is the structure suitable for putting up on top of an oil well in the ocean? Take the depth of the ocean to be roughly 3 km, and ignore ocean currents.**

**Ans.** Here, Maximum stress = 109 Pa, h = 3 km = 3 x 103 m;

p (water) = 103 kg/m3 and g = 9.8 m/s2.

The structure will be suitable for putting upon top of an oil well provided the pressure exerted by sea water is less than the maximum stress it can bear.

Pressure due to sea water, P = hÏg = 3 x 103 x 103x 9.8 Pa = 2.94 x 107 Pa

Since the pressure of sea water is less than the maximum

**4. Can Bernoulliâ€™s equation be used to describe the flow of water through a rapid motion in a river? Explain.**

**Ans.** Bernoulliâ€™s theorem is applicable only for there it ideal fluids in streamlined motion. Since the flow of water in a river is rapid, way cannot be treated as streamlined motion, the theorem cannot be used.

**5. Does it matter if one uses gauge instead of absolute pressures in applying Bernoulliâ€™s equation? Explain.**

**Ans.** No, it does not matter if one uses gauge instead of absolute pressures in applying Bernoulliâ€™s equation, provided the atmospheric pressure at the two points where Bernoulliâ€™s equation is applied are significantly different.