NCERT Class 11 Physics Solutions: Chapter 10 – Mechanical Properties of Fluids-Part 3

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Question 10.4:

Explain why

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

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

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

A fluid flowing out of a small hole in a vessel results in a backward thrust on the vessel

A spinning cricket ball in air does not follow a parabolic trajectory

Answer:

To keep a piece of paper horizontal, you should blow over, not under, it:

Explanation:

When air is blown under a paper, the velocity of air is greater under the paper than it is above it. As per Bernoulli’s principle, atmospheric pressure reduces under the paper. This makes the paper fall. To keep a piece of paper horizontal, one should blow over it. This increases the velocity of air above the paper. As per Bernoulli’s principle, atmospheric pressure reduces above the paper and the paper remains horizontal.

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

Explanation:

According to the equation of continuity:

For a smaller opening, the velocity of flow of a fluid is greater than it is when the opening is bigger. When we try to close a tap of water with our fingers, fast jets of water gush through the openings between our fingers. This is because very small openings are left for the water to flow out of the pipe. Hence, area and velocity are inversely proportional to each other.

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

Explanation:

The small opening of a syringe needle controls the velocity of the blood flowing out. This is because of the equation of continuity. At the constriction point of the syringe system, the flow rate suddenly increases to a high value for a constant thumb pressure applied.

A fluid flowing out of a small hole in a vessel results in a backward thrust on the vessel:

Explanation:

When a fluid flows out from a small hole in a vessel, the vessel receives a backward thrust. A fluid flowing out from a small hole has a large velocity according to the equation of continuity:

According to the law of conservation of momentum, the vessel attains a backward velocity because there are no external forces acting on the system.

A spinning cricket ball in air does not follow a parabolic trajectory:

Explanation:

A spinning cricket ball has two simultaneous motions – rotatory and linear. These two types of motion oppose the effect of each other. This decreases the velocity of air flowing below the ball. Hence, the pressure on the upper side of the ball becomes lesser than that on the lower side. An upward force acts upon the ball. Therefore, the ball takes a curved path. It does not follow a parabolic path.

Question 10.5:

A girl wearing high heel shoes balances on a single heel. The heel is circular with a diameter . What is the pressure exerted by the heel on the horizontal floor?

Answer:

Mass of the girl,

Diameter of the heel,

Radius of the heel,

Area of the heel

Force exerted by the heel on the floor:

Pressure exerted by the heel on the floor:

Therefore, the pressure exerted by the heel on the horizontal floor is