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181.

182. What is the effect of temperature on coefficient of viscosity of a liquid?
Viscosity is the phenomena in which liquid molecules would try more to rest rather than move, because of their internal friction mechanism. If temperature is increased, the molecules get energetic and perform higher degree of motion. So the viscosity would be less. In reverse, if the temperature decreases, they would be more at rest, so the viscosity would increase. The relation for the viscosity at different temperatures is
ηt = η0/(1+αt+βt2)
where α and β are constant quantities.

183. Roofs of the huts are blown up during stormy days. Explain why?

 When there is storm, the speed of air outside houses is more whereas those inside is very less. According to the Bernoulli's equation, , the pressure of air will be less in the region with high velocity, i.e. outside and reverse in the region with less velocity, i.e. inside. When this difference of pressure is excessive. the roof will be lifted off and it blows away.

184. Explain in brief how an aeroplane takes off? Use Bernoulli's principle.

185. An aeroplane requires a long run on the ground before taking off. Explain why?

187. When air is blown in between two balls suspended close to each other, they are attracted towards each other. Why?

 When the balls were free, the overall speeds of air molecules all around the balls will be same, so thet the pressure will also be same. However, if air is blown through the space between them, the velocity of air there will be more, then according to the relation , the pressure decreases in that region. But in the regions around the balls, the velocity is less, so pressure is high. These two pressure zones create a pressure imbalance, which develops a force in inward direction, pushing the balls near each other.

188. People think that a fast moving train attracts a person standing nearby its track. Is it true? Explain.
When a fast moving train passes by a person standing by, the air between the person and the train moves with a high speed. According to the Bernoulli formula , the air pressure becomes low there compared to the pressure at other regions (including behind the person). So there is a gust of air from the back of the person to the front, pushing the person on the way.

189. Two rain drops - one large and the other small, fall from a height. Which one reaches the ground first?
or
A man and a child fall from the top of the Dharahara without parachutes. Who is likely to get more injured and why?
The terminal velocity of a body through a fluid is given by of an object through a liquid or any fluid is proportional to the square of its radius. Then the relation tells that if the radius increases, the terminal velocity also increases. If the body is large as a large drop, they have larger radius, so the terminal velocity will be more and it will reach the ground early. However, if the radius is small, as in the case of a small drop, the terminal velocity will be less and the body requires more time to reach the ground.

190. A man needs a parachute for the safe falling from a height; whereas, an ant does not need it for a safe falling from the same height. Why?

The need or no need for a parachute depends on the terminal velocity the falling substances can gain during their free descent. In case of a person, the dimensions are large. This is more like having more 'r'. So according to the relation , the terminal velocity is high, which if continued up to the ground can cause sure fatal injuries. In order to prevent that the upward reaction force due air is increased by the use of flatter surfaces of parachutes. However in case of ants, the dimensions are smaller, much like having small 'r'. The terminal velocity will be low thereafter which minimizes the chance of fatality. So there is no need of parachutes.

191.
Dust particles float in the air why?
Dust particles have very small mass and very small dimensions as well, which means they have small 'r' (though they are exactly not spherical). since the terminal velocity depends on 'r'; as shown by the relation , their terminal velocity will be very small. This means they fall at a very small rate, so small that the fall is almost not noticeable. That's why they seem to float in the air.

192. When a part of the mouth of water tap or pipe is made smaller with fingers, the velocity of the water flow increases water. Why?
The flow of incompressible fluids like water is governed by the relation of continuity, i.e. A1v1 = A2v2, which makes the product of 'A' and 'v' constant. That means 'A' and 'v' are inversely proportional to each other. When the mouth of the pipe is wide, the cross section available for flow is very large. Therefore the velocity will be less. If it is squeezed or is made small by any means, the cross-section available for flow will be less and so the velocity will be high.

193. Why do machine parts get jammed in winter?
A lubricating oil is used between machine parts to reduce friction between various structures. In winter the outer temperature decreases, so the viscosity of the oils used also increases (viscosity increases with decrease of temperature and also decreases with increase of temperature.). This makes the movement of the machinery parts over one another difficult. This jams them.

194. Why do deep rivers run slow?
The flow of incompressible fluids like water is governed by the relation of continuity, i.e. A1v1 = A2v2, which makes the product of 'a' and 'v' constant. That means 'a' and 'v' are inversely proportional to each other. In deep rivers, the cross section available for flow is very large. Therefore the velocity will be very less. This is even more prominent in seas, where the cross section is so large that the flow is almost nonexistent.

195.

196. Why do small air bubbles rise slowly while big bubbles rise rapidly through the liquid?

or
Why do small balls fall slowly and large balls fall rapidly in a liquid?
The terminal velocity of a body through a fluid is given by of an object through a liquid or any fluid is proportional to the square of its radius as given by the relation . If the radius increases, the terminal velocity also increases. If the bubbles are large, they have larger radius, so the terminal velocity will be more. However, if the radius is small, as in the case of a small bubble, the terminal velocity will be less.

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