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.
197.
198.
199.
