fluids | Aster Classes

III CHAPTER FLUIDS

I. Choose the correct answer.

Question 1.

The size of an air bubble rising up in water ……………. .

(a) decreases

(b) increases

(c) remains same

(d) may increase or decrease

Answer:

(a) decreases


Question 2.

Clouds float in atmosphere because of their low ……………. .

(a) density

(b) pressure

(c) velocity

(d) mass

Answer:

(a) density


Question 3.

In a pressure cooker, the food is cooked faster because ……………. .

(a) increased pressure lowers the boiling point

(b) increased pressure raises the boiling point

(c) decreased pressure raises the boiling point

(d) increased pressure lowers the melting point

Answer:

(a) increased pressure lowers the boiling point


Question 4.

An empty plastic bottle closed with an airtight stopper is pushed down into a bucket filled

with water. As the bottle is pushed down, there is an increasing force on the bottom as shown in graph. This is because

(a) more volume of liquid is displaced

(b) more weight of liquid is displaced

(c) pressure increases with depth

(d) all the above

Answer:

(d) all the above


II. Fill in the blanks.

  1.  The weight of the body immersed in a liquid appears to be ………….. than its actual weight
  2.  The instrument used to measure atmospheric pressure is …………….. .
  3.  The magnitude of buoyant force acting on an object immersed in a liquid depends on ………….. of the liquid.
  4.  A drinking straw works on the existence of …………….. .

Answer:

  1.  less
  2.  barometer
  3.  density
  4. Pressure

III. True or False.

Question 1.

The weight of fluid displaced determines the buoyant force on an object.

Answer:

True.


Question 2.

The shape of an object helps to determine whether the object will float.

Answer:

True.


Question 3.

The foundations of high-rise buildings are kept wide so that they may exert more pressure on the ground.

Answer:

True.


Question 4.

Archimedes’ principle can also be applied to gases.

Answer:

False.

Correct statement: Archimedes’ principle is about fluid displacement which does not involve gases.


Question 5.

Hydraulic press is used in the extraction of oil from oil seeds.

Answer:

True.


IV. Match the following.

Column – IColumn – II
(a) Density(i) hpg
(b) 1 gwt(ii) Milk
(c) Pascal’s law(iii) MassVolume
(d) Pressure exerted by a fluid(iv) Pressure
(e) Lactometer(v) 980 dyne

Answer:

(a) (iii)

(b) (v)

(c) (iv)

(d) (i)

(e) (ii)


V. Answer in brief.

Question 1.

On what factors the pressure exerted by the liquid depends on?

Answer:

Pressure exerted by a liquid at a point is determined by,

  •  depth (h)
  •  density of the liquid (p)
  •  acceleration due to gravity (g).

Question 2.

Why does a helium balloon float in air?

Answer:

Helium is much less denser than ordinary air which gives them buoyancy and thus float in air.


Question 3.

Why it is easy to swim in river water than in sea water?

Answer:

Due to the presence of dissolved salts in sea water is denser than river water which makes floating easier and hence swimming is easier.


Question 4.

What is meant by atmospheric pressure?

Answer:

The pressure exerted by the atmospheric gases on its surroundings and on the surface of the earth is called atmospheric pressure.


Question 5.

State Pascal’s law.

Answer:

Pascal’s law states that an increase in pressure at any point inside a liquid at rest is transmitted equally and without any change, in all directions to every other point in the liquid.


VI. Answer in detail.

Question 1.

With an appropriate illustration prove that the force acting on a smaller area exerts a greater pressure.

Answer:

Consider standing on loose sand. Your feet go deep into the sand. Now, when you lie down on the sand, you will find that your body will not go that deep into the sand. In both the cases the force exerted on the sand is the weight of your body which is the same. This force acting perpendicular to the surface is called thrust. When you stand on loose sand, the force is acting on an area equal to the area of your feet.

When you lie down, the same force acts on an area of your whole body, which is larger than the area of your feet. Therefore the effect of thrust, that is, pressure depends on the area on which it acts. The effect of thrust on sand is larger while ‘ standing than lying.


Question 2.

Describe the construction and working of mercury barometer.

Answer:

The instrument used to measure atmospheric pressure is called barometer.

A mercury barometer, first designed by an Italian Physicist Torricelli, consists of a long glass tube (closed at one end, open at the other) filled with mercury and turned upside down into a container of mercury.

This is done by closing the open end of the mercury filled tube with the thumb and then opening it after immersing it in to a trough of mercury.

The barometer works by balancing the mercury in the glass tube against the outside air pressure.

If the air pressure increases, it pushes more of the mercury up into the tub and if the air pressure decreases, more of the mercury drains from the tube. As there is no air trapped in the space between mercury and the closed end, there is vacuum in that space. Vacuum cannot exert any pressure. So the level of mercury in the tube provides a precise measure of air pressure which is called atmospheric pressure. This type of instrument can be used in a lab or weather station.


Question 3.

How does an object’s density determine whether the object will sink or float in water?

Answer:

Whether an object will sink or float in a liquid is determined by the density of the object compared to the density of the liquid. If the density of a substance is less than the density of the liquid it will float. For example, a piece of wood which is less dense than water will float on it. Any substance having more density than water (for example, a stone), will sink into water.


Question 4.

Explain the construction and working of a hydrometer with diagram.

Answer:

A direct-reading instrument used for measuring the density or relative density of the liquid is called hydrometer. Hydrometer is based on the principle of flotation, i.e., the weight of the liquid displaced by the immersed portion of the hydrometer is equal to the weight of the hydrometer.

Hydrometer consists of a cylindrical stem having a spherical bulb at its lower end and a narrow tube at its upper end. The lower spherical bulb is partially filled with lead shots or mercury. This helps hydrometer to float or stand vertically in liquids. The narrow tube has markings so that relative density of a liquid can be read directly.

The liquid to be tested is poured into the glass jar. The hydrometer is gently lowered into the liquid until it floats freely. The reading against the level of liquid touching the tube gives the relative density of the liquid.


Question 5.

State the laws of flotation.

Answer:

Laws of flotation are,

  1. The weight of a floating body in a fluid is equal to the weight of the fluid displaced by the body.
  2. The centre of gravity of the floating body and the centre of buoyancy are in the same vertical line.

VII. Assertion and Reason.

Mark the correct answer as

(a) If both assertion and reason are true and reason is the correct explanation of assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of assertion.

(c ) If assertion is true but reason is false.

(d) If assertion is false but reason is true.


Question 1.

Assertion (A): To float, body must displace liquid whose weight is equal to the actual weight.

Reason (R): The body will experience no net downward force in that case.

Answer:

(a) If both assertion and reason are true and reason is the correct explanation of assertion.


Question 2.

Assertion (A): Pascal’s law is the working principle of a hydraulic lift.

Reason (R): Pressure is thrust per unit area.

Answer:

(b) If both assertion and reason are true but reason is not the correct explanation of assertion.


VIII. Numerical problems.

Question 1.

A block of wood of weight 200 g floats on the surface of water. If the volume of block is 300 cm3 calculate the upthrust due to water.

Answer:

Upthrust of floating object = weight of the water displaced

Weight = mg

= 0.200Kg × \(\frac { 10m }{ s2 }\)

= 2N


Question 2.

Density of mercury is 13600 kg m– 3. Calculate the relative density.

Answer:

Relative Density = 

DensityofMercury

Densityofwater

 at 4°C

R.D. = 13.6


Question 3.

The density of water is 1 g cm– 3. What is its density in S.I. units?

Answer:

S.I. unit of density of water = \(\frac { 1000kg }{ m3 }\)


Question 4.

Calculate the apparent weight of wood floating on water if it weighs 100g in air.

Answer:

Apparent weight = Weight of the body – Weight of liquid

Since the body is floating the two are equal. So, apparent weight is zero.

100 – 100 = 0


IX. HOTS

Question 1.

How high does the mercury barometer stand on a day when atmospheric pressure is 98.6 kPa?

Answer:

H= 

P

max

P

H

g

 = 98.6 × 10 × 10 × 10

(N/m

)

2

13.6×10×10×10Kg/

m

3

= \(\frac { 9.8m }{ s2 }\) = 740mm.


Question 2.

How does a fish manage to rise up and move down in water?

Answer:

Fish has an internal swim bladder which is filled with gas. When it needs to rise or descend, it changes the volume and its density by filling this bladder with oxygen collected from the surrounding water through gills. When the bladder is filled with oxygen gas, the fish has a greater volume, with minimal increase in weight. When the bladder is expanded, it displaces more water and so experiences a greater force of buoyancy.

When the bladder is completely inflated, the fish has maximum volume and is pushed to the surface. When the bladder is completely deflated, the fish has minimum volume and sinks to the ocean floor.


Question 3.

If you put one ice cube in a glass of water and another in a glass of alcohol, what would you observe? Explain your observations.

Answer:

Ice floats in water and not in alcohol. This is because the density of ice is 

0.917g

cc

 which is lower than that of water which is 1. Whereas, the density of ethanol (alcohol) is only 

0.789g

cc

 which is lesser than ice, hence it floats in alcohol.


Question 4.

Why does a boat with a hole in the bottom would eventually sink?

Answer:

A boat with a hole at the bottom allows water to enter it, thus increasing its weight and hence it sinks. As water starts entering the boat through the hole, the boat starts to get heavier, so it starts to sink, trying to displace more water. But the water keeps coming as the hydrostatic pressure at the hole is always higher than the atmospheric pressure pushing down on the surface of the water in the boat.


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