Selina solutions concise physics class 9 icse Solutions for Chapter 4 chapter 4 pressure in fluids and atmospheric pressure

Exercises in Chapter 4 chapter 4 pressure in fluids and atmospheric pressure Grade 9

Exercise 4A

Exercise 4B

Questions in Exercise 4A

Define the term thrust. State its S.I. unit.

What is meant by pressure? State its S.I. unit.

(a) What physical quantity is measured in a bar?

(b) How is the unit bar related to the S.I. unit pascal?

Define one pascal (Pa), the S.I. unit of pressure.

State whether thrust is a scalar or vector?

State whether the pressure is a scalar or vector?

Differentiate between thrust and pressure.

How does the pressure exerted by a thrust depend on the area of the surface on which it acts? Explain with a suitable example.

Why is the tip of an allpin made sharp?

Explain the following:

(a) It is easier to cut with a sharp knife than with a blunt one.

(b) Sleepers are laid below the rails

What is fluid?

What do you mean by the term fluid pressure?

How does the pressure exerted by a solid and a fluid differ?

Describe a simple experiment to demonstrate that a liquid enclosed in a vessel exerts pressure in all directions.

State three factors on which the pressure at a point in a liquid depends.

Write an expression for the pressure at a point inside a liquid. Explain the meaning of the symbols used.

Deduce an expression for the pressure at a depth inside a liquid.

How does the pressure at a certain depth in sea water differ from that at the same depth in river water? Explain your answer.

The pressure at the free surface of a water lake is P1, while at a point at depth h below its free surface is P2. (a) How are P1 and P2 related? (b) Which is more P1 or P2?

Explain why a gas bubble released at the bottom of a lake grows in size as it rises to the surface of lake.

A dam has broader walls at the bottom than at the top. Explain.

Why do sea divers need a special protective suit?

State the laws of liquid pressure.

A tall vertical cylinder filled with water is kept on a horizontal tabletop. Two small holes A and B are made on the wall of the cylinder, A near the middle and B just below the free surface of water. State and explain your observation.

How does the liquid pressure on a diver change if:

(i) The diver moves to the greater depth, and

(ii) The diver moves horizontally?

State Pascal’s law of transmission of pressure.

Name two applications of Pascal’s law.

Explain the principle of a hydraulic machine. Name two devices which work on this principle.

Name and state the principle on which a hydraulic press works. Write one use of the hydraulic press.

The diagram in the figure shows a device which makes use of the principle of transmission of pressure.

(i) Name the parts labelled by the letters X and Y.

(ii) Describe what happens to the valves A and B and to the quantity of water in the two cylinders when the lever arm is moved down.

(iii) Give reasons for what happens to the valves A and B in part (ii).

(iv) What happens when the release valve is opened?

(v) What happens to the valve B in cylinder P when the lever arm is moved up?

(vi) Give a reason for your answer in part (v)

(vii) State one use of the above device.

Draw a simple diagram of a hydraulic jack and explain its working.

Explain the working of a hydraulic brake with a simple labelled diagram.

Complete the following sentences:

(a) Pressure at a depth h in a liquid of density ρ is ___________

(b) Pressure is ________ in all directions about a point in

a liquid

(d) Pressure at a point inside a liquid is __________ to its

depth.

(e) Pressure of a liquid at a given depth is _______ to the

density of liquid.

The S.I. unit of pressure is:

(a) N cm-2

(b) Pa

(d) N m2

The pressure inside a liquid of density ρ at a depth h is:

(a) h ρ g

(b) h/ ρ g

(d) h ρ

The pressure P1 at a certain depth in river water and P2 at the same depth in sea water are related as:

(a) P1 > P2

(b) P1 = P2

(d) P1 – P2 = atmospheric pressure

The pressure P1 at the top of a dam and P2 at a depth h from the top inside water (density ρ) are related as:

(a) P1 > P2

(b) P1 = P2

(d) P2 – P1 = h ρ g

A hammer exerts a force of 1.5N on each of the two nails A and B. The area of cross-section of the tip of the nail, A is $2mm^2$ while that of nail B is 6 mm2 . Calculate pressure on each nail in pascal.

A block of iron of mass 7.5kg and of dimensions 12cm x 8cm x 10cm is kept on a tabletop on its base of side 12cm x 8cm. Calculate (a) thrust and (b) pressure exerted on the tabletop. Take 1kgf = 10N.

A vessel contains water up to a height of 1.5m. Taking the density of water $10^3 kg m^{-3}$ , acceleration due to gravity $9.8ms^{-2}$ and area of base vessel $100cm^2$ , calculate: (a) the pressure and (b) the thrust, at the base of vessel.

The area of the base of a cylindrical vessel is $300cm^2$ . Water (Density = $1000 kg m^{-3}$ ) is poured into it up to a depth of 6cm. Calculate: (a) the pressure and (b) the thrust of water on the base. (g = $10m/s^2$ ).

(a) Calculate the height of a water column which will exert on its base the same pressure as the 70cm column of mercury. The density of mercury is $13.6 g cm^{-3}$ .

(b) Will the height of the water column in part (a) change if the cross-section of the water column is made wider?

The pressure of water on the ground floor is 40,000 Pa and on the first floor is 10,000Pa. Find the height of the first floor. (Take: density of water = $1000kg m^{-3}$ , g= $10m/s^2$ )

A simple U tube contains mercury to the same level in both of its arms. If water is poured to a height of 13.6cm in one arm, how much will be the rise in mercury level in the other arm?

Given: density of mercury = $13.6\times10^3\mathrm{kg}\mathrm{m}^3$ and density of water = $10^3\mathrm{kg}\mathrm{m}^{-3}$

In a hydraulic machine, a force of 2N is applied on the piston of the area of cross-section $10cm^2$ . What force is obtained on its piston of the area of cross-section $100cm^2$ ?

The areas of pistons in a hydraulic machine are $5cm^2$ and $625 cm^2$ . What force on the smaller piston will support a load of 1250N on the larger piston? State any assumption which you make in your calculation.

Assumption: There is no friction and no leakage of liquid.

(a) The diameter of the neck and bottom of a bottle is 2cm and 10cm respectively. The bottle is completely filled with oil. If the cork in the neck is pressed in with a force of 1.2kgf, what force is exerted on the bottom of the bottle?

(b) Name the law/principle you have used to find the force in part (a)

A force of 50kgf is applied to the smaller piston of a hydraulic machine. Neglecting friction, find the force exerted on the large piston, if the diameters of the pistons are 5cm and 25cm respectively.

Two cylindrical vessels fitted with pistons A and B of the area of cross-section $8cm^2$ and $320cm^2$ respectively, are joined at their bottom by a tube and they are completely filled with water. When a mass of 4kg is placed on piston A, find: (i) the pressure on piston A, (ii) the pressure on piston B, and (iii) the thrust on piston B.

What force is applied on a piston of the area of cross-section 2cm2 to obtain a force 150N on the piston of the area of cross-section $12cm^2$ in a hydraulic machine?

Questions in Exercise 4B

What do you understand by atmospheric pressure?

Write the numerical value of the atmospheric pressure on the surface of the earth in Pascal.

What physical quantity is measured in torr? How is it related to the S.I unit of the quantity?

Name the physical quantity which is expressed in the unit ‘atm’. State its value in pascal.

We do not feel uneasy even under the enormous pressure of the atmosphere above as well as around us. Give a reason.

Describe an experiment to demonstrate that air exerts pressure.

Explain the following:

(i) A balloon collapses when air is removed from it.

(ii) Water does not run out of a dropper unless its rubber bulb is pressed.

(iii) Two holes are made in a completely filled sealed tin can to take out the oil from it.

Why does the liquid rise in a syringe when its piston is pulled up?

How is water drawn up from a well by a water pump?

A partially inflated balloon is placed inside a bell jar connected to a vacuum pump. On creating a vacuum inside the bell jar, the balloon gets more inflated. How does the pressure change: increase, decrease or remains the same, inside the (a) bell jar and (b) balloon?

What is the purpose of a barometer?

What is a barometer? How is a simple barometer constructed?

Explain how is the height of the mercury column in the tube of a simple barometer, a measure of the atmospheric pressure.

Illustrate with the help of a labelled diagram of a simple barometer that the atmospheric pressure at a place is 76cm of Hg.

Why is the barometric height used as a unit to express the atmospheric pressure?

What is meant by the statement ‘the atmospheric pressure at a place is 76cm of Hg’? State its value in Pa.

How will you show there is a vacuum above the surface of the mercury in a barometer? What name is given to this vacuum?

How is the barometric height of a simple barometer affected if

(a) Its tube is pushed down into the trough of mercury?

(b) Its tube is slightly tilted from vertical?

State two uses of a barometer.

Give two reasons for the use of mercury as a barometric liquid.

Give two reasons why water is not a suitable barometric liquid.

Mention two demerits of a simple barometer and state how they are removed in a Fortin barometer.

Draw a simple labelled diagram of a Fortin barometer and state how it is used to measure the atmospheric pressure.

What is an aneroid barometer? Draw a neat and labelled diagram to explain its construction and working.

State two advantages of an aneroid barometer over a simple barometer.

How is the reading of a barometer affected when it is taken to (i) a mine, and (ii) a hill?

How does atmospheric pressure change with altitude? Draw an approximate graph to show this variation.

State two factors which affect the atmospheric pressure as we go up.

Why does a fountain pen leak at a high altitude?

Why does nose start bleeding on high mountains?

What is an altimeter? State its principle. How is its scale calibrated?

What do the following indicators in a barometer regarding weather:

(a) Gradual fall in the mercury level

(b) Sudden fall in the mercury level

The unit torr is related to the barometric height as:

(a) 1 torr = 1cm of Hg

(b) 1 torr = 0.76cm of Hg

(d) 1 torr=1m of Hg

The normal atmospheric pressure is:

(a) 76m of Hg

(b) 76cm of Hg

(d) 76 Nm-2

The atmospheric pressure at earth surface is P1 and inside mine is P2. They are related as:

(a) P1 = P2

(b) P1 > P2

(d) P2 = 0

Convert 1mm of Hg into pascal. Take the density of Hg= $13.6 \times 10^3 kg m^{-3}$ and g= $9.8 m/s^2$

At a given place, a mercury barometer records a pressure of 0.70m of Hg. What would be the height of water column if mercury in barometer is replaced by water? Take density of mercury to be $13.6\times10^3\mathrm{kg}\mathrm{m}^3$ .

At sea level, the atmospheric pressure is 76cm of Hg. If air pressure falls by 10mm of Hg per 120m of ascent, what is the height of a hill where the barometer reads 70cm of Hg. State the assumption made by you.

At sea level, the atmospheric pressure is $1.04 \times 10^5 Pa$ . Assuming g= $10m/s^2$ and density of air to be uniform and equal to $1.3kg m^{-3}$ , fint the height of the atmosphere.

Assuming the density of air to be $1.295 kg \space m^{-3}$ , find the fall in barometric height in mm of Hg at a height of 107m above the sea level. Take the density of mercury = $13.6 \times 10^3 kg \space m^{-3 }$