WBBSE 9th Class Science Solutions Physical Science & Environment Chapter – 3 Matter : Structure and Properties

WBBSE 9th Class Science Solutions Physical Science & Environment Chapter – 3 Matter : Structure and Properties

West Bengal Board 9th Class Science Solutions Physical Science & Environment Chapter – 3 Matter : Structure and Properties

WBBSE 9th Class Physical Science & Environment Solutions

Synopsis

• Those materials whose molecules move freely past one another (or the materials which flow) are called fluid. Liquids and gases are fluid.
• Pressure of a liquid at a point inside the liquid is defined as the force applied by the liquid in a perpendicular direction on a surface of unit area surrounding that point.
• The force per unit area applied perpendicularly against a surface by the weight of the atmosphere at a point is known as atmospheric pressure.
• Barometer is the instrument by which atmospheric pressure is measured.
• Siphon is an arrangement that carries a liquid from a higher level up and over a barrier and then to a lower level. Here, the flow is maintained by gravity and atmospheric pressure as long as tube in the arrangement remains full.
• Density of a substance is defined as its mass per unit volume.
• Specific gravity of a material is defined as the ratio of its mass to the mass of an equal volume of water at 4°C. It is a dimensionless and unitless quantity.

TOPIC – A

Pressure of Liquids and Air; Archimedes’ Principle

SHORT AND LONG ANSWER TYPE QUESTIONS

1. Pressure of a liquid at a point inside the liquid depends on the depth and not on the shape of the vessel.
2. At any point inside a stationary liquid, a liquid exerts the same amount of pressure in every direction.
3. Pressure of a liquid is same at every point on any horizontal plane inside a stationary liquid.

1. If the reading of a barometer decreases slowly, there is a possibility of rainfall. Water vapour is lighter compared to air. If the amount of water vapour increases in air, density of air also decreases at that place. As a result, reading of the barometer comes down slowly.
2. If the reading of a barometer drops suddenly, there is a possibility of storm, i.e., low pressure has been created at that place.
3. If the reading of a barometer increases slowly, we may conclude that the amount of water pressure in air at that place is decreasing slowly. As a result, weather of that place becomes dry and clear.
4. If the reading of a barometer increases suddenly, may be concluded that the amount of water vapour in air at that place has decreased considerably. So, weather of that place remains dry and clear temporarily.

1. Pure mercury is easily available.
2. Mercury do not wet glass, as a result it is easier to take readings.
3. Freezing point and boiling point of mercury are -39°C and 357°C, respectively. So, within this long range of temperature, mercury remains in a liquid state.
4. Mercury is opaque and bright. So one can take the reading of mercury column easily.
5. As the pressure of mercury vapour on the top of the mercury column in a barometer tube is very less, it does not influence the actual reading.
6. As mercury is a good conductor of heat, temperature of mercury throughout the tube remains uniform.
7. As the density (13.6 g/cm³) of mercury is high, height of the mercury column does not reach so high.
8. Volume expansion, density and coefficient of thermal expansion of mercury can be measured accurately. If there is a change of. temperature in the room, it is not difficult to get the original reading after making necessary corrections.

1. The tube should be filled with the liquid which has to be transferred.
2. Level of liquid in the vessel from which liquid has to be transferred is to be kept higher than the level of water in the vessel in which it has to be transferred.
3. Due to the atmospheric pressure, liquid goes up the tube in the siphon process. So in order to keep the siphon process continuous, atmospheric pressure has to be maintained.
4. There should not be any hole in the tube.

VERY SHORT ANSWER TYPE QUESTIONS

Choose the correct answer

Answer in brief

Fill in the blanks

State whether true or false

TOPIC – B

Surface Tension

SHORT AND LONG ANSWER TYPE QUESTIONS

1. What do you mean by surface tension? Which natural phenomena gives us ideas about surface tension?

Ans. A tangential force per unit length on a liquid surface that acts perpendicularly with a line imagined on the free surface of a liquid is called surface tension of the liquid. If length of the line = l and force acting on the line = F, then surface tension, S = F/l.

Water droplet, raindrop, soap bubble, air bubble inside water, etc. are spherical in shape due to surface tension.

2. What do you mean by cohesive force and adhesive force?

Ans. Cohesive force: Molecules always attract each other. The force of attraction between like molecules of the same element is called cohesive force.

Adhesive force: The force of attraction between unlike mole-cules of different type is called adhesive force.

3. What do you understand by molecular range?

Ans. Theoretically, cohesive and adhesive forces are supposed to act up to an infinite distance but in reality, it is seen that this force is felt up to a definite distance (r ≈ 10^-7 cm) and afterwards, its influence is insignificant. The measure of this distance is called molecular range. So, molecular range is defined as the maximum distance up to which mutual cohesive and adhesive forces between the molecules of a material remains effective.

4. With the help of a simple experiment, how can you prove that surface tension is working on the free surface of a liquid?

Ans. If a needle or a blade is floated in water, it sinks. But if the same needle or blade is kept on a piece of newspaper or on a blotting paper and then floated, it is seen after some time that the paper has became heavier by soaking water. At one point of time, the piece of paper sinks in water but the needle or the blade is found floating on the surface of water.

If a small amount of pressure is given by finger on a tightly held rubber membrane, it is found that the membrane is slightly depressed around the finger. In the same way, water surface is slightly depressed along the side adjacent to the needle or the blade. From this experiment, it is proved that surface tension is working on the free surface of a liquid.

5. Write down the influence of different factors on surface tension.

Ans. Surface tension of a liquid depends on the following factors:

1. Temperature: If temperature increases, surface tension of the liquid decreases. At a particular temperature, surface tension of a liquid becomes zero. This is called the critical temperature.
2. Pollution: If the water surface is polluted by some material, there is a change of surface tension. For example, surface tension decreases if oil or fat is spread on water.
3. Presence of a solute: Surface tension of a liquid changes, if any other material is dissolved in the liquid. For example, surface tension increases if any inorganic substance is dissolved in water and surface tension decreases, if any organic substance is dissolved in water.
4. Medium above the surface of liquid: Surface tension depends on the medium present above the free surface of the liquid. At ordinary temperature, if dry air is present above water, surface tension of water is 72 dyn/cm. At the same temperature, if water vapour is present above water, surface tension is reduced to 70 dyn/cm.

6. A circular ring made of thin wire is dipped in soap water and then withdrawn. Now a small water-soaked knot of thread is kept on the ring and finally a hole is made in the knot by means of a needle. Explain what happens.

Ans. Experiment: A ring made of thin wire is dipped into soap water. It is found that a thin film of soap water has been formed in the ring. This is the free surface of the liquid. A small knot made of thread is soaked in soap water and kept on the film formed in the ring. It is seen that the knot is lying on the film in a haphazard way. [Fig. (a)]

Now a pointed pin is taken to pierce the film inside the knot. It is seen that the knot is stretched to take a spherical shape. In this condition also, the film of soap formed in between the ring and the knot of thread remains intact [Fig.(b)].

Conclusion: (1) At first, equal and opposite forces act in the inside and outside of every point of the knot of thread due to surface tension. Therefore, at every point of the thread at that time, total force due to surface tension is zero. For this reason, thread is lying on the film in a haphazard and static manner. (2) Afterwards, when the film inside the knot is pierced, equal forces act in a tangential way to the free surface at every point outside the thread of the knot. Due to this reason, the thread takes a spherical shape.

7. Why is a small water drop or a mercury Idrop spherical in shape?

Ans. Due to cohesive forces between the molecules of a liquid, there is always a tension acting on the free surface of a liquid for which a liquid wants to reduce the free surface of its area. This tendency of contraction of the area of free surface is called surface tension.

Of all the spherical bodies that may be constructed with a given specific volume, a sphere has the minimum area. A small water drop or a mercury drop takes the shape of a sphere only due to surface tension. As a water drop or mercury drop always wants to reduce the area of its free surface, it takes a spherical shape. But as there is some influence of gravity on them, a large sized water drop or mercury drop is slightly distorted.

8. If a painting brush is soaked in water and then taken out, the hairs stick to each other. Why?

Ans. If a painting brush is soake in water, it is found that the hairs stick out from each other [Fig. (a)]. Now if the brush is taken out of water, it is observed that the hairs stick together. Camphor is soluble in water. The place where water mixes with camphor, gets a reduced surface tension. Now the surrounding area of water, having higher surface tension drags the surface having low surface tension towards itself. The camphor also moves along the surface of low surface tension to the surface of high surface tension, where it again gets soluble with water and reduces the surface tension of that region. Thus in this way, a non-uniform force acts on camphor due to this non-uniform surface tension of water, causing the camphor to move in a haphazard manner on water.

9. If oil is poured on rough sea, it calms down-explain.

Ans. If oil is poured on water, surface tension of water reduces. If some amount of oil is poured on a rough sea, oil spreads in the direction of movement of air. As a result, surface tension of oily water reduces. Pure water having greater surface tension which remains just behind the oily water drags water backward with less surface tension. As a result, waves of lesser heights are produced than before and as a result, the rough sea calms down.

VERY SHORT ANSWER TYPE QUESTIONS

Choose the correct answer

1. At critical temperature, surface tension of a liquid is

A. zero

B. maximum

C. 100 dyn/cm

D. 200 dyn/cm.

Ans. A

2. Dimensional formula of surface tension is

A. MT^-2

B. MT^-1

C. M^-1T^-2

D. MLT^-2

Ans. A

3. If temperature is increased, surface tension of a liquid

A. increases

B. remains unchanged

C. decreases

D. increases at first, and then decreases

Ans. C

4. If water vapour is present above water instead of dry air, surface tension of water

A. increases

B. decreases

C. remains unchanged

D. increases or decreases depending on the amount of water vapour

Ans. B

5. A blade is floating on water. This phenomenon is mainly due to

A. viscosity

B. gravity

C. buoyancy

D. surface tension

Ans. D

6. If oil is poured on a rough sea, then the sea becomes calm. This is due to

A. viscosity

B. surface tension

C. buoyancy

D. none of these

Ans. B

7. Surface tension is a general property of

A. solids

B. liquids

C. gases

D. all of these

Ans. B

8. The upper surface of mercury is convex due to

A. surface tension

B. viscosity

C. density

D. pressure

Ans. A

9. Which of the following shapes (of equal volume) has minimum surface area?

A. Sphere

B. Cone

C. Cylinder

D. Cuboid

Ans. A

10. When a piece of camphor is put on water, it darts in different direction. This is due to

A. viscosity

B. humidity

C. vapour density

D. surface tension

Ans. D

Answer in brief

1. What do we call the force of attraction between two molecules of the same material?

Ans. Force of attraction between two molecules of the same material is called cohesion.

2. What do we call the force of attraction between two molecules of different materials?

Ans. Force of attraction between two molecules of different materials is called adhesion.

3. What do we call the tendency of the surface area of the free surface of a liquid to contract?

Ans. The tendency of the surface area of the free surface of a liquid to contract is called surface tension.

4. What is the dimensional formula of surface tension?

Ans. Dimensional formula of surface tension is MT^-2.

5. With rise of temperature, does the surface tension of a liquid increase or decrease?

Ans. Surface tension decreases with rise of temperature.

6. At what temperature, does the surface tension of a liquid become zero?

Ans. At critical temperature, surface tension of a liquid becomes zero.

7. If some inorganic material is dissolved in water, does the surface tension increase or decrease?

Ans. Surface tension increases if some inorganic material is dissolved in water.

8. If some organic material is dissolved in water, does the surface tension increase or decrease?

Ans. Surface tension decreases if some organic material is dissolved in water.

9. In place of dry air, if water vapour is present over a water surface, then does the surface tension increase or decrease?

Ans. In place of dry air, if water vapour is presen over a water surface, then surface tension decreases.

10. Does surface tension work inside a liquid?

Ans. No, surface tension does not work inside a liquid.

11. What is the power of ‘L’ in the dimensional formula of surface tension?

Ans. Power of ‘L’ in the dimensional formula of surface tension is zero.

Fill in the blanks

1. If inorganic material is dissolved in water, its surface tension ………..

Ans. increases

2. Surface tension becomes zero at ……….. temperature.

Ans. critical

3. Due to ………. equal force acts in every direction along the surface of a liquid at every point. 

Ans. surface tension

4. When a laminar flow of liquid takes place over a firmly held horizontal plane, velocity of the liquid layer in contact with the bottom surface becomes ……….

Ans. zero

5. Due to the property of ………… raindrops are spherical in shape.

Ans. surface tension

6. A blade can float on water due to the property of ……….. of water.

Ans. surface tension

7. The tendency of a liquid surface to ……….. its area is called surface tension.

Ans. scalar

State whether true or false

1. At critical temperature, surface tension of a liquid becomes zero.

Ans. True

2. When temperature increases, surface tension of a liquid decreases.

Ans. True

3. Surface tension of liquids is a molecular phenomena.

Ans. True

4. The force applied by the liquid tangentially on the surface adjacent to the liquid is known as surface tension of the liquid.

Ans. False

5. The tendency of the surface area of the free surface of a liquid to expand.

Ans. False

6. Due to surface tension capillary action observed in liquids.

Ans. True

7. Surface tension decreases if some inorganic material is dissolved in water.

Ans. False

TOPIC – C

Viscosity and Bernoulli’s Theorem

SHORT AND LONG ANSWER TYPE QUESTIONS

1. What is viscosity?

Ans. Viscosity is the property of a fluid by which it tries to reduce the relative velocity between two adjacent layers.

2. What is the relationship between viscosity and mobility of a liquid?

Ans. Viscosity is a general property of matter. It is different for different liquids. When viscosity of a fluid increases, mobility decreases.

Example: Honey has greater viscosity than water.

3. Why is viscosity also called internal friction?

Ans. When a body moves or is about to move on another body or on a surface, then the opposing force that works against this motion or tendency of motion is called friction. On the other hand, the property by virtue of which a liquid tries to reduce the relative velocity between two adjacent layers is called viscosity of the liquid. Because of this type of similarity between viscosity and friction, viscosity is also called internal friction of a liquid in many cases.

4. Viscous force depends on which factors?

Ans. Viscous force depends on- (1) nature of the liquid, (2) area of the layers in contact, (3) velocity gradient between the layers.

5. What is meant by an ideal fluid?

Ans. While flowing, a fluid tries to reduce the relative motion between its two adjacent layers due to viscous force. But for an ideal fluid, no such force acts between layers. The ideal fluid is hence nonviscous and streamline.

6. What do you mean by streamline flow or laminar flow?

Ans. If the magnitude and direction of the flow always remains unchanged at any point of the flow line during flow of a fluid, then that flow is called streamline flow or laminar flow. In this condition, there is not any collision amongst the particles of the liquid.

7. What do What do you mean by a st streamline? What is the type of streamline in case of streamline motion of a liquid through a right circular cylindrical tube?

Ans. In case of a streamline motion, the path in which a particle of the fluid flows is called a streamline. In case of a streamline motion, any particle of the fluid always has the velocity of the preceding particle. A tangent drawn at any point of a streamline expresses the direction of the velocity of the fluid at that point.

In case of streamline motion of a liquid through a right circular cylindrical tube, streamlines are parallel to the axis of the tube.

8. What are the characteristics of streamline?

Ans. Characteristics of a streamline are:

1. A streamline may be a straight line or a curved line.
2. A tangent drawn at any point of a streamline indicates the direction of the velocity of the fluid at that point.
3. Two streamlines never intersect with each other.
4. Velocity of the fluid increases at that place in the tube where the streamlines are very close to each other and velocity decreases at that place where the streamlines maintain greater distances between them.

9. What do you mean by turbulent flow?

Ans. At any point of a flow line during flow of a fluid, if the magnitude and direction of the flow change in a haphazard way, then the flow is called turbulent flow.

Suppose, a liquid flows through a tube. During this condition, if the liquid particles collide with each other continuously and also move simultaneously, then this type of flow is called turbulent flow [Fig.]. During this time, whirls are created at some places inside the liquid.

10. Why do two streamlines never intersect wo streamlines never intersect each other?

Ans. If a tangent is drawn at any point of a streamline, the tangent indicates the direction of the velocity of fluid at that point.

Now, suppose two streamlines intersect each other at the point A [Fig.]. Two tangents can be drawn at the point of intersection. As a result, two directions of the velocity of the fluid are obtained at the point A, which is not possible. Therefore, two streamlines can never intersect each other.

11. What do you mean by terminal velocity?

Ans. The velocity of a small body which falls through a viscous medium due to gravity increases steadily at the beginning. But as its velocity increases, value of the viscous resistance inside the perpendicular layers of the adjacent fluid of the body also increases. Along with this, buoyancy of the medium acts on the body in an upward direction. As a result, downward acceleration gradually decreases. At a particular point of time, when value of resistance force due to viscosity and buoyancy are equal to the gravitational force, resultant force on the body becomes zero. Then the body falls through the medium with a steady or uniform velocity. This uniform velocity is called terminal velocity.

Thus, terminal velocity is defined as the highest uniform velocity with which a body finally falls through an infinitely spread viscous medium.

12. Draw the velocity-time graph of a body falling through an infinitely spread viscous medium.

Ans. The velocity-time graph of a body falling through an infinitely spread viscous medium is shown here. Here the acceleration of the body decreases gradually. After a certain time the body falls with a constant velocity. This is the terminal velocity, shown in the figure with V₀.

13. If you blow between two pages of a book, the pages stick together instead of spreading apart – explain with reason.

Ans. If air is blown between two pages of a book, speed of air increases between them. Hence, according to Bernoulli’s theorem air pressure between the pages becomes less than outside. Due to this difference in pressure, the pages come closer and stick together.

14. If two boats in a river move side by side, they tend to come closer explain with reason. 

Ans. If two boats in a river move side by side, the speed of water between the boats becomes more than that of the other sides. According to Bernoulli’s theorem the pressure at the other sides of the boats becomes more than that between them. Therefore the boats tend to come closer.

VERY SHORT ANSWER TYPE QUESTIONS

Choose the correct answer

1. On a plane horizontal surface, some amount of water, shampoo and tar are poured slowly. They move with different speeds and the tar stops at first. This is because, among these three, tar has the minimum

A. viscosity

B. surface tension

C. fluidity

D. elasticity

Ans. A

2. Which of the following quantities is similar to friction?

A. viscosity

B. surface tension

C. buoyancy

D. all of these

Ans. A

3. Which of the following properties is applicable only for a flowing liquid material?

A. elasticity

B. surface tension

C. malleability

D. viscosity

Ans. D

4. Which of the following statements is incorrect?

A. Like friction, viscosity is a force against the motion

B. More the viscosity of a liquid, lesser is its mobility

C. After reaching its terminal velocity, a body starts falling with uniform velocity

D. If the area of a liquid surface decreases, value of viscous force also increases

Ans. D

5. Stir a liquid kept in a vessel and then leave it to itself. After some time the motion subsides because of

A. viscosity

B. surface tension

C. elasticity

D. buoyancy

Ans. A

6. Bernoulli’s theorem is based on the law of

A. conservation of momentum

B. conservation of mass

C. conservation of angular momentum

D. conservation of energy

Ans. D

7. Action of a sprayer depends on

A. Bernoulli’s theorem

B. Jurin’s law

C. Avogadro’s theorem

D. Stoke’s law

Ans. A

8. Bernoulli’s theorem is applicable for

A. viscous fluid

B. non viscous fluid

C. incompressible and non viscous fluid

D. compressible fluid

Ans. C

Answer in brief

1. Bernoulli’s theorem is established on which conservation law?

Ans. Bernoulli’s theorem is established on the law of conservation of energy.

2. If temperature is increased, does the viscosity of a liquid increase or decrease?

Ans. Viscosity of a liquid decreases, if temperature is increased.

3. If temperature is increased, does the viscosity of a gas increase or decrease?

Ans. Viscosity of a gas increases, if temperature is increased.

4. Do two streamlines ever intersect with each other?

Ans. No, two streamlines never intersect with each other.

5. What type of a fluid gives rise to a whirlwind?

Ans. Turbulent flow of a fluid gives rise to a whirlwind.

6. What is the velocity of a liquid layer in contact with the bottom surface when laminar flow of liquid takes place over a firmly fixed horizontal plane?

Ans. Velocity of the liquid layer in contact with the bottom surface is zero.

7. What is that velocity called when a body falls through a viscous medium with maximum uniform velocity?

Ans. That velocity is called terminal velocity.

8. If a body moving with terminal velocity through a viscous medium and density of the body is greater than the density of the medium, then what is the direction of terminal velocity?

Ans. As the density of the body is greater than the density of the medium, direction of terminal velocity is perpendicularly downward.

9. If a body moving with terminal velocity through a long viscous medium and density of the body is less than the density of the medium, then what the direction of terminal velocity?

Ans. As the density of the body is less than the density of the medium, direction of terminal velocity is perpendicularly upward.

10. What is the relationship between viscosity of a liquid and its mobility?

Ans. When viscosity of a liquid increases, its mobility decreases.

11. Bernoulli’s theorem is fully applicable for what type of fluid?

Ans. Bernoulli’s theorem is fully applicable for an ideal fluid.

12. What are the characteristics of an ideal fluid?

Ans. An ideal fluid is incompressible and nonviscous.

13. When a car is running very fast, it is found that light polythene packets etc. keep flying behind the running car. This phenomenon takes place due to which principle?

Ans. This happens due to Bernoulli’s theorem.

Fill in the blanks

1. Water is ……………. viscous than kerosene.

Ans. more

2. …………….. of a fluid is called its internal friction. 

Ans. Viscosity

3. When viscosity of a liquid increases, its ………………. decreases.

Ans. mobility

4. When a small ball of iron falls through water, three forces act on it, namely gravitational force, buoyant force and ………… force.

Ans. viscous

5. Ideal fluid has no ………… 

Ans. viscosity

6. The maximum velocity of a fluid, up to which the flow of the fluid is ………. and beyond which the flow becomes ………. is regarded as the critical velocity for that fluid.

Ans. streamline, turbulent

7. Two …………. never intersect each other.

Ans. streamlines

8. Raindrops fall to the ground with ………….. velocity.

Ans. terminal

State whether true or false

1. Bernoulli’s theorem follows the law of conservation of energy.

Ans. True

2. A smooth, uninterrupted flow in ordered layers, without any energy transfer between the layers is called laminar flow.

Ans. True

3. Mobility of water is greater than that of honey.

Ans. True

4. Two streamlines can intersect each other.

Ans. False

5. Viscosity is called internal friction of a liquid.

Ans. True

6. Viscosity of a fluid decreases with the rise in temperature.

Ans. True

7. For a streamline flow of an fluid the sum of the velocity head, elevation head and pressure head always remain constant at any point in the fluid.

Ans. True

8. If the relative motion between the layers in contact in a flowing liquid decreases, viscosity decreases.

Ans. True

TOPIC – D

Elasticity

SHORT AND LONG ANSWER TYPE QUESTIONS

1. What is elasticity?

Ans. Elasticity is the property of an object or material by which it resists the deformation in its shape or volume or both due to external balanced forces acting on it. The object regains its original shape or volume when these external forces are withdrawn.

2. What is a perfectly elastic body? Also define elastic limit.

Ans. A perfectly elastic body is that body which can regain its original shape, size and volume after the withdrawal of the external forces.

Elastic limit of a body is the upper limit of a deforming force acting on the body, upto which the body behaves as a perfectly elastic body. If the deforming force is increased beyond the elastic limit, the body loses its property of elasticity and get permanently deformed.

3. Define a perfectly rigid body and an inelastic body.

Ans. Perfectly rigid body: A body which has no strain in spite of application of external force of any magnitude is called a perfectly rigid body.

Inelastic body: If a body, deformed by external forces remains in the deformed state even after the withdrawal of these deforming forces, it is called an inelastic body.

4. What is stress? How can it be measured?

Ans. If an elastic body is strained due to the application of external balanced forces, there is an internal reaction force inside the body due to elasticity which resists the external force. When the external force are withdrawn, this internal reaction force helps the body to return to its original state. This reaction force produced per unit area the surface of the body is called stress.

Stress is the reaction of an external applied force on the body. According to Newton’s third law of motion, for every action, there is an equal and opposite reaction.

So, stress is measured by the applied force per unit area on the surface.

5. What do you mean by strain? How can it be measured? Why does it have no unit or dimension? 

Ans. Due to application of an external balanced force, there may be a change of form or shape or both of an elastic body. This is called strain.

Measure of strain is the comparative proportional change of initial value of length, shape or volume of an object, i.e.,

As strain is the ratio of two quantities of the same type, it has no unit or dimension.

6. Between rubber and steel, which has greater elasticity? Why? 

Ans. Between rubber and steel, steel has comparatively greater elasticity.

This is obvious from the fact that to elongate both of them to the same extent, much more force is to be applied to the steel than rubber. In scientific language, a body is said to be more elastic compared to another body if greater force has to be applied on it to bring about the same amount of strain in it. According to this, elasticity of steel is much more than that of rubber.

7. Ter Units of modulus of elasticity and stress are the same. Explain the authenticity of this statement.

Ans. Modulus of elasticity is the ratio of stress and strain, within the elastic limit. Strain has no unit, so the unit of modulus of elasticity should have the same unit as that of stress.

Units of modulus of elasticity in CGS system and Slare dyn/cm² and N/m² respectively, which are also the units of stress.

8. What do you understand by the force constant of a spring? Which property of a spring is measured by this constant?

Ans. If one side of a spring is firmly fixed to a support and tensile force is applied to the other end, there is an increase in length of the spring. If application of tensile force within elastic limit results in an increase of length x, then F xx or, F = kx.

Here, k is a constant which is called as the force constant of a spring. If x = 1, then F = k. So, necessary tensile force required to increase unit length of a spring is called the force constant of the spring..

One can measure the property of stiffness of a spring by the force constant of the spring.

9. With the help of a simple experiment, discuss how would you measure the force constant of a spring.

Ans. A spring is hung from a firm support. To the other end of the spring, a hook is attached and a mass hanger is hung from there. A parallel indicator is fixed with the hook. A scale is also fixed vertically by the side of the spring so that the end of the indicator touches the scale. Now, without any weight in the hanger, reading of the scale is taken. Suppose the reading is 2.5 cm. Now a 10 g weight is placed in the mass hanger. Due to this weight, the spring is elongated along its length. Suppose, the present reading is 2.7 cm. Applied tensile force, on the spring is F = 10 gf = 10 × 980 dyn and due to this, increase of length is x = (2.7 – 2.5)cm = 0.2 cm

∴ force constant of the spring,

10. It is possible to manufacture thin and fine ornaments with small amount of gold or silver whereas it is not possible to manufacture such thin and fine ornaments with equal amount of iron or copper. Why?

Ans. If the malleability of a material is high, it can be converted into a thin sheet by simple hammering. Gold and silver have greater malleability and ductility as compared to iron or copper. So, though it is possible to manufacture ornaments by iron or copper, it is not possible to manufacture fine and thin ornaments like those of gold and silver.

11. If a vessel of glass or ceramic drops accidentally from hand, it breaks into pieces but if a steel glass falls from the same height, it does not break. Which property of material is expressed through the above incidents? 

Ans. If a vessel of glass or ceramic drops from hand accidentally, it breaks into pieces but if a steel glass falls from the same height, it does not break. It is inferred from these two incidents that glass or ceramic is more brittle than steel. It may be mentioned that though a vessel of glass or ceramic breaks down easily, there is not any significant strain of individual parts. That is why glass or ceramic behaves like a perfectly rigid body to a great extent. On the other hand, if a steel vessel falls from a height, it may not break but gets deformed, i.e., permanent strain may appear. That is why steel may be considered as a partially elastic body.

12. To manufacture thin chains, materials like iron, copper, gold, silver etc. are used but lead is never used. Why?

Ans. Ductility of materials like iron, copper, gold, silver are much higher than that of lead. So these can be drawn to form sufficiently thin wires. As the ductility of lead is comparatively low, it is not possible to draw it into thin wire.

VERY SHORT ANSWER TYPE QUESTIONS

Choose the correct answer

1. Which of the following properties is applicable for any type of material?

A. buoyancy

B. surface tension

C. elasticity

D. viscosity

Ans. C

2. As the value of the force constant increases, spring becomes more

A. brittle

B. ductile

C. malleable

D. stiff

Ans. D

3. Dimensional formula of Young’s modulus is equal to which of the following quantities?

A. force

B. momentum

C. stress

D. acceleration

Ans. C

4. If temperature increases, value of Young’s modulus also

A. increases

B. decreases

C. increases at first, then decreases

D. decreases at first, then increases

Ans. B

5. Which of the following statements is incorrect?

A. Young’s modulus is a measure of the property of elasticity for any material

B. Elastic property of a metal changes if some impurity is mixed with the pure metal

C. Elastic limit of steel is much higher than that of rubber

D. Every material is perfectly elastic upto the elastic limit

Ans. A

6. Young’s modulus is the

A. characteristic of solid only

B. characteristic of liquid only

C. characteristic of gas only

D. characteristic of solid, liquid and gas

Ans. A

7. The force constant of a spring is 200 N/m. If the spring is divided into two parts, each part has a force constant of

A. 200 N/m

B. 300 N/m

C. 400 N/m

D. 100 N/m

Ans. C

8. The force constant of a spring is 300 N/m . If the spring is divided into three parts, each part has a force constant of

A. 100 N/m

B. 300 N/m

C. 600 N/m

D. 900 N/m

Ans. D

9. If elastic limit of a material is 10 N, then highest limit the applied force upto which this material behaves like a perfectly elastic material is

A. 5 N

B. 10 N

C. 15 N

D. 20 N

Ans. B

10. Young’s modulus of a perfectly rigid body is

A. 0

B. 1

C. infinite

D. depends on stress

Ans. C

11. The most elastic among the following substance is 

A. rubber

B. steel

C. glass

D. copper

Ans. B

Answer in brief

1. How does a body behave up to its elastic limit?

Ans. A body behaves like a perfectly elastic body up to its elastic limit.

2. If there is no strain of a body due to an external balanced force of any magnitude, what do we call the body?

Ans. The body is called a perfectly solid body.

3. In between stress and strain, which one is fundamental?

Ans. In between stress and strain, strain is fundamental.

4. Stress is not fundamental but strain is fundamental. Why?

Ans. If strain is generated, then only stress is evolved. So, in between stress and strain, strain is fundamental.

5. Does liquid and gaseous materials have Young’s modulus?

Ans. Only solid materials have length and as Young’s modulus is a measure of the elastic property of solid materials, so there is no existence of Young’s modulus for liquid and gaseous materials.

6. Is there any change in Young’s modulus with increasing temperature?

Ans. The value of Young’s modulus is reduced if temperature is increased.

7. What is the value of Young’s modulus for a perfectly solid body?

Ans. Value of Young’s modulus for a perfectly solid body is infinite.

8. It is possible to draw a thin wire of iron but it is not possible to do the same for lead due to which property of a material?

Ans. This is due to the property of ductility of a material.

9. Which property of a material is more prevalent in gold than in iron so that it is possible to manufacture fine gold ornaments but not iron ornaments?

Ans. Malleability of gold is more than that of iron.

10. Are the values of Young’s modulus for a thin and a thick iron wire of the same length different?

Ans. No, the values of Young’s modulus of both the values are the same because both are manufactured from the same material.

11. State whether the values of Young’s moduli for thin and thick iron wires of different length will be different.

Ans. Young’s moduli of two iron wires of different length and of different thickness cannot be different because Young’s modulus only depends on the nature of material of the wire.

Fill in the blanks

1. Within the ……….. limit, stress is directly proportional to strain.

Ans. elastic

2. Elasticity of steel is …………. than that of rubber.

Ans. more

3. Coefficient of elasticity is defined as the ratio of the applied ……….. to the change in shape of an elastic body.

Ans. stress

4. The minimum value of stress required to break a wire is called the ………. of that wire. 

Ans. breaking stress

5. ……….. is the reaction force that is produced per unit area of cross section of any material due to the application of an external force.

Ans. Stress

6. Brittleness of glass is …………. than that of iron 

Ans. more

7. More the …………. of a material, greater is the possibility of converting it to a thinner sheet by hammering.

Ans. malleability

8. k is the force constant of a spring. If the spring is cut into three equal parts, force constant of each part becomes ……….

Ans. 3k

State whether true or false

1. Required tensile force to increase the length of spring by unity is called force constant and its unit in SI is N/m².

Ans. False

2. Within elastic limit, stress is directly proportional to strain.

Ans. True

3. Young’s modulus of a perfectly rigid body is finite.

Ans. False

4. Rubber is more elastic than steel.

Ans. False

5. In the case of an elastic body strain is more fundamental than stress.

Ans. False

6. Required external force to increase the length of a spring by unity is called force constant.

Ans. True

7. Glass is a brittle substance. 

Ans. True

8. The elasticity of a material is decreased on hammering it.

Ans. True

9. Strain has no unit.

Ans. True