Samacheer Kalvi 8th Science Solutions Chapter 2 Pdf

Samacheer Kalvi 8th Science Solutions Chapter 2 Pdf

Tamilnadu Board Samacheer Kalvi 8th Science Solutions Chapter 2: Tamilnadu State Board Solution Class 8 Science Chapter 2 – Force and Pressure.

Samacheer Kalvi 8th Science Solutions Chapter 2: Overview

Board	Samacheer Kalvi
Class	8
Subject	Science
Chapter	2
Chapter Name	Force and Pressure

Samacheer Kalvi 8th Science Solutions Chapter 2 Pdf

Choose the best answer.

(1 ) if we apply force against the direction of motion of the body, then the body will

a) stop moving

b) move with an increased speed

c) move with a decreased speed

d) move in a different direction

(2) Pressure exerted by a liquid is increased by

a) the density of the liquid

b) the height of the liquid column

c) Both a and b

d) None of the above

(3). Unit of pressure is

a) Pascal b) Nm -2

c) Poise d) Both a and b

(4) The value of the atmospheric pressure at  sea level is

a) 76 cm of mercury column

b) 760 cm of mercury column

c) 176 cm of mercury column

d) 7.6 cm of mercury column

(5) Pascal’s law is used in

a) hydraulic lift

b) brake system

c) pressing heavy bundles

d) All the above

(6) Which of the following liquids has more viscosity ?

a) Grease b) Water

c) Coconut oil d) Ghee

(7) The unit of viscosity is

a) Nm 2 b) poise c) kgms -1        d) No unit

II.) Fill in the blanks.

a) The pressure of a liquid column increase with the depth of the column.

b) Hydraulic lift works under the principle of Pascal’s Law .

c) The property of surface tension of a liquid surface enables the water droplets to move upward in plants.

d) A simple barometer was first constructed by Torricelli .

III .) State true or false. If false , correct the statement.

1) Force acting on a given area is called pressure.

This statement is true.

2) A moving body comes to rest due to friction alone.

This statement is true.

3) A body will think if the weight of the body is greater than buoyant  force.

This statement is true.

4) One atmosphere is equivalent to 1,00,000 newton force acting on one square metre.

This statement is true.

5) Rolling friction is slightly greater than the sliding friction.

This statement is false.

Explanation :

Rolling friction is not slightly greater than the sliding friction but rather it is slightly lesser than the sliding friction.

6) Friction is the only reason for the loss of energy.

This statement is true.

7) Liquid pressure decreases with the decrease of depth.

This statement is true.

8) Viscosity depends on the pressure of a liquid .

This statement is true.

IV ). Match the following.

a.)

Static friction	Viscosity
Kinetic friction	Least friction
Rolling friction	Objects are in motion
Friction between the liquid layers	Objects are sliding
Sliding friction	Objects are at rest

b.)

Barometer	reduce friction
Increasing area of contact	Atmospheric pressure
Decreasing area of contact	cause of friction
Lubricants	Increases friction
Irregular surface	decreases friction

Answer

a .)

Static friction	Objects are at rest
Kinetic friction	Objects are in motion
Rolling friction	Least friction
Friction between the liquid layers	Viscosity
Sliding friction	Objects are sliding

b.)

Barometer	Atmospheric pressure
Increasing area of contact	increases friction
Decreasing area of contact	decreases friction
Lubricants	reduce friction
Irregular surface	cause of friction

V.) Complete the analogy.

1 ) knot in a thread : Static friction :: Ball bearing : Rolling friction

2 ) Download force : Weight :: Upward force offered by liquid : Buoyant force

VI.) Numerical Problem .

1 ) A stone weighs 500 N . Calculate the pressure exerted by it, if it makes contact with a surface of area  25 cm2 .

Answer :

Weight of a stone (F) is equivalent to 500 N .

Area A is equal to 25cm 2= 25× 10 -4 m2

We have to calculate Pressure P

So, Pressure P is not known (Pressure P = ? )

Formula of Pressure P = F / A

Here, F denotes Force and A denotes Area

Pressure P = 500/ 25×10- 4 = 20 × 10 4N/m 2or 20 × 104 pa .

VII.) Consider the statements given below and choose the correct option.

(1 ) Assertion : Sharp knives are used to cut the vegetables.

Reason :  Sharp edges exert more pressure.

Answer :

a ) Both assertion and reason are true and reason is the correct explanation of assertion.

(2) Assertion : Broad straps are used in bags.

Reason :  Broad straps last for long.

Answer :

b) Both assertion and reason are true, but reason is not the correct explanation of assertion.

Explanation :

The bags weight exerts larger pressure on our shoulder. For this reason, broad straps are used because it exerts less pressure. So, as a result of which lesser pressure is created.

(3)  Assertion : Water strider slides easily on the surface of water.

Reason :  Water strider experiences less buoyant force.

Answer

b) Both assertion and reason are true, but reason is not the correct explanation of assertion.

Explanation :

It is the surface tension of water, which enables the water strider to slide smoothly on the surface of water.

VIII.) Answer very briefly.

1 ) Give two examples to verify that a force changes the shape of a body.

• The shape of a sponge changes, by squeezing it.
• A rubber band becomes longer, by pulling it.

So, these are the two examples which clearly depicts that a force can alter the shape of a body.

2 ) Give two examples to verify that a force tends to change the static condition of a body.

The static condition of a body changes by applying force.

• It is due to the application of force, a rubber ball which is in a resting condition begins to move.
• While taking a penalty kick in football match, player applies a force on the stationary football. The application of force by the player enables the football to proceed in the direction of the ultimate goal.

3 ) How do you feel when you touch a nail immediately after it is hammered into a wooden plank? Why?

When we touch a nail instantly, after it is hammered into a wooden plank, the nail becomes hot and this is due to friction. Hence, there is transformation of kinetic energy to heat  because of friction.

4 ) How does the friction arise between the surfaces of two bodies in relative motion?

The force of friction is the result of interlinking of the inconsistencies of the two surfaces.

5 ) Name two instruments which help to measure the pressure of a fluid.

In order to measure the pressure of a fluid, Manometer and Pressure gauge are used .

Explanation :

Manometer

A manometer is an instrument that is used for the purpose of measuring pressure in a fluid. To measure pressure in the fluid two manometers  are included. In order to measure the difference in pressure between two vessels, simple U – tube manometer is used.

Pressure gauge

Pressure gauge are devices that are mainly used for the purpose of measuring the pressure of a liquid or gas.

6 ) Define one atmosphere.

The pressure that is applied by the mercury column of height 76 cm in the barometer is termed as one atmospheric pressure. It is almost equivalent to 1.01× 10 5 Nm-2. 1 atm is equal to 1, 00, 000 pascal in SI system . Nm-2 or pascal is the SI unit of atmospheric pressure.

7 ) Why are heavy bags provided with broad straps?

Heavy bags are provided with broad straps because it exerts less pressure on our shoulder, by giving a greater area of contact with our shoulder. So, as a result of which less pressure is created.

8 ) How does surface tension help of plant?

Due to surface tension, water molecules in plants rise up. In plants, Xylem tissues are present which are extremely narrow vessels. There is absorption of water molecules by the roots and due to ‘ capillarity action ‘, these vessels plays an important role in rising the water upward, which is the consequence of the surface tension of water.

9) Which has greater viscosity, oil or honey ? Why ?

Honey has greater viscosity when compared to oil.

Explanation :

The viscosity of the thicker liquids are much more when compared to the thinner liquids. So, greater frictional force will be acting on honey ,as it has more viscosity.

IX.) Answer briefly .

1 ) Define friction. Give two examples of the utility of fiction in day to day life.

Friction may be defined as a force that prohibits  the movements of the stationary objects or stagnates the objects from moving.

1 ) Due to friction ,it is possible for the cars and buses to run safely on the road because it acts between the strided tires  and the road surface.

2 ) Due to friction, it is possible for us to write on paper because it acts between the pen or pencil and paper.

So, these are the two usefulness of friction in our day to day life activities.

2 ) Mention any three ways of minimising friction.

Three effective ways of reducing friction are as follows :

• In order to decrease the friction between the surfaces, lubricants are applied.

Castor oil is one such example.

• In order to smoothen the surface of the carrom board, we sprinkle fine powder on its surface , which ultimately enables the striker to slide smoothly on the surface.
• As rolling friction is smaller than sliding friction, we use lead shots in bearing of a cycle hub.

3 ) State Pascal’s law and mention its applications.

According to Pascal’s law, the pressure exerted at any moment of a liquid which is in a resting condition, in an enclosed system, will be dispensed equally in almost every directions of the liquid.

Applications of Pascal’s law

The applications of Pascal’s law are as follows :

• The hydraulic lift operates according to Pascal’s law ,as it is mainly designed for the purpose of lifting the vehicles upward in automobile service stations .
• According to Pascal’s law , automobile brake system operates.
• The hydraulic press is used for the purpose of compressing bundles of cotton or cloth, so that it does not occupy much space.

4 ) Why is a ball bearing used in a cycle hub?

The sliding friction is larger than the rolling friction, so there is a replacement of sliding by rolling with the use of ball bearings .So for this reason, there is usage of lead shots in the bearing of a cycle hub.

X.) Answer in detail.

 1 ) Friction is a necessary evil-  Explain.

Friction is essential for our daily activities. In most of the situations of our day to day life activities it is extremely desirable.

• Due to friction ,it is possible for us to hold any objects in our hand.
• Because of friction, it is possible for us to walk on the road. We can walk without the risk of stumbling because friction acts between footwear and the ground.
• Due to friction, it is easy for us to write on the paper because friction acts between the pen or pencil and paper.
• Automobiles can move safely because of friction. The friction between the tires and the road enables the automobiles to move securely. Due to frictional resistance on brake shoes, brakes can be applied.
• It is the friction, which enables us to light a matchstick , sew clothes or fix a nail on the wall.

Though friction contributes in making our life convenient, but still it possess some negative impacts too. For this reason, it is termed as ‘necessary evil ‘.

 

Disadvantages of friction

• Friction is responsible for the wearing out of the surfaces that are rubbed with each other ,as for instance screws and gears in machines or soles of shoes.
• The heat which are produced by friction creates physical destruction to the machines.

2 ) Give the different types of friction and explain each with an example.

Friction can be categorized into two essential types: static friction and kinetic friction.

Static friction

The bodies which experiences  friction, that are in resting condition is termed as static friction.

Kinetic friction

When there is existence of friction during the motion of bodies ,then it is termed as kinetic friction. Sliding friction and rolling friction are the two different types of kinetic friction.

When there is sliding of one body over the surface of another body, there is fiction that acts between the surfaces in contact ,which is then termed as sliding friction.

When there is rolling of one body over the surface of another body, there is fiction that acts between the surfaces in contact ,which is then termed as rolling friction. Sliding friction is more than rolling friction. For this reason, there are wheels in vehicles, trolleys and suitcase etc.

3 ) Describe an experiment to prove that friction depends on the nature of a surface.

Firstly , a small amount of various kinds of liquids such as  coconut oil,  honey ,water and ghee are taken. Now , single drop of every liquid will be placed on a distant glass plate. Then, one end of the glass plate will be raised smoothly which will enable the liquid to slide down the plate’s surface . Then, each liquid’s speed will be observed.

Observations :

Each of these three liquids travels with a diverse speed.  Water flows quickly when compared to other liquids . Coconut oil flows with an average speed.  The flowing speed of ghee is extremely slow.

Inferences:

Between the surface of the liquid ,which is not in a static condition, there is a frictional force that is collateral to the surface of the liquid. This frictional force acts as an opposition to the motion of the liquid layers although they are in motion.

4 ) Explain how friction can be minimised.

a ) Using of ball bearing

The sliding friction is larger than the rolling friction, so there is a replacement of sliding by rolling with the use of ball bearings . For the similar reason, there is usage of lead shots in the bearing of a cycle hub.

b) Using lubricants

Lubricant is a substance that is used for the purpose of minimising the frictional force. Grease, coconut oil are some of the perfect examples. The gaps in the irregular surfaces between the bodies which are in contact ,are filled by lubricants. This gives a smooth surface thusprohibiting a direct contact between their rugged surface.

5 ) Describe an experiment to prove that the pressure in a liquid increases with depth.

A plastic bottle should be taken. In the similar direction, three holes should be punched on its sides, but at distant heights. Now, some water should be poured into it and allow it to flow with the help of holes. Then, the flow of water should be observed carefully.

Inference:

Water which is flowing from the lowest hole, is coming out with the largest force and water which is flowing from the uppermost hole, is coming out with the minimal force.

Reason

This activity clearly states that there is variation in the pressure of a liquid with the depthness of the point of observations within it.

XI.) Higher Order Thinking Questions.

1 ) Why it is not advisable to use a fountain pen while travelling in an aeroplane?

Fountain pen are constructed in such a manner that the atmospheric pressure at sea level can be balanced by the pressure that is  within them. When there is an increase in the height above the sea level,there is a gradual decrease in atmospheric pressure , so as a result of which, the pressure within the pen seems to be quite higher then compared to the air pressure that is inside an aeroplane. So, ultimately the pen begins to leak. For this reason, it is not advisable to use a fountain pen while travelling in an aeroplane.

2 ) Is there any possibility of making a special device to measure the magnitude of friction directly?

The answer will be yes. Tribometer is a special instrument that is mainly designed for the purpose of measuring the magnitude of friction directly.

3 ) Vidhya feels that mercury is costly.So ,instead of mercury she wants to use water as a barometric liquid . Explain the difficulty of constructing a water barometer.

In barometers, mercury is used because it’s great density signifies the columns height could be an appropriate size in order to measure atmospheric pressure. In order to acquire the same pressure difference,  a water barometer would require to be 13.6 times higher than compared to a mercury barometer. The reason behind it is that, the density  of mercury is 13.6 times more when compared to water.