# 8th Science Guide Unit 2 Forces and Pressure

## 8th Standard Science Guide Lesson.2 Forces and Pressure | English Medium

8th Science Guide Unit 2 Forces and Pressure book back answers. 8th Standard Science Guide English Medium Book in Answers. 8th Science Samacheer kalvi guide Tamil Medium and English Medium Book Back and additional questions and answers. 8th Text Book download pdf. 8th std All Subject Guide

## 8th Science Guide Unit 2 Forces and Pressure

### I. Choose the best answer

#### 1. If we apply a force against the direction of motion of a body, thenstop moving

1. move with an increased speed
2. move with a decreased speed
3. move in a different direction

Ans: stop moving

#### 2. Pressure exerted by a liquid is increased by

1. the density of the liquid
2. the height of the liquid column
3. Both (a) & (b)
4. None of the above

Ans: base the height of the liquid column

#### 3. Unit of pressure is the body will

1. pascal
2. N m–2
3. poise
4. Both (a) & (b)

Ans: Both (a) & (b)

#### 4. The value of the atmospheric pressure at sea level is

1. 76 cm of mercury column
2. 760 cm of mercury column
3. 176 cm of mercury column
4. 6 cm of mercury column

Ans: 76 cm of mercury column

#### 5. Pascal’s law is used in

1. hydraulic lift
2. brake system
3. pressing heavy bundles
4. All the above

Ans:  All the above

1. Grease
2. Water
3. Coconut oil
4. Ghee

Ans: Ghee

1. N m2
2. poise
3. kg m s–1
4. no unit

Ans: poise

### II. Fill in the blanks.

1. The pressure of a liquid column _____ with the depth of the column. Ans: increase
2. Hydraulic lift works under the principle of _____. Ans : Pascal’s law
3. The property of _____ of a liquid surface enables the water droplets to move upward in plants Ans: surface tension
1. A simple barometer was first constructed by _____. Ans : Torricelli

### III. True or False.

1. Force acting on a given area is called pressure. Ans: True
2. A moving body comes to rest due to friction alone Ans: False
3. A body will sink if the weight of the body is greater than the buoyant force. Ans : True
4. One atmosphere is equivalent to 1,00,000 newton force acting on one square metre Ans: True
1. Rolling friction is slightly greater than the sliding friction. Ans: False

Cu. An. : Atomic clocks are used in GPS Devices.

1. Friction is the only reason for the loss of energy. Ans : False
2. Liquid pressure decreases with the decrease of depth. Ans : True
3. Using barometers, one can measure the height of a building Ans : False
4. Surface tension causes the spherical nature of a water drop. Ans : True
5. Viscosity depends on the pressure of a liquid Ans : False

### IV. Match the following:

1. Static friction – Viscosity
2. Kinetic friction – Least friction
3. Rolling friction – Objects are in motion
4. Friction between the liquid layers – Objects are sliding
5. Sliding friction – Objects are at rest

Ans: 1 – e, 2 – c, 3 – b, 4 – a, 5 – d

1. Barometer – reduce friction
2. Increasing friction – atmospheric pressure
3. Decreasing friction – cause of friction
4. Lubricants – increasing area of contact
5. Irregular surface – decreasing area of contact

Ans: 1 – b, 2 – d, 3 – e, 4 – a, 5 – b

### VI. ANALOGY

1. Knot in a thread : _____ friction; ball bearing : _____ friction Ans : static, rolling
2. Downward force : weight ; Upward force offered by liquid : _____Ans : Buoyant force

### VII. Problems:

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

 Weight a Stone = 500 N area = 25 cm2 = 0.25m2 Pressure = force / area = 500 / .25 = 50000 / 25 Pressure = 2000 Nm2

### VIII. Assertion & Reason.

#### 1. Direction: Mark the correct choice as

1. If both assertion and reason are true and the reason is the correct explanation of the assertion.
2. If both assertion and reason are true, but the reason is not the correct explanation of the assertion.
3. If the assertion is true, but the reason is false.
4. If the assertion is false, but the reason is true

#### Reason : Sharp edges exert more pressure.

• Ans : If both assertion and reason are true and the reason is the correct explanation of the assertion.

#### Reason : Broad straps last for long life.

• Ans : If both assertion and reason are true, but the reason is not the correct explanation of the assertion.

#### Reason : Water strider experiences less buoyant force.

• Ans : If the assertion is true, but the reason is false.

### IX (A). Answer the following in one or two sentences (LOT):

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

Force can change the static condition of a body.

• If you squeeze a sponge, its shape changes.
• If you pull a rubber band, it becomes longer.

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

Force can change the static condition of a body.

• A rest rubber ball begins to move, when a force applied on it.
• Player applies a force on the stationary football while taking a penalty kick is football match. The force applied by player makes the football move towards the goal.

• Force

#### 4. What do you feel when you touch a nail immediately after it is hammered into a wooden plank? Why?

• The nail becomes hot due to friction. Friction changes kinetic energy to heat.

#### 5. How does the friction arise between the surfaces of two bodies in relative motion?

• The force of friction is arised by the interlocking of the irregularities of the two surfaces.

#### 6. Name two instruments, which help to measure the pressure of a fluid.

1. Manometer
2. Pressure gauge.

#### 7. Define one atmosphere.

• The pressure exerted by this mercury column is considered as the pressure of magnitude ‘one atmosphere’ (1 atm).

#### 8. Why are heavy bags provided with broad straps?

• Broader straps are provided on a back – pack for giving less pressure on the shoulders by providing a larger area of contact with the shoulder.

#### 9. How does surface tension help a plant?

• Water molecules rise up due to surface tension. Xylem tissues are very narrow vessels present in plants. Water molecules are absorbed by the roots and these vessels help the water to rise upward due to “capillarity action” which is caused by the surface tension of water.

#### 10. Which has greater viscosity, oil or honey? Why?

Honey has greater viscosity.

Reason : Thicker liquids are more viscous than thinner liquids. As honey has greater viscosity, more frictional force will be acting on it.

### X. Answer the following questions with a few sentences (MOT):

#### 1. Define friction. Give two examples of the utility of friction In day to day life.

Frictional force or friction arises when two or more bodies in contact move or tend to move, relative to each other.

• Wheels provided in vehicles.

#### 2. Write down three ways of minimising friction.

• Decreasing the area of contact.
• Using lubricants.
• Using ball bearing.

#### 3. How do sallors protect thelr ship during a heavy storm?

• During a heavy storm, sailors pour soap powder or oil into the sea near their ship to decrease the surface tension of sea water. This process reduces the impact of the violent water current against the all of ship.

#### 4. Write down three applications of Pascal’s law.

• In an automobile service station, the vehicles are lifted upward using the hydraulic lift, which works as per Pascal’s law.
• The automobile brake system works according to Pascal’s law.
• The hydraulic press is used to make the compressed bundles of cotton or cloth so as to occupy less space.

#### 5. Why ls a ball bearing used in a cycle hub?

• Since, the rolling friction is smaller than sliding friction, sliding is replaced by rolling with the usage of ball bearings. We can see lead shots in the bearing of a cycle hub.

### XI. Answer the following questions in detail:

#### 1. “Friction is a necessary evil”- explain.

Friction is a necessity in most of our day to day activities. It is desirable in most situations of our daily life.

• We can hold any object in our hand due to friction.
• We can walk on the road because of friction. The footwear and theground help us to walk without slipping.
• Writing easily with a pen on paper is due to friction.
• Automobiles can move safely due to friction between the tyres and the road. Brakes can be applied due to frictional resistance on brake shoes.
• We are able to light a matchstick, sew clothes, tie a knot or fix a nail in the wall because of friction.
• Though it is giving a negative effect, in most of our day to day life friction helps us to make our life easy. So, it is called as “necessary evil”.

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

Friction can be classified into two basic types:

• Static friction
• Kinetic friction.

Static friction :

The friction experienced by the bodies, which are at rest is called static friction. (E.g : All the objects rigidly placed to be at rest on the Earth, a knot in a thread.

Kinetic friction :

Friction existing during the motion of bodies is called kinetic friction.

Further, kinetic friction can be classified into two:

• Sliding friction
• Rolling friction.

Sliding friction :

When a body slides over the surface of another body, the riction acting between the surfaces in contact is called sliding friction.

Rolling friction

When a body rolls over another surface, the friction acting between the surfaces in contact is called rolling friction. Rolling friction is less than sliding friction. That is why wheels are provided in vehicles, trolleys, suitcases etc.

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

To understand about the frictional force between the layers of liquid in motion.

Materials required : Different kinds of liquid (coconut oil, honey, water, ghee), glass plates – 4 nos.

Procedure :

• Take a small quantity of different kinds of liquid like coconut oil, honey, water and ghee etc., in a cup.
• Place one drop of each liquid on a separate glass plate.
• Next, gently raise one end of the glass plate, one by one, so as to allow the liquid to slide down the smooth surface of the plate.
• Observe the speed of each liquid.

Observation:

Each liquid moves with a different speed. Water flows faster than other liquids. Coconut oil flows with a moderate speed. Ghee flows very slowly.

#### 4. Explain how friction can be minimised.

Using lubricants:

• A substance, which reduces the frictional force, is called a lubricant. Example: Grease, coconut oil, graphite, castor oil, etc.
• The lubricants fill up the gaps in the irregular surfaces between the bodies in contact. This provides a smooth layer thus preventing a direct contact between their rough surfaces.

Using ball bearing:

Since, the rolling friction is smaller than sliding friction, sliding is replaced by rolling with the usage of ball bearings. We can see lead shots in the bearing of a cycle hub.

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

Forces and Pressure Book Back Solution

• Take a plastic bottle. Punch three holes on its side in the same direction, but at different heights.
• Now pour some water into it and let it flow through the holes. Observe the flow of water.
• The water comes out from all the holes with a different force and falls on the table at points that are at variable distances from the bottle.
• Water from the lowest hole comes out with the greatest force and falls at a point that is at the maximum distance from the bottle.
• Water from the topmost hole comes out with the least force and falls at the point that is at the minimum distance from the bottle.
• This activity confirms that the pressure in a liquid varies with the depth of the point of observation in it