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#### Page No 171:

(a) newton
Newton is the SI unit of force.

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(a) gravity
The pull of the Earth is a force called gravity. Gravitational force is the force of attraction between all material objects.

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(a) Poles
The weight of the body is maximum at the poles.

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(a) 9.80 N
1 kg wt is equal to 9.80 N.

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(a) mass
The mass of a body remains the same everywhere.

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(c) magnetic force
Magnetic force can be felt from a distance. Hence, it is a non-contact force.

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(a) N
The SI unit of thrust is newton because thrust is a kind of force.

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(a) Pressure
Force per unit area is called pressure.

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(b) 1 Nm–2
1 pascal is equal to 1 Nm-2.

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(c) 105 Pa
At the sea level, the atmospheric pressure is 105 Pa.

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1. The weight of the body is measured by weighing balance.
2. SI unit of force is newton.
3. The brakes of all the automobiles work only because of Pascal's law.
4. Force can change the speed of a moving body.
5. Weight is the force with which the Earth  pulls a body towards its centre.
6. The total force acting on a given surface is called thrust force.
7. The force acting per unit area of the surface is called pressure.
8. Hydraulic devices are based on Pascal's law.
9. Increasing the area of the surface, pressure decreases for a given applied force.
10. Pressure increases as we move from low density to high density.

#### Page No 172:

If one of the windows of a jet plane breaks due to an accident, the plane may undergo explosive decompression due to low atmospheric pressure. At such low pressure, we can have problems like nose bleeding because the pressure exerted by the blood in our body will be much higher than the pressure outside. This can also cause blood vessels to burst open.

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If no external force is being applied on the body, then there must be two equal and opposite forces acting on the body. These forces are:
1. Weight of the body, W = mg (in the downward direction)
2. Reaction force, N = W (in the upward direction)
The resultant force becomes zero.
Also, the surface is smooth; therefore, there is no frictional force.

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As we go deeper into a liquid, the weight of the liquid above us keeps on increasing. However, the area upon which this weight (force) is acting remains unchanged. Thus, with this increasing force, the pressure exerted by the liquid increases.

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When an iron object is kept near the magnet, the magnet exerts magnetic force on the iron object and attracts it from a distance. Thus, contact between the iron object and the magnet is not required for the magnetic force to act. Therefore, magnetic force is a non-contact force.

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When a number of forces act on a body, then the body will move in the direction of the resultant force.

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When an object is thrown upwards, the force of gravity starts acting it. This force works in the direction opposite to the direction of the movement of this object, i.e. downwards. When gravity overcomes the force applied to throw the object, the object reverses the direction of its motion and finally comes down.

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At least two forces are required to give rise to a force of interaction.

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When we press the dropper, with its nozzle in the water, the pressure inside the nozzle increases. Because of this, the air moves out of the nozzle into the water forming bubbles. When we release the bulb, with the nozzle still in the water, the pressure inside the nozzle becomes lower than the atmospheric pressure, which makes the water to rush inside the nozzle.

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When we rub a balloon with a woollen cloth, the balloon acquires a negative charge. This negatively charged balloon, when placed near a wall, attracts the positive charges on the wall and sticks to it.  Electrostatic force is responsible for this attraction between the balloon and the wall.

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(a) The pressure will get reduced to half if the area is doubled and the force is kept constant because pressure varies inversely with area.
(b) If the force is doubled, keeping the area constant, then the pressure will get doubled because pressure varies directly with force.

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The force applied to a hot piece of iron when it is hammered changes the shape of the piece of iron. Hot iron is softer and can be moulded into any desired shape.

#### Page No 172:

1.True

2.True

3.False
Mass is the quantity of matter contained by a body, but weight is the force with which the Earth pulls a body towards its centre.

4.True

5.True

6.False
Pressure is the force per unit area acting on a surface. Thrust is the total force acting on a given surface area.

7.True

8.True

9.False
Pressure exerted by the gases decreases with height.

10.False
Atmospheric pressure decreases at higher attitudes.

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 Column A Column B 1. Spring balance (b) Weight 2. Force exerted by muscles (a) Muscular force 3. Thrust (SI unit) (d) N 4. Hydrostatic pressure (e) Nm–2 5. Weight (c) kg wt

#### Page No 173:

When water in a can is heated, the steam that is produced expels most of the air from inside the can while moving out. When this can is allowed to cool down with its mouth covered with an air tight lid, the amount of air present inside cannot exert enough pressure on the walls of the can to counter the atmospheric pressure. Thus, the shape of the can gets deformed.

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Our body is adapted to bear pressure equal to or close to the atmospheric pressure. The blood in our vessels exerts pressure on the walls of our arteries and veins. A low pressure zone or area, for e.g. a room as mentioned in the question, may have adverse effects on our body as the arteries may burst open causing haemorrhages or bleeding from the nose. We may also encounter breathing problems due to low pressure in the room.

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It is more difficult to pull a boat on the beach than on the sea. This is because the sand on the beach exerts a much larger force of friction on the boat as compared to that exerted by the water in the sea.

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When we drink a soft drink with the help of a straw pipe, the atmospheric pressure acting on the surface of the soft drink forces it to move upwards.

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In case of circular motion, centripetal force acts on a body, which makes it move in a circle and the direction of motion of the body changes continuously, while its speed remains constant. Therefore, it is possible for a force to change the direction of a body without changing its speed.

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No, force can produce motion in a body only when it dominates the friction force. When friction force is more than the applied force, a body cannot move.

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Yes, weight is a force. It is the force with which the Earth pulls a body towards its centre.

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Force may be defined as the pull or push on a body. For example, if we want to make a football move, we have to apply force on it through a kick.
.

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Contact force may be defined as the force that comes into play when two objects are in physical contact with each other.
For example, friction force is a contact force because it acts only when the two objects are in contact with each other and one of them tries to move over the surface of the other.

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Different types of forces are:
1. Muscular force
2. Frictional force
3. Mechanical force
4. Magnetic force
5. Electrostatic force
6. Gravitational force

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We may define pressure as the force applied per unit surface area.

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The SI unit of pressure is pascal (pa).

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 Mass Weight 1. Mass is the quantity of matter contained by a body. 1. Weight is the force with which the Earth pulls a body towards its centre. 2. Mass of the body remains the same at all places. 2. Weight of a body does not remains the same at all places on the Earth. 3. Mass is measured in kg. 3. Weight is measured in kgf or N.

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 Electrostatic Force Magnetic Force Electrostatic force is a force that exists between two charged particles. Magnetic force is a force of attraction or repulsion exerted by a magnet on another magnet or magnetic material.

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 Contact Force Non-contact Force 1. Contact forces are the forces that come into play when two bodies are in contact with each other. 1. Physical contact of two bodies is not needed for non-contact forces to come into play. 2. Frictional force, muscular force and mechanical force are examples of contact force. 2. Magnetic force, gravitational force and electrostatic force are examples of non-contact force.

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 THRUST PRESSURE 1. Thrust is the total force acting on a surface. 1. Pressure is the force applied per unit area of a surface. 2. The SI unit of thrust is N. 2. The SI unit of pressure is Nm–2.

The unit of thrust is newton.
The unit of pressure is Nm-2.
Relation between thrust and pressure:
Pressure = Thrust/Area

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Pascal's law states that when pressure is applied to any part of a liquid, it gets equally transmitted to all the parts of the liquid.
Let us take some liquid in a glass vessel having holes at different places. When we apply a force to the piston, the pressure applied gets equally transmitted throughout the water and the water comes out from all the holes with equal force.

#### Question 1:

Calculate the weight of the body whose mass is 30 kg. Take g = 10 m/s2.

Mass of the body, m = 30 kg (Given)
g = 10 m/s2 (Given)

Weight of the body, W = mg
W = 30 × 10
W = 300 N

#### Question 2:

A girl weighing 50 kg is standing on a pencil heels. Each having an area of cross-section 1cm2. An elephant weighing 200 kg and foot area of cross-section of 250 cm2 standing on the floor. Which of the two exerts more pressure on the floor? (Given acceleration due to gravity = 10 m/s2)

Weight of the girl = 50 kg (Given)
Area of the cross-section = 1 cm2 (Given)

$\mathrm{P}=\frac{\mathrm{Force}}{\mathrm{Area}}$

Force = Weight of the body = 50 kg

1 cm = $\frac{1}{100}$

P = 500000 Nm–2

Weight of the elephant, W = 200 kg
Area of the cross-section of foot A = 250 cm2
A = 0.0250 m2

Pressure exerted by the elephant on land:

#### Question 3:

Where is pressure greater, 20 m below the sea level or 35 cm below?

Depth of water, h1 = 20 m
Density of water, $\mathrm{\rho }$ = 1000 kg/m3
Pressure of the water, P = h1$\mathrm{\rho }$
P =  20 196000 Pa

Depth of the water, h2 = 35 cm
= 35/100 m
= 0.35 m
Density of water $\mathrm{\rho }$ = 1000 kg/m3
Pressure of the water, P = h2$\mathrm{\rho }$
P =

It is clear that at 35 cm below the sea level, the pressure will be lower.

#### Question 4:

Calculate the pressure exerted by 76 cm column of mercury.

We know:
P = h$\mathrm{\rho }$g

Density of mercury, $\mathrm{\rho }$ =
Height of the column of mercury, h = 76 cm = 0.76 m

P = h$\mathrm{\rho }$g

#### Question 5:

A container contains water column of height 50 m. He connects two taps –– one at the bottom while the other 10 m above the bottom. He opens both the tabs simultaneously from which tap, water will touch the ground at a greater distance?

Water flowing out from the bottom of the container will touch the ground at a greater distance. Pressure increases with an increase in depth. Thus, water flowing from 10 m above the bottom will have a lower pressure than water at the bottom.

#### Question 6:

Two boys pull a rope from two ends with a force equal to 100 N each. What is the resultant force?

The resultant force will be zero as the two forces are equal in magnitude and are acting on in opposite directions.

#### Page No 174:

A nail is made thinner at the end that is to be driven inside an object. An end with a thinner area easily penetrates the surface of the object into which the nail has to be hammered. This is because a thinner area increases the pressure tremendously. The other end of the nail is made thicker because as the area increases, the pressure on this end, which bears all the hammering, is reduced and the nail is prevented from breaking down.

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The rear wheels of a tractor are made very wide in order to increase the area of contact of the wheels with the land. This helps in reducing the pressure exerted on the land by the tractor, thereby preventing the tractor from sinking into the mud in a field.

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Pressure of a liquid depends upon the following:
(a) Density of the liquid
(b) Depth of the liquid from where the observation is being taken

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Railway tracks are laid on wooden or iron sleepers so that the heavy thrust of the train is spread over the large surface area of the sleepers. Thus, they help in reducing the pressure on the ground by the rail and prevent the rail from sinking into the earth under the heavy pressure of the train.

#### Page No 174:

The following are the effects of atmospheric pressure:
(a) It is difficult to breath at higher altitudes due to lower atmospheric pressure.
(b) Injection syringes work due to atmospheric pressure.
(c) Sucking of water from a glass is possible due to atmospheric pressure.

#### Page No 174:

A pull or a push on an object is called force. Force can change the shape, direction or state of motion of a body.
The SI unit of force is called newton (N).
Gravitational unit of force is called kilogram force or kgf.
A kilogram force is the force with which a body of mass 1 kg is attracted towards the Earth.

#### Page No 174:

(i) Atmospheric pressure is the pressure exerted on the surface of the Earth by the weight of the column of air above the Earth's surface.

(ii) At high altitudes, the air pressure outside the pen is lesser than the air pressure inside the fountain pen. This difference in pressure makes the ink to flow from high pressure to low pressure.

#### Page No 174:

A manometer works according to Pascal's law, which states that when pressure is applied to any part of a liquid, it gets equally transmitted to the other parts of the liquid.

Construction: It consists of a U-shaped tube of glass filled with a liquid. It is simply a pressure gauge and it measures the difference in pressure exerted at the two ends of the apparatus.

Working: Initially, both ends of the tube are open and the liquid is at the same height in each leg. When pressure is applied to the liquid in one leg, it gets transferred to the liquid in the other leg, thereby raising it up in the column. The difference in the heights of the liquid in both the columns is used to measure the pressure.

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The following are the effects of force on a body:

1.  Force can change the direction of motion of a body.
For example, when we hit a moving ball with a bat, the force applied by us in hitting the ball changes the direction of the ball.

2. Force can produce motion.
For example, gravity makes all objects fall towards the surface of the Earth.

3. Force can change the shape of an object.
For example, when we beat a metal rod with a hammer, the shape of the metal rod changes. This shows that force can change the shape of the object.

4. Force can change the speed of on object.
For example, if we receive a push from behind while riding a bicycle, the cycle starts moving faster. This shows that force can change the speed of an object.

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According to the principal of gravitational force, all the bodies in the universe attract each other.
Gravitational force can be measured by measuring the weight of the bodies involved and the distance between them.
Gravitational force is measured in newton (N).

Gravitational force = $\mathrm{G}\frac{{\mathrm{m}}_{1}{\mathrm{m}}_{2}}{{\mathrm{r}}^{2}}$
Here, G is gravitational constant whose value is 6.67 x 10-11 Nm2/kg2.
m1 and m2 are the masses of the two bodies.
r is the distance between the two bodies.

#### Page No 174:

Force of friction is a contact force because it comes into play only when two bodies are in contact with each other and try to move over each other's surface.
A rusty machine does not move smoothly because of the huge friction between its parts, which acts in the direction opposite to that of the motion of the body, thereby making the motion of machine parts difficult.

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Contact Forces: Contact forces are the forces that come into play when two bodies are in contact with each other. For example, frictional force and mechanical force are contact forces.

Non-contact Forces: In non-contact forces, contact between two bodies is not required for the force to act. For example, gravitational force and magnetic force are non-contact forces.