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Page No 146:

Question 1:

A force can move a stationary object. If force is applied to a stationary object, does it always move?

Answer:

No, it is not necessary that the stationary object will move always when a force is applied. If there is a friction force between the object and the ground then the friction force will oppose the motion of the object. Hence, the stationary object will move only if the applied force is greater than the friction force between the object and the ground. 

Page No 146:

Question 2:

Two equal forces act on an object. Is this information enough to say which way the object will move? Give reasons.

Answer:

No, this information is not enough. Two equal forces may or may not cause an object to move. If both the forces are acting on the same side of the object and are large enough to overcome the friction force between the object and the ground, then the object will move. If both the forces are acting on the object from the opposite directions then the object will not move as the net force on the object will become zero. 

Hence, the direction of the force and value of the friction force must be defined in order to determine the movement of the object.

Page No 146:

Question 3:

Name the force that acts on all bodies on the earth at all times.

Answer:

Gravitational force is the force which acts all the time on all the bodies present on the earth. It's equal to the weight of a body and always directed towards the center of the earth. 

Page No 146:

Question 4:

Name a force that always opposes motion.

Answer:

The friction force is the force which always opposes the motion. Friction force acts due to irregularities and roughness of the surface.

Page No 146:

Question 5:

Can a force act on an object even when it is not in contact with the object? Give examples.

Answer:

Magnetic force is the force which can act on an object without making any contact with the objects. Example: Force exerted by a magnet on another magnet.



Page No 151:

Question 1:

A force acts perpendicular to a given surface. What is the quantity 'force per unit area' known as?

Answer:

The quantity 'force per unit area' is known as 'pressure'. 

Page No 151:

Question 2:

How is pressure related to the area over which a force acts?

Answer:

The pressure is defined as the force per unit area. So, the pressure is indirectly proportional to the area over which the force acts. 

Page No 151:

Question 3:

What is the force of 1 N acting over an area of 1 m2 called?

Answer:

Force per unit area is defined as the pressure. If 1 N of force is acting over an area of 1 m2, then it would be called unit pressure or pressure of 1 Pa.
 

Page No 151:

Question 4:

How does the pressure exerted by a liquid vary with  a. direction? b. depth?

Answer:

(a) The pressure exerted by a liquid is the same in all the directions at the same height.
(b) But it is directly proportional to the depth in the liquid. As the depth increases, the pressure exerted by the liquid increases too.

Page No 151:

Question 5:

You are put into a room where the air pressure is very low. What effect will this have on your body?

Answer:

The air present in the atmosphere exerts pressure on our body, but that pressure is compensated by our blood pressure which acts from inside out. If the air pressure of the surroundings will become very low then our blood vessels will burst out due to excess pressure of the blood inside our body.



Page No 152:

Question 1:

There is one force which is exerted by all matter on all other matter. Which force is this?
(a) gravitational force
(b) magnetic force
(c) electrostatic force
(d) frictional force

Answer:

(a) Gravitational force

According to Newton, every object in this universe attracts the other object with gravitational force.

Page No 152:

Question 2:

Which of these is a contact force?
(a) friction
(b) magnetic force
(c) gravitational force
(d) electrostatic force

Answer:

(a) Friction

Frictional force is a contact force which resists the relative motion between two surfaces.



Page No 153:

Question 3:

Which of the following is true for the pressure exerted by a liquid?
(a) pressure does not depend on depth.
(b) pressure is only exerted in the downward direction.
(c) Pressure is exerted both downwards and sideways but downwards pressure is greater than sideways pressure.
(d) At the same depth, pressure is same in all directions.

Answer:

(d) At the same depth, pressure is the same in all directions.

Pressure exerted by a liquid increases with depth and acts in all directions, but it is the same at all points in a horizontal plane at a given depth in a stationary liquid.

Page No 153:

Question 4:

The SI unit of pressure is
(a) kg/m3.
(b) kg/m2.
(c) pascal.
(d) newton.

Answer:

(c) Pascal

The SI unit of pressure is the pascal (Pa).

Page No 153:

Question 5:

Two forces A and B act on an object in opposite. A is bigger than B. The net force on the object is
(a) A + B acting in the direction of A.
(b) A − B acting in the direction of A.
(c) A + B acting in the direction of B.
(d) A − B acting in the direction of B.

Answer:

(b) A - B acting in the direction of A

When two forces are applied on an object in opposite directions, the net force on the object is the difference between the forces; this net force acts in the direction of the greater force.

Page No 153:

Question 6:

Which of these can a force acting on an object not change?
(a) direction of motion
(b) state of rest
(c) shape
(d) mass

Answer:

(d) Mass

Mass is the amount of matter in an object; it cannot be changed by the application of a force.

Page No 153:

Question 7:

The weight of an object is due to
(a) gravitational force.
(b) frictional force.
(c) both gravitational and frictional force.
(d) neither gravitational nor frictional force.

Answer:

(a) gravitational force

The weight of an object is nothing but the gravitational force with which it is attracted towards the centre of the Earth.

Page No 153:

Question 8:

A force of 10 N acts on an area of 0.1 m2. The force is kept the same but the area is reduced to half. Which of the following is true?
(a) The pressure does not change.
(b) The pressure reduces to half.
(c) The pressure increases by 1.5 times.
(d) The pressure doubles.

Answer:

(d) The pressure doubles.

Pressure (P) = Force (F)/Area (A)
= 10 N / 0.1 m2 = 100 N/m2
= 100 Pa
Now, according to the question,
New area, A' = A/2 and F' = F
∴ P' = F'/A' = 2F/A = 2P
= 2 × 100 = 200 Pa
Thus, the pressure gets doubled.

Page No 153:

Question 1:

All pushes and pulls are forces. True or false?

Answer:

True.

All pushes and pulls are forces and the direction in which an object is pushed or pulled is considered the direction of the force.

Page No 153:

Question 2:

If a force acts on a body it will move in the direction in which the force acts. Ture or false?

Answer:

False.

Suppose you are driving a bicycle and you apply the brakes. Although the stopping force will be applied on the direction opposite to the direction of motion of the bicycle, the bicycle will move forward. Thus, the bicycle will move in the opposite direction of the applied force.

Page No 153:

Question 3:

A force with magnitude A and another with magnitude B act on an object in the same direction. What is the net force acting on the object?

Answer:

The net force acting on the object is A + B.
If two forces are applied on an object in the same direction, then the net force acting on the object is a single force whose magnitude is the sum of the two forces.

Page No 153:

Question 4:

A force with magnitude A and another smaller force with magnitude B act on an object in the opposite directions. What is the net force acting on the object?

Answer:

If two forces are applied on an object in opposite directions, then the net force acting on the object is the difference between the two forces. Thus, in the given case, the net force acting on the object is A - B.

Page No 153:

Question 5:

Does a force acting on a body always cause a change in its state of motion?

Answer:

No. The net force of all forces determines the acceleration of the body.

Page No 153:

Question 6:

Only the earth exerts gravitational force on all objects. True or false?

Answer:

False.

All matter in the universe exert gravitational force on other matter.

Page No 153:

Question 7:

What measures the earth's gravitational pull on an object− its weight or mass?

Answer:

An object’s weight is a measure of the Earth's gravitational force acting on it.

Weight = Force of the Earth's gravity acting on an object 
            = Mass × Acceleration due to gravity

Page No 153:

Question 8:

This force acts from a distance and affects only objects made of certain metals such as iron. Name the force.

Answer:

Magnetic force
A magnet can influence a magnetic material, such as a piece of iron, even when they are not in contact.

Page No 153:

Question 9:

Which type of force is exerted by an electrostatic charge?

Answer:

Electrostatic force is exerted by an object with electrostatic or static charge.

Page No 153:

Question 10:

Which force tends to slow down objects or prevent them from moving?

Answer:

Friction is a force that slows down objects or prevents them from moving.

Page No 153:

Question 11:

The larger the area over which a force acts, the __________ is the pressure.

Answer:

The larger the area over which a force acts, the lesser the pressure is.

Page No 153:

Question 12:

Does pressure exerted by a liquid increase or decrease with depth?

Answer:

Pressure exerted by a liquid increases with depth and acts in all directions.



Page No 154:

Question 13:

Every square centimetre of your body experiences a force equal to the weight of 1 kg due to the atmospheric pressure. True or false?

Answer:

True.

Due to the atmospheric pressure of air, we experience a force equal to that exerted by a mass of 1 kg on every square centimetre of our body.

Page No 154:

Question 14:

Atmospheric pressure increases with height. True or false?

Answer:

False.

Atmospheric pressure is caused by the weight of air molecules above the atmosphere. As we go to higher altitudes, the air becomes thinner and the atmospheric pressure decreases.

Page No 154:

Question 1:

What two things fully describe a force?

Answer:

Force is a vector quantity. It can be described by two things: its magnitude and the direction in which it acts.

Page No 154:

Question 2:

If several forces act in different directions on a body, in which direction will the body move?

Answer:

If several forces act on a body in different directions, the effect on the body will be due to the magnitude and direction of the net force acting on it. So, the body will move in the direction of the resultant force.

Page No 154:

Question 3:

If an object is thrown up, it finally comes down. Why?

Answer:

Gravitational force is the force with which the Earth pulls everything towards itself. So, if an object is thrown up, it slows down and then comes down because of the gravitational force of the Earth.

Page No 154:

Question 4:

What is the difference between mass and weight?

Answer:

 

Mass Weight

The mass of a body is the quantity of matter contained in it.

The weight of a body is the force with which it is attracted towards the centre of the Earth.

It is a scalar quantity that has only magnitude, no direction. Weight is a vector quantity having magnitude as well as direction.
The SI unit of mass is the kilogram (kg). The SI unit of weight is the newton (N).




 

 

Page No 154:

Question 5:

Why can a magnet act from a distance?

Answer:

Every magnetic substance is surrounded by its own invisible magnetic field, depending on its pole strength. A magnet can act from a distance because of the field of magnetic force.

Page No 154:

Question 6:

What is pressure? What does pressure depend on?

Answer:

Pressure is the force acting perpendicularly on a unit area of an object.
Pressure = Force/Area
Pressure depends on two factors:
(i) Force applied
(ii) Area over which the force acts

Page No 154:

Question 7:

What is atmospheric pressure? What is it caused by?

Answer:

Atmospheric pressure is the force per unit area exerted on the surface of the Earth by the atmosphere. Atmospheric pressure is caused by the weight of air molecules in the atmosphere.

Page No 154:

Question 8:

Why is it difficult to cut vegetables with a blunt knife?

Answer:

It is difficult to cut vegetables with a blunt knife because it has more area compared to a sharp knife, which means that the pressure exerted by the blunt knife is less than that of the sharp knife. In the case of blunt knife, even a good effort may go futile, as the pressure applied by its edge may not be sufficient to cut vegetables.

Page No 154:

Question 1:

What is force? What are the four main effects of force?

Answer:

Force:
Force can be defined as an influence that tends to set a stationary body in motion or stop a body from moving. The SI unit of force is kg m/s² or newton.

Effects of force:
1. It can make a stationary object move and stop.
2. It can change the speed of a moving object.
3. It can change the direction of a moving object.
4. It can change the shape or size of an object.

Page No 154:

Question 2:

What is the difference between contact and non-contact forces? Explain with the help of one example of each.

Answer:

 

Contact Force Non-Contact Force
Contact force is the force in which a direct contact between two bodies is required. Non-contact force is the force in which two bodies interact without any direct contact.
For example: When you stop pedalling a bicycle, it slows down and stops after some time. This is because of the frictional force or contact force. For example: When a ball is thrown upwards, it ultimately falls down because of the gravitational force of the Earth. In this case, the gravity of the Earth forces the ball to fall down without any direct contact. So, it is a non-contact force.

Page No 154:

Question 3:

What is friction? Explain with an example.

Answer:

Friction is a resistance caused by a surface. It is a contact force that resists the relative motion between two surfaces. For example, when you stop pedalling a bicycle, it slows down and stops after some time. This is because of the frictional force.

Page No 154:

Question 4:

Why are caterpillar tracks used in battle tanks instead of tyres?

Answer:

Caterpillar tracks are used in battle tanks instead of tyres because they considerably increase the area of contact. The larger the area of contact, the smaller the effect of force. Thus, a caterpillar track helps a battle tank in distributing its weight more evenly over a large surface area than wheels can. This prevents it from sinking in areas where wheeled vehicles of the same weight can easily sink.

Page No 154:

Question 5:

How does the pressure exerted by a liquid change with depth? Explain with a diagram.

Answer:


Suppose we have a liquid column of height h.



The pressure exerted by the liquid is given by the following expression:
P = ρgh

Here,
ρ = Density of the liquid
g = Acceleration due to gravity
h = Height/depth of the liquid column

Here, pressure is directly proportional to the height of the liquid column. If we go down into the the liquid, the depth(h) of the liquid column increases. Thus, the pressure in the liquid increases with increasing depth.

Page No 154:

Question 6:

When you press a rubber sucker on a smooth surface, it sticks to the surface. Explain why this happens?

Answer:

When we press a rubber sucker on a smooth surface, we remove some of the air between the sucker and the smooth surface. This deficiency of air lets the air outside to exert pressure on the surface of the sucker. Force due to the pressure keeps the sucker stuck to the surface.

Page No 154:

Question 1:

Two forces act on an object. Explain what the net force acting on the object will be in different situations.

Answer:

If two forces act on an object in different directions, then the effect on the body will be due to the magnitude and direction of the net force acting on it. Thus, the body will move in the direction of the resultant force.
We can have two situations for two forces:
(i) If forces A and B are applied on an object in the same direction, then they will add up.
(ii) If forces A and B are applied on an object in opposite directions, then the net force on the object will be the difference between these forces.

Page No 154:

Question 2:

Explain the effects that a force acting on an object can have on the object.

Answer:

Effects of force:

1. Force can make a stationary object move or change its position of rest.

2. Force can change the speed of a moving object.

3. Force can change the direction of a moving object.

4. Force can change the shape or size of an object.

Page No 154:

Question 3:

Describe an experiment to show that air exerts pressure in all directions.

Answer:

Take a tin and boil some water in it. When steam begins to come out, cork it and stop heating it further. Put the tin in a trough and pour some cold water mixed with ice on it. You will observe that the tin gets crushed and becomes irregular in shape. This is because the cold water condensed the steam inside the tin to water. This left no air inside the tin. Thus, the pressure inside the tin became much lesser than that on the outside. The air pressure from outside acted on the tin from all directions and crushed it.

The above experiment shows that air exerts pressure in all directions.

Page No 154:

Question 1:

If a body is moving with uniform speed in a particular direction on a perfectly smooth surface, then no force is acting on it. True or false? Explain.

Answer:

False.

If no external force is applied on the body, then there must be two equal and opposite forces acting on the body. Both these forces add up to zero.

1. Weight of the body, W = mg (In downward direction)
2. Reaction force, N = W (In upward direction)

Because the surface is smooth, there is no frictional force.

Page No 154:

Question 2:

If you use a stick to push an object, the muscular force acting on the object is a non-contact force since your body is not in contact with the object. True or false? Explain.

Answer:

False.

Contact force is the force in which a direct contact is required between two bodies. The contact may also be with the help of a stick or a piece of rope.
If we use a stick to push an object, we make the contact with the help of stick. This push or force is caused by the action of the muscles in our body. This muscular force is a contact force.

Page No 154:

Question 3:

Gravitational force exists between you and a building. Why are you not pulled towards the building?

Answer:

The magnitude of gravitational force depends on the masses of two objects and the distance between them.
Because the Earth has the greater mass, objects on the Earth are pulled towards it. Also, there exists gravitational force between us and any building. However, because of the smaller mass of the building as compared to the earth, this force is very small to experience. Therefore, we are not pulled towards the building.

Page No 154:

Question 4:

Why do you think it is necessary to define a separate quantity called pressure? Why is defining only force not enough?

Answer:

It is necessary to define pressure as a separate quantity in order to evaluate the force acting on a surface. To measure and describe the effect of a force acting on a surface, we need both the amount of force applied and the area over which it is applied. So, only force is not enough for this purpose. We need a quantity that includes both the above things, and this quantity is pressure, which is defined as the force exerted per unit area.



Page No 155:

Question 5:

Suppose you are going in a jet plane, and one of the windows breaks due to an accident. What problems do you think you will face?

Answer:

If one of the windows in a jet plane breaks due to an accident, the plane may undergo explosive decompression. 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 cause blood vessels to burst.

Page No 155:

Question 6:

If you press a rubber sucker on a rough surface, it does not stick to the surface. Why?

Answer:

If we press a rubber sucker on a rough surface, it will not stick to the surface because an airtight seal cannot be achieved between the surfaces. It is so because air rushes into the space between the rubber sucker and the rough surface.

Page No 155:

Question 1:

Pallavi is a weightlifter. The first picture shows her on earth and the second on the moon. Can you answer the question that is troubling Pallavi?

Answer:

The value of gravity on the moon is one-sixth of its value on the earth. So, the objects weigh less on the surface of the moon as compared to the surface of the earth. The weight which Pallavi couldn't lift on the earth, she can easily lift on the moon because it feels lighter on the surface of the moon. 



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