Living Science 2020 2021 for Class 8 Science Chapter 11 - Force And Pressure

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. 

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.

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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. 

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

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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.

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

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

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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.
 

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(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.

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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.

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(a) Gravitational force

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

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(a) Friction

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

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(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.

Answer:

(c) Pascal

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

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(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.

Answer:

(d) Mass

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

Answer:

(d) The pressure doubles.

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

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.

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.

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.

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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.

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

Answer:

False.

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

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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 $\times$ Acceleration due to gravity

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

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

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

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

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

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.

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.

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

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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.

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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.

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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.

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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

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.

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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.

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.

Answer:

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.

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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.