# STD 8 Science Chapter-3

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• 1.

### The SI unit of force is---------

• A.

Dyne

• B.

Newton

• C.

Joule

• D.

Option 4

B. Newton
Explanation
The correct answer is Newton. Newton is the SI unit of force, named after Sir Isaac Newton. It is defined as the force required to accelerate a mass of one kilogram at a rate of one meter per second squared. This unit is commonly used in physics to measure forces such as gravitational force, friction, and applied forces.

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• 2.

### The air pressure on our body is equal to ---------pressure

• A.

Atmospheric

• B.

Sea bottom

• C.

Face

• D.

Option 4

A. Atmospheric
Explanation
The air pressure on our body is equal to atmospheric pressure because the air around us exerts pressure in all directions. Atmospheric pressure is the force per unit area exerted on a surface by the weight of the air above it. This pressure is what we feel on our body and is essential for our survival.

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• 3.

### For a given object the buoyant force is liquid of difference---------- is different

• A.

The same

• B.

Density

• C.

Area

• D.

Option 4

C. Area
Explanation
The buoyant force on an object is determined by the density of the liquid it is submerged in and the volume of the liquid it displaces. The area of the object does not affect the buoyant force. Therefore, the buoyant force is the same regardless of the area of the object.

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• 4.

### The SI unit of pressure is-------------

• A.

N/m3

• B.

N/m2

• C.

Kg/m2

• D.

Option 4

B. N/m2
Explanation
The SI unit of pressure is N/m2 because pressure is defined as force per unit area. The unit N/m2 represents the force in Newtons exerted on a surface area of 1 square meter. This unit is commonly known as Pascal (Pa) and is widely used in scientific and engineering calculations involving pressure.

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• 5.

### -------------Surrounds the Earth from all side

• A.

Air

• B.

Water

• C.

Land

• D.

Option 4

A. Air
Explanation
Air is the correct answer because it surrounds the Earth from all sides. The Earth is enveloped by a layer of gases known as the atmosphere, which is composed mainly of nitrogen, oxygen, and trace amounts of other gases. This layer of air extends from the surface of the Earth to outer space, providing protection, regulating temperature, and supporting life on our planet.

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• 6.

### Object are attracted to the Earth due to the force of gravity

• A.

True

• B.

False

A. True
Explanation
Objects are attracted to the Earth due to the force of gravity. Gravity is a fundamental force in nature that exists between any two objects with mass. The Earth has a large mass, so it exerts a gravitational pull on objects near its surface. This force of gravity is what keeps objects on the ground and causes objects to fall when dropped. Therefore, it is true that objects are attracted to the Earth due to the force of gravity.

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• 7.

### The tendency of an object to remain in its existing state is called its inertia

• A.

True

• B.

False

A. True
Explanation
The explanation for the given correct answer is that inertia is indeed the tendency of an object to remain in its existing state. This means that an object at rest will stay at rest, and an object in motion will continue moving at a constant velocity unless acted upon by an external force. Inertia is a fundamental property of matter and is described by Newton's first law of motion. Therefore, the statement "The tendency of an object to remain in its existing state is called its inertia" is true.

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• 8.

### Force does not give motions to a stationary object

• A.

True

• B.

False

B. False
Explanation
Force does give motion to a stationary object. According to Newton's first law of motion, an object at rest will remain at rest unless acted upon by an external force. Therefore, force is necessary to initiate motion in a stationary object.

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• 9.

### Rubber band when stretched expand

• A.

True

• B.

False

A. True
Explanation
Rubber bands are known for their elasticity, meaning they can stretch and expand when a force is applied. This is due to the molecular structure of rubber, which allows it to return to its original shape after being stretched. Therefore, it is true that rubber bands expand when stretched.

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• 10.

### The force exerted perpendicularly on a unit area is called inertia

• A.

True

• B.

False

B. False
Explanation
The force exerted perpendicularly on a unit area is actually called pressure, not inertia. Inertia refers to an object's resistance to changes in its motion. Therefore, the given statement is false.

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• 11.

### Air surrounds the Earth from all side this layer of air is called

• A.

Atmosphere

• B.

Hydrosphere

• C.

Lithosphere

• D.

Option 4

A. Atmosphere
Explanation
The correct answer is Atmosphere. The atmosphere is a layer of air that surrounds the Earth from all sides. It is composed of various gases, such as nitrogen, oxygen, carbon dioxide, and others. The atmosphere plays a crucial role in supporting life on Earth by regulating temperature, protecting against harmful radiation, and providing oxygen for respiration.

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• 12.

### The density of sea water is------ then the density of freshwater

• A.

Lower

• B.

Higher

• C.

Non above this

• D.

Option 4

B. Higher
Explanation
Sea water is denser than freshwater because it contains a higher concentration of dissolved salts and minerals. These dissolved substances increase the mass of the water, making it denser. This is why objects float more easily in freshwater compared to seawater.

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• 13.

### When a--------- is applied on a stationary object  it move

• A.

Force

• B.

Inertia

• C.

Pressure

• D.

Option 4

A. Force
Explanation
When a force is applied on a stationary object, it causes the object to move. Force is a push or pull that can change the state of motion of an object. In this case, the force overcomes the inertia of the object, which is the tendency of an object to resist changes in its motion. Therefore, the correct answer is Force.

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• 14.

### The pressure created by this air column on Earth is known as

• A.

Unit of pressure

• B.

Atmospheric pressure

• C.

Gas pressure

• D.

Option 4

B. Atmospheric pressure
Explanation
The pressure created by the air column on Earth is known as atmospheric pressure. This pressure is exerted by the weight of the Earth's atmosphere and is responsible for various atmospheric phenomena such as weather patterns and wind. Atmospheric pressure is measured in units such as millibars or inches of mercury, and it decreases with increasing altitude.

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• 15.

### The atmospheric exist to about -------km height

• A.

16 km

• B.

20 km

• C.

18 km

• D.

Option 4

A. 16 km
Explanation
The atmosphere exists up to a height of approximately 16 km.

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