9th Class Physics Chapter 7 Short Questions with Answers Solved

Question: 1

What is meant by buoyant force or up trust in fluids?

Answer: 1

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Buoyant Force or Upthrust:

The upward force exerted by the fluid on the object which is immersed in it is called upthrust or buoyant force. It is denoted by "Fb"

For Example:

If we try to push a pieces of crock underwater. We feel an up awrd, force that is pushign the cork back up. So, when we release the cork, it will rise to the surface and floats due to upthrust or buoyant force . Here , the buoyant force is greater than the weight of the cork, so it will move upward and float.

Question: 2

Why is the cutting edge of the knife made very thin?

Answer: 2

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As we know that

P = F/A............................(i)

eq (i) shows that pressur eis inversely proportonal to the area. That is greater the area, smaller will be the pressure and vice ersa. So in this way cutting edge of the knife is made thin to aply more pressure and things can be cut easily.

Question: 3

What is atomospheric Pressure?How is it measured by using a mercury barometer?

Answer: 3

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Atmospheric Pressure: A thick layer of air around the earth is called atmosphere. The pressure exerted by this atmopshere on the surface of earth called atmospheric pressure.

Question: 4

Define Plasma State.

Answer: 4

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Plasma State: Plasma is the fourth state of matter. It consists of free electrons and atoms from which the electrons have been removed. Plasma exists in the sun, where the rmouclear reactions take place at very high temperatures.

Question: 5

Write Properties of Gases.

Answer: 5

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i- A gas is made up of molecules which are is constant random motion.

ii. The distance between molecules is larger as compared to size of molecules.

iii. The attractive forces between the molecules are negligible except during collisions.

iv Their volume depends upon temperature and pressure.

v. The molecules of gases are having mximum kinetic energy.

Question: 6

Which material is more elastic, steel or rubber and why?

Answer: 6

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Steel is more elastic than rubber beause steel produce more resistance against the deformign force than rubber, So, more deforming force is required to deform steel while rubber can be easily deformed. As we know that the young's modulus of rubber is 1.40 x 10⁶N/m2 and steel is 20 x 10²⁰N/m2. Therefore, steel is more elastic than rubber.

Question: 7

Define and Explain Stress and Unit.

Answer: 7

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Stress: The sgtress is defined as the force applied per unit area of cross section on an elastic body to produce deformation. It is denoted by sigma.

Mathematical Form:

Mathematically it an be written as

Stress = Force /Area of cross section

Sigma = F/A

Unit: The SI Unit of stress is Nm-2 or Pascal

Question: 8

Define Atmospheric Pressure and weather:

Answer: 8

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Atmospheric Pressrue and weather: Baremeters ketp int he same plae at the same heigt above the sea level show some variations in atomsphereic pressure from day to day. There pressure variations are shown on wheather maps.

Isobars: The lines in the map joining all those places with the same atmospheric pressure are called isobars.

Unit: The unit for pressure used in a weather maps is called millibar or bar. It is exactely equal to 100000 Pa or 100 kPa which is approximately equal to normal atmospheric pressure.

Question: 9

Define Standard Atomospheric Pressure:

Answer: 9

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Standard Atomospheric Pressure: The pressure exerted by 760 mm of mercury column at sea level is known is standard atmospheric pressure or 1 atmospheric i.e.

1 atm = 1.013 x 10⁵ Pa

Question: 10

What is Hydraulic Brake and how does it works?

Answer: 10

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Hydraulic Brakes: The brarkes of vehicles work on the principle of Pascal. In such type of brakes, when the driver pushes doesn the pedal, the piston in the master cylinder exerts a force on the brake fluid and the resuling pressure is transmitted equally to the brake of the wheels. These force the brake shoes or pads against the wheels and stop the car. But when pressure is released from the pedal. The springs pull back the brake shoes and wheel again gtrun freely.

Question: 11

You would have probably experienced your ears 'popping' while driving in the mountains. Why eas 'pop'.

Answer: 11

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In plane area, the pressure of air inside the ears and outside remains the same, so we feel comfortable.

When we drive a car in mountains as height increases the atmospheric pressure decreases. At this stage the pressrue of air insid the ear is higher than outside pressure.So the ear drum is stressed outwards due to higher inner pressure of the ear.

Question: 12

State of application of Atmospheric pressure.

Answer: 12

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1- Drinking through Straw. The action of sucking increases the volume of lungs, so air pressure reduce in the lungs and the mouth. The atmospheric pressure acting on the surface of the liquid will be greater than the pressure in the mouth , thus forcing the liquid to rise up the straw into gthe mouth.

Question: 13

How pressure varies with depth in Liquids?Expain.

Answer: 13

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We know that p = pgh. It is clear from the eq that pressure varies with depth and density of liquid.

Explanation:

i. From first hole "A" water exert with low speed because of less depth from top of container.

ii. It means of hole "A" due to less depth, pressure "P" is low .

iii. From second hole "B" water exert with greatest speed. It means that pressure "P2" at hole "B" is greater than hole"A"

Question: 14

Walnuts can be broken in the hand by squeezing two together but not one. Why?

Answer: 14

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When two walnuts are places in hand, the area of the palm of hand is greater than the area of surfaces of walnuts in contact, When pressure is provided to squeeze these walnuts together, the pressure between the surfaces in contact increseswhich causes the walnuts to break.

Question: 15

Write Properties of Liquid.

Answer: 15

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i- TIn liquids, the moleculaes are close together but the pattern of molecules are not fixed which means that the molecules in pattern keep changing their positon.

ii- The liquids have no definite shape but have volume which means that the molecules are able to move and change its shape.

iii. Liquids can adopt the shape of the container on which it is pour.

iv. Their kinetic energy of molecules is little higher than that of solids.

Question: 16

State and explain Hoook's law.

Answer: 16

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Hook's Law: This law states that " Within elastic limit the extension or compression in a body is directly proportional o the restoring force".

Explanation: When a body is stretched or compressed this extension or compression is directly proportional to the applied force .This relationship is known as Hooke's law.

Mathematically it can be written as.

Fres = - k x

Where "k" is known as the force constant or the "modulus of elasticity" Its value depends upon the nature of material and system of units. The negative sign shows that force is directed against displacement.The unit of force constant "k" is Nm-1

Question: 17

Properties of Solids

Answer: 17

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1- Solids are made up of molecules which are rrranged closely in a fixed pattern

ii. Solid have definite shape and volume.

iii. Molecules in solids vibrate abut their means positions.

iv. The attractive forces between the molecules are very strong.

v. They have minimum kinetic energy.

Question: 18

Explan how and why camels have adapted to allow them to walk more easily in desert conditions?

Answer: 18

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The camels have feet with large surface area, therefore the camel's feet exert less pressure on sand and proprly distributed its weight on sand. As a result of less pressure they can walk in desert easily.

Question: 19

What is elasticity? Explain.

Answer: 19

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Elasticity: The propertyof solid materials to return to their original shape and size after removal of deformiang force is called elasticity.

Explanation: A force is required to change the shape o a solid Rubber band spring the bow and tennis ball are example of elastic bodies. When deforming fore is stored in it that enables the body to regain is original shapte. For example.if a we apply a sorce to stretch a rubber band, its length increases,When the force is remoed, the rubber band will return to its original positon.

Question: 20

Define Pressure and relations.

Answer: 20

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Pressure: Pressure is defined as force per unit area. It is denoted by "P".

Mathematically Form:

If force "F" is applied on area "A" the pressure is.

Pressure = Force /Area

P = F/A

Relation between pressure and force:

Eq P =F/A elarly shows that pressure is directly proportioal to the applied force. it means by appling greater force.pressure will be greater.

Relation between Pressure and area:

Eq (i) also shows that pressure is inversely proportional to each other. It means that smaller the area for a given foce. the greater will be the pressure. There fore, a force , a force exeeted over a small area produces more pressure than the same amount force exerted over a large area.

For Example:

A grid wearing high heel pointed shows exrts more pressure on the ground than the one wearing flate shoes.

Unit: The SI unit of pressure is Pascal which is equal to Netwon per square meter

Question: 21

Why a small needle sinks in water and huge ships travel easily in water without sinking?

Answer: 21

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The water displace by a small needle is less than its weight. So, the upthrust acting on the needle is less than the weight of the needle. While, in case of huge ship,the weight of water displaed is greater than weight of ship.So, the up thrust acting on the ship is greater than the weight of the ship. Therefore, a small needle sinks in water but a huge ship floats.

Question: 22

Define Young's Modulus with unit and mathematical.

Answer: 22

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Young's Modulus: " The strain produces in an elastic body is directly proportional to the stress with in the limit of proportionality".

Question: 23

Define Kinetic Molecular Model of matter.

Answer: 23

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Kinetic Moleclar Model of Matter: According to this theory, the matter is made up of molecules which are always in motion. There are three states of matter i.e. solids, liquids and gases. There is a force of attraction between the molccules of solid, liquid and gas the depend upton the intermolecular.

Question: 24

A rowboat is floating in a swimming pool when the anchor is dropped over the side. When the anchor is dropped, will the water level in the swimming pool increases, decrease or remain the same? Explain.

Answer: 24

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When the anchor is in the boat, the wate ris dis placed more due to combine weight of boat and anhor. When anhor is dropped in water, it will displae an amount of water equal to its volume. The water displaced by weight is more than water displaced by volume. So, when the anchor is dropped in water, the level of water in pool decreases.

Question: 25

State Pascal's principle and explain with example?

Answer: 25

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Pascal's Principle:

Statement: Pascal's Principle of fluid pressure states that whenever an external pressure is applied on a liquid in a closed container the pressure is transmitted equally inall direction to every pointof the liquid.

Explanation: Pascal's principle has discovered by a French philosopher named Blaise Pascal.This principle is used in different hydraulic machines in our daily lif i.e. Hydraulic press , hydruulic brakes, hydrualic , jacks and hydraulic lift etc.

Question: 26

Define and explain density.

Answer: 26

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Density : Density of a substance is defined as the mass of substances per unit volume.

Mathematical form:

Density = mass /volume

Example: Iron denser than air, because iron has more mass in the same voume as compared to air. We can also say that for some mass of iron and air. volume of iron will be less than air. Also an iron nail is denser than water.That's why it will sink in water because in iron more amount of mas spresent in unit volume than water.

Unit: The S.I unit of density is kgm-3 or gm. cm3

Question: 27

If you climbed a mountain carrying a mercury barometer, would the level of the mercury column in the glass tube of the barometer increase or decrease as you climb the mountian ? Explain.

Answer: 27

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The level of mercury in the column depnes upon the atomospheric pressure. Greater the atmospheric pressure, high will be the level of mercury in the column and vice versa.

As the elevationof the mountain increses. the pressure reads by baraometer decreases and henc colmn of mercury decreases or fall donw an dvice versa.

Question: 28

Define Strain and explain with unit .

Answer: 28

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Strain: The stress is defined as the extension per unit length.

Mathematical Form:

Strian = Extension/ Original Length.

As stain is the ratio of two lengths, so , it does not have a unit.

Explanation: Suppose a wire of length "i" and area "A" is streched by applying force , then the extension is produce in a wire is 'x' means its length increases that causes strain Simply when stress is applied on a wire, strain is produced in it and the length of a wire changes 'x' from the original length'i'

Question: 29

State and explain Archimedes Principle.

Answer: 29

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Archimedes Principle: Archimedes principle states that " The buoyant force acting on an object fully or partically submerged in a fluid is equal to the weight of the fluid displaced by the object

FB = W

Explanation: According to Archimedes principle, every object experience a buoyant force. Wen a solid object is dipped in a liquid, an upward buoyant force acts on the object.The magnitude of this force "FB" is gien by Archimedes Principle. For example ,if we dipped a brick in a container of water, the brick sinks and water level rises in it.

Principle of Floatation: A floating object alwyas a displaces its own weight of liquid in which it is immersed The archimedes principle applies to both floating and submerged bodies and to all fluids i.e. liquid and gases. Whether an object floats or sinks can be explained by usng this principle, the floatation or sinkingof an object depends on the density of an object relative to the fluids and the buoyant force.

According to law of Floatation:

1- When buoyant force "FB" is grater than objects weight i.e. FB>W, the object will float in liquid or if pB>po, it will float.

2- When buoyant force "FB" is smaller than object's weight i.e. FB<W the object will sink.

Question: 30

If you filled an airtight balloon at the top of a mountain, would the balloon expand or contract as you descend the mountain? Explain.

Answer: 30

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At the top of mountains, the air pressur eis lower as compared to plan area. So low pressure air will be filled in a balloon at thetop of mountain.

When wedecend the mountain the air pressure increases. Thus the outside air pressure becomes higher than air pressure inside the balloon. The higher atmospheric pressure force tghe walls of the balloon inwards, due to which the balloon contracts.

Question: 31

Why water tanks are constructed at the highest level in our houses?

Answer: 31

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Water has the ability to maintain its level. The pressure of waer increases with height. Thus for the easy flow of water in a pipe system, the water tanks are constructed at hightest level in our houses.

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9th Class Chapter 7 Physics Short Questions

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