Unit 4 - Pressure - Class 8

UNIT 4: PRESSURE

-          When you squeeze a plastic, what happens and why?

-          It shrinks and its due to force (pressure). When you squeeze, your hand covers some surface area.

-          So, The perpendicular force applied per unit area is known as PRESSURE.

-          Mathematically, Pressure = Force/Area , ( P=F/A )

-          The SI unit of Pressure is Pascal (Pa), fundamental units associated are Nm^-2 (kgm^-1s^-2).

1 Pa Pressure

-                        P = F/A

-          or,  P  = 1N/1m²

-          or,  P  = 1 Pa

-          1 N force applied perpendicularly on 1m² area is defined as 1Pa pressure.

Pressure & Area

-          Poke your hand with a finger.

-          Poke your hand with a needle.

-          In which case you feel more hurt?

-          Obviously in the second case. It means that needle has more pressure than finger on your hand.

-          So, Pressure depends up on area, less area causes more pressure and vice versa.

-Question: Why it is easier to cut vegetables using sharp knife than blunt knife?
- Ans: Because sharp knife has less area, which causes more pressure. 
-  Question: Which hurts more when poked by pencil or a finger?
- Ans: Pencil hurts more because pencil has less area that causes more pressure.
Atmospheric Pressure

-          When the wind blows strongly, you feel difficult to walk, why?

-          It is due to atmospheric pressure.

-          The earth is surrounded by a thick layer of air which is known as ATMOSPHERE about 1000 km high from the earth surface.

-          The pressure exerted by the atmosphere is known as ‘ATMOSPHERIC PRESSURE’.

-          Atmospheric pressure is measured with the help of Barometer.

-          The value of atmospheric pressure on the oceanic surface in 760 mm of Hg or 101300N/m².

Effects/Importance of Atmospheric Pressure

-          Air blows from one place to the another place. Its due to the difference in the air pressure on the earth surface.

-          An empty glass does not sink in water when its upside down position.

-          Cold drinks can easily be sucked with the help of thin straw pipe.

-          Ink can be filled in the fountain pen easily.

-          Syringe can be filled with medicines easily.

-          Air can be filled in the tube.

-          Water can easily be pulled with hand pump.
 - Atmospheric Pressure protects us from harmful uv-radiations from sun.  
    - It protects collision of meteoroids to the earth. Meteoroids burn due to the friction with the atmospheric layer.
       Answer the following questions
 - Define atmospheric pressure. Give some examples.
 - What are the advantages of atmospheric pressure?
 - Write any four importance of pressure.
 - How does air flow from one place to the another place?
 - How can we identify the atmospheric pressure?

 

















 Liquid Pressure
-          We already have discussed about pressure of solid and atmosphere.

-          Solid gives pressure on the body where it is placed.

-          Atmosphere gives pressure to the earth surface.

-          Similarly, Liquid also exerts the pressure on the bottom of the container in which it is kept.

-          The pressure of liquid is due to its weight.

-          Liquid pressure is the force exerted by a liquid per unit area on the bottom of the container due to its weight.

-          Mathematically, Liquid pressure, P = h x d x g, where, P = Pressure, h = height of the container, d = density of the liquid, g = acceleration due to gravity.

-          Let, ‘A’ is the area of the bottom of the container in which the liquid is kept.                     

-          ‘d’ is density of the liquid, ‘h’ is the height of the container and                                              

-          ‘g’ is the acceleration due to gravity.                                                                             

-          Pressure exerted by a body is, P = Force (F)/Area(A)

-          We know, Pressure of the liquid is due to the weight of the liquid. Therefore,

-                     P = Force(F)/Area(A)

-          or, P = Weigh(w)/Area(A)

-          or, P  = Mass(m) x Acceleration due to gravity(g)/Area(A) (⸪ w = m x g)

-          or, P = Volume(v) x Density(d) x Acceleration due to gravity (g)/Area(A)  (⸪ m = v x d)

-          or, P = Area(A) x Height (h) x Acceleratioin due to gravity (g)/Area(A) (⸪ v = A x h)

-          ⸫   P = h x d x g 

Characteristics of Liquid Pressure

-          The pressure of liquid increases with increase of depth.

-          Liquid exerts equal pressure perpendicularly in all direction.

-          (in another words, a liquid enclosed in a vessel exerts equal pressure perpendicularly in all direction. It is Pascal’s Law)

Density

-          Take a bucket.

-          First, fill it with water and lift it.

-          Secondly, fill it with cotton and lift it.

-          Thirdly, fill it with sand and lift it.

-          In which case you feel heavier and lighter.

-          Lighter in case of second and heavier in case of third.

-          It is because density, cotton has less density and sand has more density.

-          Density is compression of molecules (mass) in a space (in bucket there is space).

-          Therefore, The mass per unit volume of a body is known as density.

o   Mathematically,  Density (d) = Mass (m)/Volume(v)

-          SI unit of density of Kgm^-3.

-          Density of a body doesn’t depend on shape and size, bigger size may not have higher density and lesser size may not have less density.

-          Density of a body depends up on compactness of a body.

-          More compactness, more density and Less compactness, less density.

      Answer the following questions.
    - Write any three characteristics of liquid?
    - What are the factors on which liquid pressure depend?
    - Differentiate between atmospheric pressure and liquid pressure?
    - Dams made wider at the bottom, why?
    - A bucket is filled at first floor takes less time than the one filled at upper floor, why?

Floating and Sinking of a body

-          When you place a volley ball in water, it floats.

-          When you place a stone in water, it sinks, Why?

-          Similarly, an unboiled egg placed in fresh water sinks and placed in salty water, it floats.

-          Floating and Sinking of a body in water depends on density.

-          If density of a body is more than water, it sinks.

-          If density of a body is less than water, it floats.

Relative Density

-          Relative density of a substance is ratio of  ‘density of a substance to the density of water at 4⁰C.’

-          Mathematically,  Relative Density = Density of a substance/Density of water at 4⁰C

-          It does not have any unit because it is ratio.



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