pressure

PRESSURE in solid

What happen if you sticking the sharp nail compared with if you sticking to dull nail through the same force?

Off course easier  you sticking the sharp nail compared with if you sticking to dull nail, because the force acted an the first nail is smaller compared to the force acted on the second nail, so the pressure produced by the first nail is smaller compared to the pressure produced by the second nail.

Based on the explanation above:

      Pressure is magnitude of the force acting on the body for each unit surface area of pressure field.

CONCLUSION:  The magnitude of pressure on a body depends on the surface field and magnitude of force acted.

Mathematically, pressure is formulated as follow

      F

P=-----

      A

In which : P = pressure (N/m²)

                 F = force (N)

                 A = plane area/luas bidang (m²)

In SI system pressure un it is force unit per area unit, namely:

So : 1 Pa = 1 N/m²

In cgs (=centimeter gram second) system pressure unit is dyne/cm²

	1 atm = 76 cm Hg  1 N/m2 = 100.000 dyne/cm2

 

Exercise:

1. A body of pressure area 6 m² is given force of 54N. what is the pressure produced?

2. A cube has edge of 10 cm with pressure of 40 N/m². Calculate what is the magnitude of force?

3. A child of mass of 30 kg stays on the floor with base area of his feet each of  75 cm². If earth gravitational accelereation is 10 m/s². What is the magnitude of pressure of the child to the floor?

PRESSURE IN LIQUID

What happen, if dam is not strong to hold water pressure?

So, liquid also has pressure and the pressure in water to every direction.

Make hole at the plastic bottle by using nail vertically from the bottom to up word in parallel for 3 hole. In which hole does the water spout the most distance?

Of course, the most distance water spout come from the most bottom hole.

So the pressure in liquid is larger if its position is deeper.(semakin dalam)

Liquid pressure in motionless is called hydrostatics pressure.

The magnitude of hydrostatics pressure can be measured by using a HARTL DEVICE

Hydrostatics pressure depends on density, height or depth of liquid, also Earth gravitational acceleration, so it can be formulated as follow:

 

In which :   P  = Hydrostatics pressure (N/m² or Pa)

                   ρ  = density of liquid (kg/m³)

                   g  = earth gravitational acceleration (m/s²)

                   h = height or deep of the liquid (m)

exercise:

			Calculate hydrostatic pressure of water at point t (see figure besides). With depth of  h = 20 cm from water surface, if  given density of water = 500 kg/3 and earth gravitational acceleration = 10 m/s2.

 

 

1. Pascal Law:

Pascal law

Expressed that:

Pressure given to liquid in closed chamber will be continued by the liquid to every direction with uniform and the same magnitude

The pascal’s law a lot of used to make hydrolytic device which is used removing and altering the magnitude of force, exampleby giving a little force to obtaining a large force.

Such as :   - hydraulic  jack (dongkarak hidrolik)

                 - hydrolic lifter (alat pengangkat hidrolik)

                 - hydrolic brake (rem hidrolik)

Piston I has section area of A_1,                Piston II has section area of A₂

                                    A₁           <           A₂

Since A₂ > A₁ then F₂ > F_1. This causes the force working on section A₂ is large A₁

Exercise:

A car of weight 6000 N is lifted by using car lifter device. Section area of the big piston is 300 cm², while section area of small piston is 60 cm². What is the force that must be done in the small piston?

Connected vessel

If the connected vessel is filled by the same liquid and in balance condition, liquid surface in the vessel is located on one flat plane.

Connected vessel does not hold if:

1 . The vessel is filled with more than one kind of liquid

2.  The vessel is in closed condition

3. there is capillary pipe

                           P₁ = P₂

                   ρ₁ g.h₁= ρ₂ g.h₂

                       ρ₁ h₁= ρ₂ h₂

Exercise:

A connected vessel contains two liquids which are not mixed, namely and water (as in the figure). Given density of water is ρ_a = 2 g/cm³ and density ρ_m = 0,50 g/cm³. If the height h_a= 5 cm, find the height of oil surface h_m.

Known: ρ_a = 2 g/cm³

             ρ_m = 0,50 g/cm³

             h_a= 5 cm

Ask : h_m = ?

Aswer:

Archimedes Law

Stated that:

“ a body which is immersed partly or entirely into liquid, will get buoyancy force which the magnitude is equal to the weight of liquid moved the object”.

Mathematically Archimedes law is

                    F_A = ρ.V.g

In which:

F_A = buoyancy force / gaya ke atas (N)

ρ = density of liquid(kg/m³)

V= volume of liquid moved (m³)

g = Earth gravitational acceleration (m/s²)

Exercise:

A body of volume 6000 cm³ and density 1200 kg/m³ is immersed in the water at all. Density of water is 1000 kg/m³, Earth gravitational acceleration is g = 10 m/s². What is the magnitude of the buoyancy experienced by the body all and the weight of the body in the water?

Answer:

Glass beaker filled by water, When stone inserted to glass beaker, do volume of water changed?

Actually, Abody if into liquid some are

a. sinking (tenggelam)= the body is lies on the base of a liquid

b. suspending (melayang)= the body is immersed at all but does not reach base of the liquid,      or

c. floating (terapung) = if the body is immersed only a part in the liquid, so volume of liquid moved by the body (V_a) is smaller than volume of the body

sinking,

                                            

                                        w > F_a

                                  m_b . g > m_a . g

                       (V_b . ρ_b) . g > (V_a . ρ_a) . g

                         Because V_b = V_a

                                      ρ_b >  ρ_a

in which:

ρ_a =  density of liquid (N/kg)

ρ_b = density of the body

V_a =volume part of the body immer (m³)

V_b = volume of the body

suspending,

                                        w = F_a

                                  m_b . g = m_a . g

                       (V_b . ρ_b) . g = (V_a . ρ_a) . g

                         Because V_b = V_a

                                      ρ_b =  ρ_a

Floating

 

                                        w < F_a

                                  m_b . g < m_a . g

                       (V_b . ρ_b) . g < (V_a . ρ_a) . g

                         Because V_b = V_a

                                      ρ_b <  ρ_a

       

 

Exercise

A body is floating in the water. Part of the body that appear on water surface is 75% of its volume. If the density of water is 1 g/cm³, what is the density of the body?

From explanation above can be concluded several things as follow:

a. a body can sink, if the density of body is large than the density of liquid

b. a body can suspend, if the density of the body is equal to density of the liquid

c. a body can float, if the density of the body is smaller than the density of the liquid.

PRESSURE IN GAS

What is The meaning of:

a. Troposphere: 

b. Stratosphere

c. Mesosphere

d. Thermosphere

e. Exosphere

known: density of mercury = 13.600 kg/m³

                   g = 9,8 N / Kg

             height of mercury coloum = 76 cm = 0,76 m

ask Pressure of mercury= ?

answer : P = ρ. g . h       

              P = 100.000 Pa

Relationship between height of place and air pressure

The lower position of a place from see level, the large air pressure

In which : h = height of place above sea level

                 x = pressure of place above sea level

Exercise:

Someone is in a place wuth certain height. After measured by using a barometer , the mercury height in the tube shows the number of 66 cm. What is the height of the place?

Air Pressure Measuring Device

The device used to measure air pressure is barometer.

Kinds of barometer:

a. mercury barometer : is the device to measure air pressure in open air.

b. water barometer: working principle is the same with mercury barometer

                    P = ρ_air . g . h_air

In which:              

ρ = density of liquid(kg/m³)

g = Earth gravitational acceleration (m/s²)

h = height of mercury coloumn

c. Aneroid barometer (metal) for  to measure air pressure by using metal

The Use of atmospheric pressure

a. The time when sucking beverage by using straw

     That caused atmosphere pressure outside of the straw push the beverage so it can go up through the straw to the mount.

b. When releasing condensed from the can

c. Rubber Sucker

d. Injector

The device to measure air pressure in closed chamber is called manometer

Kinds of manometer:

a. Liquid Manometer

b. metal manometer

BOYLE’S LAW

“At constant temperature, gas pressure in closed chamber is inversely with its volume”.

The statement above is known with boyle law.

                          P . V = C

In which:

                        P = pressure

                        V = volume

                       C = Constant

Or can be written as follows:

                       P₁  . V₁ = P₂ . V₂

Several example : water pump, air pump, and bicycle pump

Exercise:

A closed chamber of volume 5 m³ has gas pressure of  2.5 atm. If the pressure is made 10 atm, what is its volume now?