Hydrostatics

Physics CLIL 1D a.s. 2014-2015

HYDROSTATICS

the branch of fluid mechanics

that studies

fluids at rest


HYDROSTATICS

Liquid and aeriform substances (gasses and

vapours) are fluidsPhysics CLIL 1D a.s. 2014-2015

What is a FLUID?

A fluid is a substance that can

flow


Water is a fluid

Oil is a fluid

Smoke is a fluid

Air is a fluid


Fluids conform to the boundaries of the container in which they are placed


At a given temperature, each substance has its own density

which is the ratio of a given mass of the substance and its volume:

DENSITY

𝒅𝒆𝒏𝒔𝒊𝒕𝒚=𝒎𝒂𝒔𝒔𝒗𝒐𝒍𝒖𝒎𝒆

=𝒎𝑽

𝑆𝐼 𝑢𝑛𝑖𝑡𝑜𝑓 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡 :𝒌𝒈𝒎𝟑


Normally, the higher is the temperature of a substance the smaller is its density becouse its volume increases with temperature.

DENSITY


PRESSURE


Pressure is the magnitude F of a force acting

perpendicular to a surface divided by the area S of the surface over which the force acts.

PRESSURE

𝑷=𝑭⊥

𝑺

𝑆𝐼 𝑢𝑛𝑖𝑡𝑜𝑓 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑚𝑒𝑛𝑡 :𝑵𝒎𝟐=𝑷𝒂 (𝑝𝑎𝑠𝑐𝑎𝑙)


PRESSURE

�⃗��⃗�⊥

𝑆

ForcePerpendicular force

Surface


PRESSURE

Pressure is not a vector quantity but a scalar quantity


PRESSUREFor a given pressure Force and Area

are directly proportional

𝐴(𝑚2)

𝐹 (𝑁 )


PRESSURE

For a given Pressure Force and Area are directly proportional

Weight=10N

Area=

Pressure=

Weight=20N

Area=

Pressure=


PRESSUREFor a given Force, Pressure and Area are

inversely proportional

𝐴(𝑚2)

𝑃 (𝑃𝑎)


PRESSURE

Weight=10N

Area=

Pressure= Pressure=

For a given Force, Pressure and Area are inversely proportional

Weight=10N

Area=


PRESSURE

For a given Area, Pressure and Force are directly proportional

𝐹 (𝑁 )

𝑃 (𝑃𝑎)


PRESSURE

Weight=10N

Area=

Pressure=

Weight=20N

Area=

Pressure=

For a given Area, Pressure and Force are directly proportional


PRESSURE

Now try to answer!


PRESSURE

Why does a sharp knife cut better

than a dull knife?


PRESSURE

The diagram at right shows a sharp knife and a dull knife in contact with a surface (maybe a nice, juicy steak!). Notice that the sharp knife has a very small area of contact with the surface, while the dull knife has a much larger area of contact. If both knives are pushed down with the same force, the sharp knife will exert a much greater pressure on the surface than the dull knife - and pressure cuts.So, if you are "stuck" with a dull knife, you have to exert much more force in order to generate enough pressure to cut your steak.


PRESSURE

Why do people have to use ice skates?


PRESSURE

The downward force that you exert on the ice (assuming you are standing on two feet) would be half of your weight.Notice, however, that the shoe distributes the force over a much larger area than the skate does. This means that the skate exerts a much higher pressure on the ice than the shoe does - it is this high pressure that makes ice skating possible!


PRESSURE

Why a karate chop is much more effective than an open-handed

slap?


PRESSURE

Because a reduction of surface area increases net pressure.If one were to slap a board squarely with one's palm, the only likely result would be a severe stinging pain on the hand. But if instead one delivered a blow to the board, with the hand held perpendicular the board could be split in two. In the first instance, the area of force exertion is large and the net pressure to the board relatively small, whereas in the case of the karate chop, the surface area is much smaller—and hence, the pressure is much larger.


PRESSURE

Why snowshoes are much more effective for walking in snow ?


PRESSURE

Sometimes, a greater surface area is preferable. Thus, snowshoes are much more effective for walking in snow than ordinary shoes or boots. Ordinary footwear is not much larger than the surface of one's foot, perfectly appropriate for walking on pavement or grass. But with deep snow, this relatively small surface area increases the pressure on the snow, and causes one's feet to sink. The snowshoe, because it has a surface area significantly larger than that of a regular shoe, reduces the ratio of force to surface area and therefore, lowers the net pressure


PRESSURE

https://www.youtube.com/watch?v=0-gYRc_VUEI


PRESSURE

syringe

hammer

nail

pushpin


Physics at home

PRESSURE


What you need:- 1 carton of milk (closed); a dish; some flour, a scale, a ruler

What to do:1. weight the carton of milk 2. measure the three dimensions (a, b and c) of the carton 3. calculate the area of each side (axb, bxc and axc) of the carton4. place the carton on a table and calculate the three different

pressures it exerts on the three different contact surfaces between the table and the carton

5. Pour the flour into the dish and place carefully the carton over the flour, firstly with the largest size in contact with the flour , then the medium size and eventually the smallest size (before placing the carton on the flour, flatten the surface of the flour)

What do you notice? How can you explain?

PRESSURE


PRESSURE

Pascal’s principleAny change in the pressure applied to a completely enclosed fluid is transmitted undiminished to all parts of the fluid and the enclosing walls

Blaise Pascal(1623-1662)

http://en.wikipedia.org/wiki/Blaise_Pascal


PRESSURE

Pascal’s principle

When the syringe is filled with water, pushing the plunger water comes out from all nozzles with equal speed perpendicularly to the surface of the container

http://demonstrations.wolfram.com/PascalsSyringe/


PRESSURE

A Pascal’s principle applicationTHE HYDRAULIC PRESS

experiment

http://www.aplusphysics.com/courses/honors/fluids/Pascal.html

https://www.youtube.com/watch?v=XuLO_TuvB80


PRESSURE

STEVIN’S LAW

Simon Stevin (1548 – 1620)

The pressure at a point in a liquid in static equilibrium depends only on the depth at that point:

+dgh

Where is the external pressure on the surface of the liquid, h is the depth, g is the gravity acceleration and d is the liquid density


PRESSURE

STEVIN’S LAW


PRESSURE

STEVIN’S LAW APPLICATIONS𝑪𝒐𝒎𝒎𝒖𝒏𝒊𝒄𝒂𝒏𝒕 𝒗𝒂𝒔𝒔𝒆𝒍𝒔(𝒗𝒂𝒔𝒆𝒔)

Given a set of two or more connected containers containing a homogeneous liquid, when the liquid settles, it balances out to the same level in all of the containers regardless of the shape and the volume of the containers


PRESSURE

STEVIN’S LAW APPLICATIONS

𝑷𝒂𝒔𝒄𝒂 𝒍′ 𝒔𝒃𝒂𝒓𝒓𝒆𝒍 𝒆𝒙𝒑𝒆𝒓𝒊𝒎𝒆𝒏𝒕

http://demonstrations.wolfram.com/PascalsBarrel/


PRESSURE

Watch the video

https://www.youtube.com/watch?v=KAvy5P88lms


PRESSURE

Hydrostatic pressure inside the water in a dam or a swimming pool increases with depth according to Pascal’s principle and Stevin’s law.That is the reason of the

trapezoidal shape of walls in dams and swimming pools


PRESSURE

h1

h2

𝑑2𝑑1

h2>h1→𝑃2>𝑃1→𝑑2>𝑑1

STEVIN’S LAW APPLICATIONS


ATMOSPHERIC PRESSURE

The atmosphere of Earth is a layer of gasses surrounding the planet Earth that is retained by Earth's gravity.

Three quarters of Earth atmosphere is within about 11 km from the planet surface.

The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary

Although its density is very small, it exerts a great force over all the objects on the Earth surface



Atmospheric pressure is the force exerted per unit of area on the surface of the Earth by the column of air extending vertically above it.

The average atmospheric pressure at sea level is 1 standard atmosphere (atm)=101.3 kPa



70cm (at sea level)



h = 70cm (at sea level)

https://www.youtube.com/watch?v=GgBE8_SyQCU




PRESSURE

BUOYANCY Buoyancy is an upward force exerted by a fluid that opposes the weight of a partially or completely immersed object.


PRESSURE

ARCHIMEDES’ PRINCIPLE

https://www.youtube.com/watch?v=KMNwXUCXLdk


PRESSURE


𝒃𝒖𝒐𝒚𝒂𝒏𝒕 𝒇𝒐𝒓𝒄𝒆= 𝒇𝒍𝒖𝒊𝒅𝒅𝒆𝒏𝒔𝒊𝒕𝒚 ∙𝒈 ∙𝑽𝒐𝒍𝒖𝒎𝒆𝒐𝒇𝒅𝒊𝒔𝒑𝒍𝒂𝒄𝒆𝒅𝒇𝒍𝒖𝒊𝒅

𝑭 𝑩=𝒅𝒇 ∙𝒈 ∙𝑽 𝒇

https://www.youtube.com/watch?v=ChK1en0pxi0


PRESSURE

𝒇𝒍𝒖𝒊𝒅𝒅𝒆𝒏𝒔𝒊𝒕𝒚 >𝒐𝒃𝒋𝒆𝒄𝒕 𝒅𝒆𝒏𝒔𝒊𝒕𝒚→𝒕𝒉𝒆𝒐𝒃𝒋𝒆𝒄𝒕 𝒇𝒍𝒐𝒂𝒕𝒔

s


nor floats nor sinks


PRESSURE

air ballon

submarineaircraft carrier

For a fully submerged object, Archimedes' principle can be reformulated as follows:Immersed object’s apparent weight =weight of the object – weight of the displaced fluid

Hydrostatics

Education

pressure pressure

pressure weight

given pressure force

greater pressure

pressure cuts

fluids physics clil

rest physics clil

given area