Fluids in physics and technology

Conservation of momentum in physics. – Picture from youtube.com
Forces – Upthrust. – Picture from quizlet.com

WELCOME to our sixth lecture in the Science in Action series on the topic of fluids and technology.
Before we get to our lecture, I have mentioned in our last lecture on Heat and Temperature that everybody in PNG needs a wholesome science education to invent and innovate in new products to sell around the world just like our Pacific and Asian neighbours. Before we make any workable product such as a machinery, let’s ask the question, what is wrong with the PNG education system that we are not able to make very simple food, beverages and technology products that our neighbours are producing and selling to us at a very cheap price?
I have mentioned also last week that the STEM curriculum was the right curriculum and approach to embrace because, in the stem curriculum you are going to learn about science, technology, engineering and mathematics. All of these necessities will enable Papua New Guineans to become inventors, innovators and pace setters.
Fluids in physics speaks of the two states of matter; liquid and gas. Nutritionists and medical practitioners talk about maintaining a fluid balance in the body which focuses mainly on liquids. Three substances in matter that fluids include are liquids, gasses and plasmas. Plasmas are ionised or charged particles whose electrons have been removed so they carry positive charges to attract electrons to form atoms and molecules. The plasma are able to flow as ionised gasses.

Difference between tensile and compressive force. – Picture from pediaa.com

The fluids can deform and can respond to applied force without little or no resistance. As a result the fluids can flow and similarly they can easily be affected by shear stress. The fluids’ characters can be looked at from the four equations of Navier-Strokes as on the continuity as in the law of conservation of mass, the conservation of linear momentum, the conservation of angular momentum and the equation on the conservation of energy.
Fluids, like any matter, have a free energy that they carry on their surface. This is called the surface energy for solids. In fluids, the free energy is called the surface tension. The changes in the surface tension allows for the fluids to flow.

Shear stress and strain. – Picture from people.mines.com

The fluids can deform to a certain extent of a shear stress and restore the surface tension. In solids, the shear stress is a function of the strain. Thus, strain is basically a deformation, a flow or a stretch on an object. Stress is hence, a force acting on an object per unit area that can cause a strain if it is sufficient to overcome the strength of the object that is under stress. Strain therefore is a change in shape or size resulting from external forces (shear stress). This is plasticity nature of the fluids. The solids can withstand the shear stress and deform partly or permanently depending on whether the tensile strength is exceeded or not. If it is exceeded, then the object is permanently stretched, deformed or even fractured. If it is not exceeded, then the surface energy will be maintained when the plasticity nature of the object is maintained. That is anything can deform or strain when placed under a shear stress or an external force.

Forces acting on an air plane. – Picture from anjungsainssmkss.wordpress.com

In liquids when its tensile strength is exceeded, a further condition of cavitation is created. That is at low pressures, the liquid turns into vapor.
Fluids are compressible in nature. Fluids plays a very important role in their everyday applications as in engineering. Two phenomenon that exist are the positive and negative pressures created as a result of the compression. The positive pressure is created as a result of the ideal fluids been compressed. Likewise, the negative pressure is created as a result of the tensile stress. The idea gas always try to restore their normal stresses from those pressure.
There are fluids that are known as Newtonian fluids and Non-Newtonian fluids that display stress characters. In Newtonian fluids, the stress is directly proportional to the strain. On the other hand, the Non-Newtonian fluids’ stress is not directly proportional to the strain. So when using those certain types of fluids, one has to consider the four Navier-Stroke equations regarding the continuity, conservation of linear and angular momentum with the equation on the conservation of energy to solve those problems.
Fluids can be classified be classified accordingly as in Newtonian or Non-Newtonian fluids. Under application of pressures, Newtonian fluids can undergo volume and density changes. Such fluids can become supersonic which we do not really see happening in nature but is theoretically sound. Complete or ideal and perfect fluids as such do not really exist as all known fluids can be compressed and are viscous or resist flow only slightly.

Archimedes’ Principle – density of object. – Picture from oneschool.net

Fluids including liquids, gases and plasma exert a pressure in all directions. The liquids are not bound by the container. They have a free surface.
Whilst the gases do not because their molecules quickly escape since they do not have a surface energy like the surface tension of the liquids. Water and oil are fluids that do not change their volume when there is change in pressure or flow velocity.
Pressures in fluids are spread in all directions. Pressures are very similar at the same depths regardless of whether the objects are further apart at the same altitude. The mass and density of an object are very important in determining the volume of the object. It is given in the formula density equals mass over volume (D=M/V). Density is measured in kilogram (kg) per unit volume eg m3 (kgm-3), mass is measured in kg and volume in m3.
The pressure increases with depth because there is more force known as weights added as you go downwards. In this formula pressure measured in pascals can be seen to be a derivative of force (weight) measured in Newtons divided by the area measured in unit squared like meter squared (m2) (P=F/A). That is P=Pressure, F=Force and A=Area.
The upthrust force is the flotation and is also called buoyancy force seen as an upward force resulting from the displacement of its mass in a fluid such as a gas or liquid. The force this particular body experiences upward is called the upthrust force that is equal to the magnitude of the mass of its own body displaced.
In a laboratory a displacement can be used to perform that experiment to get any equal mass of any irregular object. Tie a thread of negligible mass to a pebble or a small stone and drop it gently into an instrument called a displacement can. That displacement can had to have been fully filled with water at room temperature. While dipping the object slowly into the can, hold onto the other end of the rope outside of the can. When the pebble is dropped into the displacement can, a particular mass of water equal to the mass of the pebble will be displaced. When you measure the mass of the displaced water, it will equal to the mass of the pebble.
My prayer for PNG today is; “The Lord has made His Salvation known, faithful to His promises of old, let the ends of the earth and all the sea and all it holds, make music, before our King”.

Next week: Technology and sciences of the atmosphere

Michael John Uglo is the author of the science textbook Science in PNG, Pacific, Asia and Caribbean” and a lecturer in Avionics, Auto- Piloting and Aircraft Engineering. Please send comments to: [email protected]