Supersonic and shock wave

When the speed of objet is faster than speed of sound, it is called supersonic. Usually, it can be said as mach. For example, if the speed of sound and object 340 m/s and 680 m/s respectively, so it is called mach 2.

When the object has speed faster than sound of speed, it will be followed by shock wave. What is shock wave?

Sound is wave, when source emit sound, it will spread to any direction (figure a). If the source moves with low speed, we will get Doppler effects. If the speed is increased it will be like figure c. When the speed of object is increased until faster than speed of sound, as we can see at figure d, shock wave is formed.

Shock wave (Giancoli: Physics Principles with Applications)

The analogy of this phenomenon is when boat makes waves because its speed is faster than water waves.When aeroplane approach the speed of sound, sound barrier is in front of it. To be faster than speed of sound, aeroplane needs extra thrust to through the sound barrier. Once sound barrier is broken by aeroplane, sonic boom is formed. Sonic boom is produced only when the aeroplane breaks the sound barrier. 

Buoyancy Force application

How does boat float on the water?

Boats which are very big and heavy can float on the water, but needles are drowned at water. Why?

Every material in fluid (such as air and water) has buoyancy force. The direction of buoyancy is always upward. It means if buoyancy force is bigger than weight of the object, it will float.

When we put empty plastic bottle on the water, it will floats. This is because of the buoyancy force is bigger than weight of the bottle. Actually, there is small portion of bottle that drowned, but it is the key how it can float. Pressure occurs inside water with any direction. This pressure increases as the depth of water. Force can be formed if there is a surface in the water. As we know, pressure is force divided by area.

Buoyancy forces are formed by this phenomenon. Archimides principle said that buoyancy force is directly proportional with drowned volume. Mathematically, Buoyancy force is: F = ρ . g . V, where ρ is the density of fluid (water), g is gravity, and V is volume which is drowned.

Boat can float on water based on this principle. Boat has hull to get buoyancy force and makes the boat float. So it is very important to keep hull safe. In the sea, corals are sometimes found in the sea. Hull can crash the coral and causes the hull leakage. This condition can causes boat drowned since hull is the source of buoyancy force.

How to measure fluid flow

When fluid flows in pipe, there are several ways how to measure its flow. Ultrasonic is the one of technology that can measure fluid flow inside pipe. The measurement principle using ultrasonic doesn’t affects fluid flow because it is non-contact measurement. The other ways to measure fluid flow are contact measurement. This kind of measurement will affect the fluid flow.

Contact measurement of fluid flows usually use the principle that convert from flow to different pressure. Venturi tube, orifice, and pitot tube are the example of fluid flow measurement using conversion flow to different pressure.

Venturi meter is pipe narrow constriction. Fluid flows from pipe whose diameter is bigger, then enter the pipe whose diameter is smaller. Based on continuity law, flow rate will be same but the velocity will be different. 

Q1=Q2

A1.v1 = A2 . v2

As we can see at the simple equation above, if area of pipe is smaller, velocity will increase. How to measure flow?

Based on Bernoulli principle, pressure in small diameter pipe is different wit big diameter. This difference is used to determine fluid flow inside pipe. By using Bernoulli principle the difference of pressure can be converted to different velocity between big pipe and small pipe. This velocity difference is then used to determine flow rate (volume rate). Moreover, it also can be used to determine flow rate (mass rate) by using relation between density and volume of the fluid.

Fiber optic

Fiber optic is often used in telecommunication network. Comparing with electrical cable, signal transmission by fiber optic is faster because it uses light as a signal. Besides that, it is also immune to electromagnetic interference, transmits longer distance, and higher bandwidth data rates. How can fiber optic be a path for light?

Light can travel from edge of fiber optic to another edge. The principle of fiber optic is total internal reflection. Before discuss about total internal reflection, let us review about light refraction. Light will be refracted when light through different medium which their refraction index value is different. For example, if light is emitted from inside the water with certain angle, then it will be refracted when comes out from water to the air. The phenomenon of light refraction is the angles are different. If refraction index of medium where the light comes from higher than refraction index of medium where light goes on, the angle will be bigger. If the initial angle is enlarged, there will be a critical angle, that is the angle when the light is not transmitted to the second medium. Moreover, if the angle is still enlarged, the light will be reflected as total internal reflection.

Fiber optic consists of core and cladding. Core has higher refraction index than cladding, then if light travel inside core with the angle more than critical angle, so there will be total internal reflection. By this phenomenon light can travel from one edge to another edge.