Hello Steemians, I hope you had a great weekend? It’s Sunday again and the beginning of another week. Make sure you have some goals for the week and try as much as possible to achieve them. In all, discipline is the key. As they say - <em>all work and no play makes Jack a dull boy</em>. So for me, I’m gonna be starting the week with the FA Cup quarter-finals match. Even in the wake of a three nil defeat at Barcelona last Wednesday, I am still a Chelsea fan. Besides, we were the better team that night (please don’t start an argument!). Let’s just say it was not our match to win.  We are taking on the Foxes today and I am confident we will triumph. It’s our best hope of winning a silverware. Well, enough of the sport talk! By the way, feel free to share with me how you intend to begin the week. Now let’s talk science.

I was reading an article authored by @lemouth on anti-matters. An anti-matter is simply a matter that is the reverse of its natural existing counterpart.  For instance, on the sub atomic level, anti-proton is the anti-matter of proton. Anti-Hydrogen is that of hydrogen on the atomic level and so on. He mentioned that the motivation behind the production of anti-matters is based on the efforts to validate the principle of CPT symmetry (a concept in quantum field theory). You probably do not have an inkling on what CPT symmetry means. In a layman’s term the idea of CPT symmetry is simply that a reverse or opposite case of any situation can exist. Nonetheless, I’m not here to discuss quantum field theory. I was only fascinated and inspired by this concept and I decided to make an article on a theory that shares similar concept.

<em>**Food for thought:**</em>
> Do you know why a fully inflated, unsealed balloon suddenly flies off when released? Do read on to find out.

 Let’s start on a lighter note. Back in high school, I always wonder why we do or study some things in our academics. For instance, I cannot account for the number of times I have been asked to find “x” and “y”. Perhaps, when you come across them, you can draw my attention to them. But wait! Weren’t you also looking for “x” and “y”. Too bad, we all once had to look for “x” and “y”. Perhaps you are still even searching for them. As for me, I have given up on them. Notwithstanding, on a more serious note, and with deeper consideration of these things, you will agree with me that they have meaningful implications and applications. We only like to whine and make fun out those things back then.

![STS-30 LAUNCH](https://upload.wikimedia.org/wikipedia/commons/5/56/STS-30_launch.jpg) 
<center>[Wikimedia Creative Commons: STS-30 LAUNCH](https://commons.m.wikimedia.org/wiki/File:STS-30_launch.jpg)</center>


Scientists like Sir. Isaac Newton laid foundations upon which a lot of today’s technology were built. You probably remember him for the three laws of motion. I first came across these laws at the inception of my senior year in High school.  The first law is the law of inertia, the second law led to the definition of Force and the third law described the relationship between bodies interacting with one another with respect to Force. In this article, we are going to focus on the third law and a vital application of this law, in the aviation and aerospace industry, but before then, let’s have a quick review of some physical quantities.

**LET’S DO SOME BASIC PHYSICS**
It is essential that you acquaint yourself with the following quantities as they are key to the main discussion.

* **Mass**
Mass is the quantity of matter contained in a body. The more this quantity of matter, the more the mass of the body. The mass of an object is a constant attribute of that object. That is, the mass does not change, irrespective of the location. A body will have the same mass on earth as that which it will have on Jupiter. The unit of mass is kg.

* **Mass Flow rate**
Mass flow rate is a concept that is usually used in the dynamics of fluids. It is the mass of a substance that is flowing per unit time. It is measured in kg/s. 

* **Velocity**
Velocity is the rate at which a given displacement is covered with respect to time. It is the ratio of the distance travelled to the time take to travel that distance. The unit of velocity is m/s. If a car covers a distance of 50 m in a second, it is said to have a velocity of 50 m/s. Well, I wish you all the best if you journey in such car.

* **Momentum and Force**
Momentum is the product of mass and velocity. Hence a body with a fixed mass which changes position with respect to time has a momentum that is equal to its mass times the rate at which it changes position (i.e. velocity). 

> Momentum = mass x velocity	(kg.m/s)

Let’s assume that suddenly, the velocity of such body with a fixed mass changes (in magnitude and/or direction) for a given period of time, probably by collision with another body (this is otherwise known as acceleration or maybe a deceleration depending on the case). There will be a change in momentum by virtue of the force applied by the other body. This applied Force is directly proportional to the change in momentum per time of occurrence. Remember the unit of Force is Newton (N).

> Force = Momentum change per time

For a case where the body has a fixed mass we have:
> Force =mass x velocity change per time 

In another case, the momentum change for a given period of time may arise as a result of changing mass. This is usually the case when we are considering a fluid. Remember in the previous case we have a fixed mass. In this case the mass is changing. Such is the case of water gushing out of a water hose. In such case:

> Force = (mass flow rate) x velocity change

Let’s assume a car washer is holding a hose ejecting water a rate of 1.5 kg/s perfectly horizontal and the water strikes a vertical portion of the car at right angles. Let us assume that the water comes to rest after hitting the car body. To know the force exerted by the water on the car we only need to account for quantity of water coming out of the hose in each second (i.e. the mass flow rate) as well as the initial velocity of the water at the exit of the hose, say 4 m/s. The force is then given by:
> Force on car body = 1.5 x (4 – 0) = 6 N.

Hence, Force is produced by a change in momentum with respect to time which can be either by acceleration or change in mass per time.

**DEFINITION OF THE STATEMENT OF THE THIRD LAW**
The statement of the third law reads:
>Action and Reaction are equal and opposite.

<em>**Food for thought**:</em>
>Do you know why you feel pain when you hit an object? Why is the pain more when you hit harder? Let’s move on.

The key elements in the statement of the third law are the words “Action” and “Reaction”. As we all know, Action precedes Reaction. One dictionary meaning of “Action” is “any act that is carried out to achieve a purpose”. Reaction is next after any Action. Simply say that it is the response or feedback to any Action.


<div class="pull-right"><img src="https://steemitimages.com/DQmRNKRwVGWzs42gJqsqw5PcD8h95DFj8ShWcsuHfFhakej/Screenshot_20180318-123743.png" 
><center>Image created by me (@temitayo-pelumi)</center></div>


However, in the context of the third law, “Action and Reaction” refers to two forces, which are equal in magnitude but are opposite in sense. In this case, the Reaction occurs instantaneously following the Action. This law implies that when a body A applies a force on another body B, body B applies a force of equal magnitude on body A, but in the opposite direction. See the image to the right. 

<div class="pull-left"><img src="https://upload.wikimedia.org/wikipedia/commons/a/ab/Breaking_technique.jpg" 
><center><a href="https://commons.m.wikimedia.org/wiki/File:Breaking_technique.jpg">Wikimedia Creative Commons: Breaking Techniques</a></center></div>

Now let’s answer the above question. When you hit an object, you have applied a force. This force is the Action. Logically we might want to think that since we apply the force, then the effect of the force should be only on the object. Well you are partially right. However, from Newton’s third law, the object does not only “feel” the effect of the application of your Action, but is also capable of producing a force of same magnitude in a direction opposite to your Action i.e. a Reaction. This reactive force is the cause of the pain you feel whenever you hit an object. Now if you hit harder, that is, you increase your Action, equally the Reaction from the object will increase. I’m sure you know what that means. This simple physics however finds application in much complicated systems as we will see below:

**AIR VEHICLE PROPULSION SYSTEMS**
We often see planes fly over us, but we never considered what the driving force behind this wonderful creations is. Basically, the propulsion of aircrafts is modelled on the Newton’s third law of motion. Generally, this propulsion is brought about by the action of gases entering and leaving a “propulsion tube”. The propulsion tube is always found on both wings of an airplane.

![Jet Engine](http://maxpixel.freegreatpicture.com/static/photo/1x/Aviation-Airplane-Engine-Aircraft-Plane-Jet-1293503.jpg) 
<center>[Creative Commons Zero - CC0: Aviation Aircraft Engine](http://maxpixel.freegreatpicture.com/Aviation-Airplane-Engine-Aircraft-Plane-Jet-1293503)</center>

Now, how does the Propulsion System work? In a simple sense, a stream of relatively low speed gas enters into the “propulsion tube” and is accelerated within this tube and then exits at a higher speed. The gas at the inlet of the tube has a particular momentum and a much higher momentum at the exit due to the increase in its speed. Thus by virtue of this change in momentum with respect to the time spent by the gas within the tube a rearward force is produced (see the “LET’S DO SOME BASIC PHYSICS” section of the article). By Newton’s third law, another force of equal magnitude is generated but in the opposite i.e. forward direction. It is this forward force, also called Thrust that propels the aircraft. To increase the Thrust, we can either increase the amount of air induced or the change in velocity of the inlet and outlet gases or both.

Propulsion systems vary in types and complexity, but all have one thing in common which is a fan at the inlet of the propulsion tube which draws in the gas. The gas at the inlet is usually air and that at the exit may be combustion products or air. The term propeller propulsion is used when the exit gas is air, while a propulsion system in which combustion product exit from the tube are referred to as Jet propulsion system. In propeller propulsion, the fan is driven by an internal combustion engine remotely located from the propulsion tube. Here the fan is designed to accelerate substantially, the air stream in its wake. Therefore, the air exit at much greater speed to produce the Thrust force to propel the plane. 

In the Jet propulsion system however, combustion of the fuel takes place inside the propulsion tube. Thus we can say that the propulsion tube here is the engine. In fact it is called the Jet Engine. The fan at the inlet merely draws in air. The air is later compressed and then mixed with fuel. The mixture is burnt, and the combustion products exit at very high speed giving rise to the Thrust force. Jet propulsion systems are capable of much higher speed than the propeller types. A typical Jet propulsion system can produce at Thrust of 500, 000 N. So much Force, right?

The principle of spacecraft propulsion is similar to the Jet Engine propulsion system. However, instead of using atmospheric oxygen. The vehicle carries its own oxygen in addition to the fuel. The term Rocket Engine is applied here. Spacecraft propulsion systems are capable of unimaginable Thrusts beyond those of airplanes.

**CONCLUSION**
Now you have seen the Implications and Applications of the Third law of motion. There are many ways in which this law can be applied ranging from trivia to more important cases. By the way, about the balloon flying off, I believe you can now answer that yourself. Please feel free to leave your explanation about it in the comments. You can also add you own application of the third law.

<em>Finally, few days ago the management of @steemstem went on a break to resolve the issue of lack of engagement within the steemSTEM community which is hindering the growth of the community. Even though we are growing in numbers, should that be our definition of growth? It is true that majority of us write articles and then sit back to watch our reward grow. However, its high time we realized that the community is beyond making dollars. This community is all about developing ourselves and others by creating good content and making good contributions. We need to read other people’s article as we will want them to read ours. Every author has made an Action, it is your duty to give a Reaction. Leave meaningful comment, ask question and give advice. Have a splendid week.</em>


**REFERENCES**
* Nelson, M. and Parker, P. (1995). Advanced Level Physics, 7th Edition. CBS PUBLISHERS AND DISTRIBUTORS, INDIA.
* [EXPLAINTHATSUFF!: Jet Engines](http://www.explainthatstuff.com/jetengine.html)
* [Wikipedia: Jet Engine](https://en.m.wikipedia.org/wiki/Jet_engine)
* [NASA: Propeller Propulsion](https://www.grc.nasa.gov/www/k-12/airplane/propeller.html)