Question: One of my friends and I had an argument over this topic. He stressed the fact that in real life many forces exist, whereas in physics we deal only with ideal situations. He put the following arguments:-

> 
> 1. Newton's First Law is invalid because friction exists in real life.
> 2. Newton's second law is invalid due to the same reasons.
> 3. Newton's third law is invalid because in a trampoline, there is excessive reaction.
> 
> 
> 

In defence, I put forward the following arguments:-

> 
> Newton's laws are true but the equations have to be modified to take into account the other forces in real life.
> 
> 
> For example, if a force $F$ is applied on a body of mass $m$, and $f\_s$ is the force of friction, then, the equation becomes $F - f\_s = ma$. Thus, we have just modified the equation $F = ma$.
> 
> 
> So basically I mean to say that we have to adjust the laws to suit our purpose.
> 
> 
> 

In the end, there was a stalemate between us. Even now I am confused after this argument. Please clarify my doubt.

Answer: 
Wolphram johnny gave good explanation but for trampoline case there can be more explanation

Whenever you jump from earth, according to "third law", the force you give to the earth is equal to the force (reaction force) given by earth to you which makes you go off from the ground, the force you gave to earth actually moves earth (negligibly due to its high mass) the reaction force is the force which makes you jump... So i guess you are clear about where the third law acts now.

Coming to the trampoline case when you jump and land on trampoline your kinetic energy is stored due to its spring like property, so when you jump again the force is exerted by you and trampoline, both so you have more acceleration (newton second law) this net force is higher compared when you jump from the ground alone (when you jump and land on ground the energy is not stored like in that of trampoline and no extra force). Here the net force is more and the reaction force given to the earth is more and the earths moves more (yet negligible).
Your friend actively misunderstands the Newton laws and the ideas behind it. He is about to argue that formula $E=mc^2$ is invalid because it states that Youngs modulus ($E$) is equal to mass ($m$) times hypotenuse squared ($c$). And he is supporting his arguments by several [argument fouls](https://www.reddit.com/r/realsocialengineering/comments/2g9661/the_art_of_mental_trickery_and_manipulation/).

[First law](http://www.phdcomics.com/comics/archive.php/archive/archive.php?comicid=221) states that when there's **no** forces applied to the body it remains still or move with constant velocity. The argument by friction is foul, because friction **is** force and must be included to the equation. It also states that if the velocity is **not** changing there is zero **net force** - all applied forces cancel each other completely.

[Second law](http://www.phdcomics.com/comics/archive.php/archive/archive.php?comicid=222) describes the relation between **net force** and change in **net momentum** of [point mass](https://de.wikipedia.org/wiki/Massepunkt)\*. For real bodies we have to introduce angular momentum ($E\_{rot}=J\omega^2$). Note that $F=\frac{dp}{dt}$. For constant mass (non-relativistic universe) we can **derive** $F=ma$.

[Third law](http://www.phdcomics.com/comics/archive.php/archive/archive.php?comicid=223) states that for every force (action) there is equal force (reaction) in opposte direction. In case of the trampoline (idalised) there is gravity ($G=mg$), the force of the trampoline pushing you ($F\_{tr}=-k\Delta y$) and the force your body that pulls the trampoline down ($F\_{body}=ma=m\ddot {\Delta y}$). If you draw proper image you will see that $G+F\_{tr}+F\_{body}=0$, that means $F\_{tr}=-(G+F\_{body})$. Call whatever side action, the other side is reaction.  

 We can also state that third law is result of laws of conservation of momentum and energy. It just say that if the **net** momentum and **net** angular momentum does **not** change, the **net** force is zero.

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* I could not find proper english word to czech "hmotný bod", german "Massepunkt", polish "punkt materialny". If such word(s) exist, feel free to edit.