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: 
As the saying goes: "All models are wrong, but some of them are useful."

For me, laws of physics are actually just models, i.e., simplifications of reality that give satisfactory answers to certain questions. Newton's laws are *useful* to build bridges, build skyscrapers, land rockets on the Moon, etc.

You could also argue that:

* In reality, a force is never perfectly constant (e.g., combustion engines produce force in bursts)
* In reality, mass is never constant (e.g., even the [prototype kilogram loses mass](https://en.wikipedia.org/wiki/Kilogram#Stability_of_the_international_prototype_kilogram))

Then again, how often do you "care" in reality about a change of $50µg$? If you do calculus of a car, you might decide to model its mass as being constant during its trip. As long as the model gives you are reasonable answer it is useful. Otherwise, you need to adjust or change the model (e.g., add fictions forces, relativistic effects, etc.)

P.S. No clue who had the idea of giving models in physics such a solemn name as "laws". Scientific marketing, I guess. :D
Remember the **sum** $\sum$ symbol:
$$\sum \vec F=0$$
$$\sum \vec F=m\vec a$$

A very important detail.

It's not about "adjusting" anything to make the laws work, it's just about including *all* forces. Which might be tricky in real life, but that's another story. 

And by the way, you can tell your friend that friction is not at all always present - space is a quite good example.