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:-

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> 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.
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In defence, I put forward the following arguments:-

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> Newton's laws are true but the equations have to be modified to take into account the other forces in real life.
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> 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 $F = ma$.
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> So basically I mean to say that we have to adjust the laws to suit our purpose.
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In the end, there was a stalemate between us. Even now I am confused after this argument. Please clarify my doubt.

Answer: 
Newtons laws are a good approximation for how the world works when the velocity is less than 1% the speed of light, the gravity isn't too strong, when the number of elementary particles an object is composed of isn't too small as an object needs to be large enough for quantum uncertainty to be insignificant, and the amount of space is small compared to the observable universe so that the effect of the expansion of space is insignificant. Friction doesn't disprove Newtons Laws as Friction itself is a force. Because we live on a planet where we tend to be unable to get objects off the ground for long periods of time without technology friction tends to effect everything in our daily lives to the point that until people performed proper experiments on motion it wasn't obvious that friction was effecting objects, which is why it wasn't obvious that moving objects would continue to move in the absence of any force.

Friction does not disprove the second law as friction has to be taken into account to find the total force acting on an object.

The trampoline example doesn't disprove Newtons third law because it states that for every force there is an equal and opposite force, which is different from the effect being equal. Because F=ma the acceleration of a lighter object will be greater than that of a heavier object even when the same force is applied to the lighter object than the heavier object.
Newton's laws are valid for all situations where velocities are small (compared to the speed of light, ie relativity is not important) and where quantum effects are negligible (mostly where objects are much bigger than elementary particles). The problem with your argument is that you and your friend are using idealized expressions for Newton's laws, not their most general form. That is completely understandable because the more general forms require mathematical concepts that are mostly restricted to physicists and mathematicians (in fact, Newton invented the calculus in order to formulate these laws). Rest assured that the laws are not just valid for those idealized situations that are expressed in terms of elementary math.