Are Newton's laws invalid in real life?
Regardless of relativistic effects:
- Newton's First Law is invalid because friction exists in real life.
False, the first law talks about the case when no forces are present, if forces are present go to the second law.
- Newton's second law is invalid due to the same reasons.
False, you add friction to the total force.
- Newton's third law is invalid because in a trampolin, there is excessive reaction.
False, why do you think there is excessive reaction?
Newton's First Law is invalid because friction exists in real life.
Let's review what Newton's first law says:
When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a net force.
Your friend is right that friction exists in real life. But your friend is wrong that the first law is invalid because of it. The friction is that net force being acted upon the object. The law holds out in space where there is (virtually) no friction, and it holds places on earth where there is a lot of friction.
Newton's second law is invalid due to the same reasons.
Again, let's review the second law:
In an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration vector a of the object: F = ma.
Once again, friction is a force that goes into that vector sum. So like he said, for the same reasons. Except it's valid for the same reasons, not invalid.
Newton's third law is invalid because in a trampoline, there is excessive reaction.
Just bought a trampoline. There isn't any excessive reaction. There's just more bounce. Say you jump from the ground. You push off with some force, go up in the air, hang for a sec, and fall back down. At the top, you have lots of potential energy and at the bottom you've got lots of kinetic energy. And because the earth is made of very rigid rock it doesn't flex very much and absorbs the energy you put into it. Probably your knees and ankles absorb some too.
On a trampoline, this isn't what happens. Your potential energy turns to kinetic energy as you fall as before, but instead of that energy going into the ground with a thud, it goes into the trampoline as potential energy. At the bottom of the trampoline bounce, the resistance of the trampoline overcomes your motion and starts pushing you back up.
So the difference between the ground and the trampoline is that on the ground your energy from jumping goes into shockwaves in the ground, while jumping on a trampoline goes into potential energy that's used to push you back up.
If you want to try this, try taking a rubber bouncy ball and a stuffed animal. Throw them both at a wall and see which one bounces back more. Same energy going into both of them, but one is able to turn that kinetic energy into potential energy and reverse the flow while the other just kinda flops. Newton's laws account for both scenarios quite well.
We may have been able to find scenarios where Newton's laws don't apply (i.e. relativity) but the laws remain fundamental to engineering and have very, very real world applications to this day.
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).