Imagine there is a massless spring of spring constant 10 newton/metre
Now, a person's hand apply a pulling force of constant 100 newton on the spring
Now, according to Newton's 3rd law, the spring should also exert an equal amount of pulling force on the man, but right at this MOMENT/INSTANT, the spring has 0 elongation, and according to Hooke's law, the force applied by spring is equal to spring constant multiplied by its elongation/compression, since elongation is 0, the restoring force of string is 0, therefore the restoring force of spring on the hand of the man is 0
How is Newton's 3rd law being followed here? Since the man pulled the end of the spring with 100N, the end of that spring should also pull the man with 100N, but as we calculated the spring force is 0 (according to Hooke's law)
Consider another situation
In this same scenario, because of 100N force, the spring must have elongation, let's look at the moment when elongation of spring is 1 metre
As we can see, man is still pulling on the spring with a constant force of 100N, but at this moment when elongation of spring is 1 metre, the reaction force applied by spring due to action force of man's pull is 10N (according to Hooke's law, as spring restoring force is spring constant multiplied by acceleration, which is $10\times1=10$)
Therefore ,here also newtons 3rd law is being invalid, as man is pulling on the string with a force of 100N but the spring only pulls on the man with a force of 10N
But, when the elongation of spring is 10 metres, there is a whole different story
Here, as we can see, Newton' s 3rd law is valid,as the man is pulling on the spring with a force of 100N, and the spring also pulls the man with a force of 100N (as spring force is spring constant times elongation, which is $10\times10=100$) at this moment
So Newton's 3rd law was invalid initially, but it became valid after spring elongated a length of 10metres
Can someone explain what's happening here? I'm pretty sure I'm doing something wrong as a law can't be wrong anytime, am I considering wrong action reaction pair of forces? Or is there some other mistake?