We know a thread or string experiences the same tension force throughout the whole length,but when we tie a knot in the thread,why does the tension force become different in the two sides of the knot? Is it possible to explain it through newtonian mechanics?
Unless that knot is tying the string to some stationary object (in which case it's more properly called a "hitch" rather than a "knot"), the tension force is necessarily the same on both sides. Otherwise, there'd be a net force one way or the other, and the knot would accelerate in that direction. (Moreover, if just tying a knot in a string resulted in any such net force, you could trivially construct a perpetual motion machine, thereby solving the energy crisis, remediating climate change, etc, etc.)