Dear Dieu,
"A professor's story about his students' reaction when first encountering Newton's Third Law touched me and inspired me to keep on fighting for the truth.
He wrote: "During 1st and 2nd years of graduate school, grad students were required to spend a few hours per week in a large conference room to supply tutorial help for undergraduate students... the biggest issue by far was Newton's 3rd law. The most common question was "If the force and reaction force are equal and opposite, then how can there be any motion if the net force is always zero?"
This legit question and many others..."
The undergraduate students were showing confusion about when Newton's Second, and Third Laws apply. I am sure the grad students explained the differences clearly. The Third Law and the Second Law can apply simultaneously for the same overall action although at different physical points. An example is when one pulls horizontally on a rope that is attached to a block that is sliding horizontally. If the block is increasing its speed, then both the Second and Third Laws apply. One needs to know where in the example each applies.
James Putnam