I have always had trouble with students misunderstanding Newton's third law.
I have lately come to realize that they don't know what a force is.
Their only experience is the "effect" of forces. Since I have approached
Newton's 3rd from the this viewpoint they seem to understand it better.
When I start Newton's third, I stress that what they observe is NOT the
two equal forces but the effect of those forces.
Andy Jones
Physics Teacher
1600 Loop 256
Palestine Texas 75801
andjones@tenet.edu