I don't know if this may be part of the confusion but note that the forces forming an action-reaction pair never act on the same object!!! Therefore, they never cancel out! (Unless you consider the two objects as a combined object). For a car, there is a friction force on the car exerted by the pavement and a reaction force on the pavement produced by the car. They act on different objects.ok, so the box is in dynamic equilibrium...so what's the difference between the force of the box pushing back and the force of friction?
Zero, assuming the box isn't accelerating. Since static friction is normally greater than dynamic friction, it would have taken more than 100N to get the box to start sliding, and once it was sliding the force could be reduced to equal that of dynamic (sliding) friction, which in this case is 100N, and the box would not accelerate.ok, so the box is in dynamic equilibrium...so what's the difference between the force of the box pushing back and the force of friction?
That's right: Action and reaction forces never cancel out because they don't act on the same body.I just cannot understand how the forces do not cancel out directly yet do in fact cancel out in some way. I understand that the action and reaction forces act on different bodies, but how then do they end up canceling each other out? Do they never cancel out at all?