Ahhhh, but when we start talking about movement and displacement, Newton's 1st and 2nd laws come into play and the important thing about them is that they apply to the *external* forces exerted *on* a single object. The classic example of the N3 (Newton's 3rd) dilemma is the horse and cart - when the horse pulls the cart, the cart exerts an equal and opposite force on the horse, so how do they move?

Now if we look at the horse (that is define our single object to be "horse", the external forces acting on it include the force exerted on the horse by the cart and frictional forces between the horse and the ground, but it *does not* include the force of the horse pulling the cart because that is a force exerted *by* the horse and not *on* the horse.

But if we define our single object to be "horse + cart", the force exerted on the cart by the horse and on the horse by the cart are ignored when looking at the motion of the system because they are now *internal* forces. The only forces we worry about now are external ones such as friction which are acting on the "horse + cart" object.

Hope that helps

Amanda