4.6 Newton’s Third Law
newtons third law motion you can’t pull on a doorknob without the doorknob pulling a football, the forward force that your foot exerts on the ory,but you also feel the force the ball exerts back on your the pain you feel is due to the force that the boulder exerts the force that you exert on the other body is in the opposite body exerts on you. Experiments show that whenever two bod- that they exert on each other are always equal in magnitude This fact is called Newton’s third law of motion. In Fig. body A (first subscript) on body B (second subscript), by body B (first subscript) on body A (second subscript) . of newton’s third law action pair. This is not meant to imply any cause-and-effect either force as the “action” and the other as the “reaction.” the forces are “equal and opposite,” meaning that they have posite directions . forces described in Newton’s third law act on different bodies. involving Newton’s first or second law, which involve the For instance, the net force acting on the football in Fig. 4-18 weight of the football and the force FA onB exerted by the kicker. force A because this force acts on the kicker, not on the reaction forces are contact forces that are present only touching. But Newton’s third law also applies to long-range physical contact, such as the force of gravitational attraction . upward gravitational force on the earth that’s equal in mag- gravitational force the earth exerts on the ball. When you drop the earth accelerate toward each other. The net force on each but the earth’s acceleration is microscopically small great. Nevertheless, it does move!
An apple sits on a table in equilibrium. What forces act on it? What is the reaction force to each of the forces acting on the apple? What are the action-reaction pairs?
SOLUTION Figure 4-19a shows the apple on the table, and Fig. 4-19b shows the forces acting on the apple. The apple is A, the table T, and the earth E. In the diagram, Fe on A is the weight of the apple; that is, the downward gravitational force exerted by the earth E (first subscript) on the apple A (second subscript). Similarly, F T on A is the upward force exerted by the table T (first subscript) on the apple A (second subscript). As the earth pulls down on the apple, the apple exerts an equally strong upward pull FA on E on the earth, as shown in Fig.
4-19d. E and FE on A are an action-reaction pair, rep the mutual interaction of the apple and the earth, so FA on B Also, as the able pushes up on the apple with force corresponding reaction is the downward force FA the apple on the table (Fig. 4-19c). So we have The two forces acting on the apple are FT on A and FE •.they an action-reaction pair? No, they aren’t, despite be’ and apposite. They do not represent the mutual interact . two bodies; they are two different forces acting on the same. A are still there (the gravitational interaction is still present), Since is now zero, it can’t be the negative of , and these two forces can’t be an action-reaction pair.
Finally, we emphasize once more a fundamental truth: The two forces in reaction pair never act on the same body. Remembering this simple fact can you avoid confusion about action-reaction pairs and Newton’s third law.