7.1 Interacting Objects
FIGURE 7.1 shows a hammer hitting a nail. The hammer exerts a force on the nail as it drives the nail forward. At the same time, the nail exerts a force on the hammer. If you’re not sure that it does, imagine hitting the nail with a glass hammer. It’s the force of the nail on the hammer that would cause the glass to shatter.
FIGURE 7.1 The hammer and nail are interacting with each other.
In fact, any time an object A pushes or pulls on another object B, B pushes or pulls back on A. When you pull someone with a rope in a tug-of-war, that person pulls back on you. Your chair pushes up on you (the normal force) as you push down on the chair. These are examples of an interaction, the mutual influence of two objects on each other.
To be more specific, if object A exerts a force on object B, then object B exerts a force on object A. This pair of forces, shown in FIGURE 7.2, is called an action/reaction pair. Two objects interact by exerting an action/reaction pair of forces on each other. Notice the very explicit subscripts on the force vectors. The first letter is the agent; the second letter is the object on which the force acts. is a force exerted by A on B.
FIGURE 7.2 An action/reaction pair of forces.
NOTE The name “action/reaction pair” is somewhat misleading. The forces occur simultaneously, and we cannot say which is the “action” and which the “reaction.” An action/reaction pair of forces exists as a pair, or not at all.
The hammer and nail interact through contact forces. Does the same idea hold true for long-range forces such as gravity? Newton was the first to realize that it does. His evidence was the tides. Astronomers had known since antiquity that the tides depend on the phase of the moon, but Newton was the first to understand that tides are the ocean’s response to the gravitational pull of the moon on the earth.