All forces are interactions in which objects exert forces on each other. If A pushes on B, then B pushes back on A. These two forces form an action/reaction pair of forces. One can’t exist without the other.

《 LOOKING BACK Section 5.5 Forces, interactions, and Newton’s second law

We will often analyze a problem by defining a system—the objects of interest—and the larger environment that acts on the system. An interaction diagram is a key visual tool for identifying action/reaction forces of interaction inside the system and external forces from agents in the environment.

A common way that two objects interact is to be connected via a rope or cable or string. Pulleys change the direction of the tension forces. We will often model

• Ropes and strings as massless;

• Pulleys as massless and frictionless.

The objects’ accelerations are constrained to have the same magnitude.

Newton’s third law governs interactions:

• Every force is a member of an action/reaction pair.

• The two members of a pair act on different objects.

• The two members of a pair are equal in magnitude but opposite in direction.

The dynamics problem-solving strategy of Chapter 6 is still our primary tool.

• Draw a free-body diagram for each object.

• Identify and show action/reaction pairs.

• Use Newton’s second law for each object.

• Relate forces with Newton’s third law.

《 LOOKING BACK Section 6.2 Problem-Solving Strategy 6.1

We started our study of dynamics with only the first two of Newton’s laws in order to practice identifying and using forces. But objects in the real world don’t exist in isolation—they interact with each other. Newton’s third law gives us a much more complete view of mechanics. The third law is also an essential tool in the practical application of physics to problems in engineering and technology.