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The momentum  of any particle-like body cannot change unless a net external force changes it. For example, we could push on the body to change its momentum. More dramatically, we could arrange for the body to collide with a baseball bat. In such a collision (or crash, in everyday language), the external force on the body is brief, has large…

Now that we have defined linear momentum for a single particle, let us extend the definition to a system of particles. Consider a system of n particles, each with its own mass, velocity, and linear momentum. The particles may interact with each other, and external forces may act on them as well. The system as a whole…

In this section, we discuss only a single particle instead of a system of particles, in order to define two important quantities. Then in Section 9-5, we extend those definitions to systems of many particles. The first definition concerns a familiar word—momentum—that has several meanings in everyday language but only a single precise meaning in physics…

Now that we know how to locate the center of mass of a system of particles, we discuss how external forces can move a center of mass. Let us start with a simple system of two billiard balls. If you roll a cue ball at a second billiard ball that is at rest, you expect…

We define the center of mass (com) of a system of particles (such as a person) in order to predict the possible motion of the system.  The center of mass of a system of particles is the point that moves as though (1) all of the system’s mass were concentrated there and (2) all external forces were…

Every mechanical engineer hired as an expert witness to reconstruct a traffic accident uses physics. Every trainer who coaches a ballerina on how to leap uses physics. Indeed, analyzing complicated motion of any sort requires simplification via an understanding of physics. In this chapter we discuss how the complicated motion of a system of objects,…