240 Chapter 9 Mass and Mass-Related Parameters
In Equation (9.14), represents momentum vector,mis mass, and is the velocity vector.
Because the velocity of the moving object has a direction, we associate a direction with mo-
mentum as well. The momentum’s direction is the same as the direction of the velocity vector
or the moving object. So a 1000-kg car moving north at a rate of 20 m/s has a momentum
with a magnitude of 20,000 kgm/s in the north direction. Momentum is one of those
physical concepts that are commonly abused by sports broadcasters when discussing sporting
events. During a timeout period when athletes are standing still and listening to their coaches,
the broadcaster may say, “Bob, clearly the momentum has shifted, and team B has more
momentum now going into the fourth quarter.” Using the preceding definition of momen-
tum, now you know that relative to earth, the momentum of an object or a person at rest
is zero!
Because the magnitude of linear momentum is simply mass times velocity, something
with relatively small mass could have a large momentum value, depending on its velocity.
For example, a bullet with a relatively small mass shot out of a gun can do lots of harm and
penetrate a surface because of its high velocity. The magnitude of the momentum associ-
ated with the bullet could be relatively large. You have seen stunt actors falling from the
top of tall buildings. As the stunt performer approaches the ground, he or she has a rela-
tively large momentum, so how are injuries avoided? Of course, the stunt performer falls
onto an air bag and some soft materials that increase the time of contact to reduce the
forces that act on his or her body. We will discuss the relationship between linear mo-
mentum and linear impulse (force acting over time) in Chapter 10 after we discuss the
concept of force.V
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