Game Development Reference
In-Depth Information
CHAPTER 3
Force
This chapter is a prerequisite to Chapter 4 , which addresses the subject of kinetics. The
aim here is to provide you with enough of a background on forces so you can readily
appreciate the subject of kinetics. This chapter is not meant to be the final word on the
subject of force. In fact, we feel that the subject of force is so important to realistic
simulations that we'll revisit it several times in various contexts throughout the re‐
mainder of this topic. In this chapter, we'll discuss the two fundamental categories of
force and briefly explain some important specific types of force. We'll also explain the
relationship between force and torque.
Forces
As we mentioned in Chapter 2 , you need to understand the concept of force before you
can fully understand the subject of kinetics. Kinematics is only half the battle. You are
already familiar with the concept of force from your daily experiences. You exert a force
on this topic as you hold it in your hands, counteracting gravity. You exert force on your
mouse as you move it from one point to another. When you play soccer, you exert force
on the ball as you kick it. In general, force is what makes an object move, or more
precisely, what produces an acceleration that changes the velocity. Even as you hold this
book, although it may not be moving, you've effectively produced an acceleration that
cancels the acceleration from gravity. When you kick that soccer ball, you change its
velocity from, say, 0 when the ball is at rest to some positive value as the ball leaves your
foot. These are some examples of externally applied contact forces.
There's another broad category of forces, in addition to contact forces, called field forces
or sometimes forces at a distance . These forces can act on a body without actually having
to make contact with it. A good example is the gravitational attraction between objects.
Another example is the electromagnetic attraction between charged particles. The con‐
cept of a force field was developed long ago to help us visualize the interaction between
objects subject to forces at a distance. You can say that an object is subjected to the
 
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