Physics for Game Developers, 2nd Edition

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 remainder of this book. 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 book 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 ...