Objects move: Balls bounce, cars speed, and spaceships accelerate. We are so familiar with the concept of motion that we use sophisticated physics terms in everyday language. For example, we might say that a project has reached “escape velocity” or, if it is going less well, that it is in “free fall.”

In this chapter, you will learn more about motion, a field of study called kinematics. You will become familiar with concepts such as velocity, acceleration and displacement. For now, the focus is on how things move, not what causes them to move. Later, you will study dynamics, which centers on forces and how they affect motion. Dynamics and kinematics make up mechanics, the study of force and motion.

Two key concepts in this chapter are velocity and acceleration. Velocity is how fast something is moving (its speed) and in what direction it is moving. Acceleration is the rate of change in velocity. In this chapter, you will have many opportunities to learn about velocity and acceleration and how they relate. To get a feel for these concepts, you can experiment by using the two simulations on the right. These simulations are versions of the tortoise and hare race. In this classic parable, the steady tortoise always wins the race. With your help, though, the hare stands a chance. (After all, this is your physics course, not your literature course.)

In the first simulation, the tortoise has a head start and moves at a constant velocity of three meters per second to the right. The hare is initially stationary; it has zero velocity. You set its acceleration − in other words, how much its velocity changes each second. The acceleration you set is constant throughout the race. Can you set the acceleration so that the hare crosses the finish line first and wins the race? To try, click on Interactive 1, enter an acceleration value in the entry box in the simulation, and press GO to see what happens. Press RESET if you want to try again. Try acceleration values up to 10 meters per second squared. (At this acceleration, the velocity increases by 10 meters per second every second. Values larger than this will cause the action to occur so rapidly that the hare may quickly disappear off the screen.)

It does not really matter if you can cause the hare to beat this rather fast-moving tortoise. However, we do want you to try a few different rates of acceleration and see how they affect the hare’s velocity. Nothing particularly tricky is occurring here; you are simply observing two basic properties of motion: velocity and acceleration.

In the second simulation, the race is a round trip. To win the race, a contestant needs to go around the post on the right and then return to the starting line. The tortoise has been given a head start in this race. When you start the simulation, the tortoise has already rounded the post and is moving at a constant velocity on the homestretch back to the finish line.

In this simulation, when you press GO the hare starts off moving quickly to the right. Again, you supply a value for its acceleration. The challenge is to supply a value for the hare's acceleration so that it turns around at the post and races back to beat the tortoise. (Hint: Think negative! Acceleration can be either positive or negative.)

Again, it does not matter if you win; we want you to notice how acceleration affects velocity. Does the hare's velocity ever become zero? Negative? To answer these questions, click on Interactive 2, enter the acceleration value for the hare in the gauge, press GO to see what happens, and RESET to try again. You can also use PAUSE to stop the action and see the velocity at any instant. Press PAUSE again to restart the race.

We have given you a fair number of concepts in this introduction. These fundamentals are the foundation of the study of motion, and you will learn much more about them shortly.