# Motion Along A Straight Line

## WHAT IS PHYSICS?

One purpose of physics is to study the motion of objects—how fast they move, for example, and how far they move in a given amount of time. NASCAR engineers are fanatical about this aspect of physics as they determine the performance of their cars before and during a race. Geologists use this physics to measure tectonic-plate motion as they attempt to predict earthquakes. Medical researchers need this physics to map the blood flow through a patient when diagnosing a partially closed artery, and motorists use it to determine how they might slow sufficiently when their radar detector sounds a warning. There are countless other examples. In this chapter, we study the basic physics of motion where the object (race car, tectonic plate, blood cell, or any other object) moves along a single axis. Such motion is called one-dimensional motion.

Motion

The world, and everything in it, moves. Even seemingly stationary things, such as
a roadway, move with Earth’s rotation, Earth’s orbit around the Sun, the Sun’s orbit
around the center of the Milky Way galaxy, and that galaxy’s migration relative
to other galaxies.The classification and comparison of motions (called kinematics)
is often challenging.What exactly do you measure, and how do you compare?
Before we attempt an answer, we shall examine some general properties of
motion that is restricted in three ways.

1. The motion is along a straight line only.The line may be vertical, horizontal, or
slanted, but it must be straight.

2. Forces (pushes and pulls) cause motion. In this chapter we discuss only the motion itself and changes in the motion.

Does the moving object speed up, slow down, stop, or reverse direction? If the
motion does change, how is time involved in the change?

3. The moving object is either a particle (by which we mean a point-like object
such as an electron) or an object that moves like a particle (such that every
portion moves in the same direction and at the same rate). A stiff pig slipping
down a straight playground slide might be considered to be moving like a particle;
however, a tumbling tumbleweed would not.

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