Four Basic Types of Motion

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Physics encompasses a great many topics, but at heart, it is the study of how things move. At the "macro" level (concerning everyday and visible things, as opposed to "micro," or atomic and subatomic, phenomena) many physicists and engineers classify motion into four basic types: linear, rotary, reciprocating and oscillating. Machines with moving parts exhibit one or more of these.

Note that these are not to be confused with hard-fast laws of motion; they are accepted in many conventions, but by no means represent ironclad categories. Astronomers, for example, often discuss motion in terms of revolution, rotation, cosmic expansion and orbiting systems. The four kinds of mechanical motion are nevertheless a good starting point to learn how things get from one point to another in physical space in fully familiar ways.

Linear (Translational) Motion

Linear motion, sometimes more broadly called translational motion, is simply the shifting of an object from one point in space to another. Schematically, on a typical graph with x- and y-axes, if a point shifts from the origin at (0, 0) to the point (3, 4), the Pythagorean theorem may be used to show that the point has undergone 5 units of linear motion (the square root of 32 + 42 is 5). An arrow shot from a bow undergoes linear motion.

Many objects experience more than one type of motion simultaneously, with the predominant form being used as the overall descriptor. For example, a baseball thrown from a pitcher to the catcher 60 feet away has undergone translational motion, but the ball has likely rotated a number of times along its path from the pitcher's mound to home plate.

Rotary (Rotational) Motion

When something rotates, roughly speaking, it spins around in a circle. A child standing in one spot on a playground and spinning in a circle until she reaches her original starting point has undergone rotational motion, but she need not complete the circle for this to be true; they key point is that her body has rotated about a well-defined geometric axis – in this case, one running from the top of her head to the ground at her feet.

Rotation is the foundation of automotive transport. For a car as a whole to be translated from, say, New York City to Los Angeles, its wheels must rotate around the car's axles, and many of the internal parts of the automobile's combustion engine rotate as they do work. The Earth itself rotates about its own axis between the North and South Poles once almost every 24 hours.

Reciprocating Motion

Reciprocating motion is related to other forms of motion, in particular oscillating motion. In this form of motion, an object is translated, or moved linearly, in one direction and then back along the same path in the opposite direction until it returns to its starting point; the cycle is then repeated. One example is a power saw. A less obvious example is a person who drives to work and then drives home along the same route eight or so hours later, and then repeats this day after day. These may seem like very different endeavors, but in reality they differ only in time and distance scales; the saw may move through an amplitude of only half a meter and travel through its entire in-out path several times per second, while a commuter may travel 20 miles twice a day.

Oscillating Motion

Things that move in a reciprocating manner, but with elements of rotational motion like swinging, are said oscillate. A pendulum, which swings from a fixed point of attachment and traces an arc, is a classic example. A sprinkler or oscillating fan does the same thing, except that these oscillate in a horizontal plane rather than a vertical plane and are powered by motors rather than gravity.

For completeness' sake, imagine a sprinkler of this sort mounted on the back of a car that traces 120 degrees of arc while moving back and forth along a 50-meter stretch of railroad track. This device has readily identifiable translational, rotational, reciprocating and oscillating forms of motion, and most real-world moving objects exhibit more than one form of motion when they move.

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About the Author

Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.

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