Speed

Speed is the rate of change in an object’s position measured in units of distance per unit of time. Speed is commonly expressed in miles per hour (mph) in the United States and kilometers per hour (km/h) throughout the rest of the world. Scientists usually express speed in terms of meters per second (m/s). The fastest possible speed is the speed of light in a vacuum, which is c = 299,792,458 meters per second (approximately 1,079,000,000 km/h or 670,600,000 mph).

There are two aspects of speed: average speed and instantaneous speed. Average speed is the distance that an object travels divided by the amount of time it was traveling. Instantaneous speed is the speed at which an object is traveling at any given point in time. The equation for average speed is s = d / t, where s is speed, d is distance, and t is time.

Speed is closely related to velocity. Velocity is speed with the added component of direction. Speed is a scalar (meaning it is just a quantity), while velocity is a vector (meaning it is a quantity and a direction).

Background

Galileo Galilei (1564–1642), an Italian physicist and mathematician, is sometimes credited with being the first to define speed mathematically as the distance traveled over a period of time. Galileo conducted numerous experiments on speed, most famously disproving the Aristotelian belief that, even in a vacuum, heavier objects fall faster than lighter ones.

Physicists in the ensuing centuries identified a number of benchmark speeds in the natural world. In the late seventeenth century, Sir Isaac Newton roughly estimated the speed of sound in air, and by the twentieth century the estimate became much more precise: sound travels at roughly 344 meters per second (769 mph) in dry air at around 70 degrees Fahrenheit (the temperature and humidity of the medium affect the speed of sound). The speed of an object divided by the speed of sound is known as the object’s Mach number; in other words, an object traveling at the speed of sound is traveling at Mach 1; twice the speed of sound is Mach 2; and so on. Before the twentieth century, the speed of sound had been broken by small objects, such as the tip of a whip or bullets fired from a gun. But self-propelled vehicles did not definitively reach supersonic (faster than sound) speeds until World War II, most famously in the case of the German V-2 ballistic missile. Chuck Yeager, a captain in the United States Air Force, became the first human to travel at supersonic speed in horizontal flight in 1947.

The top speed in the universe is the speed of light in a vacuum; it has been measured to a very high degree of accuracy at close to 300 million meters per second, or roughly 186,000 miles per second. The symbol for the speed of light is c, a key part of many calculations in physics.

Speed Today

The concept of speed has important applications on diverse levels of human experience, from the highly abstract, such as theoretical physics, to the very practical, such as traffic safety. In physics, understanding the speed of light as a physical constant has been central to understanding how the universe works. In everyday life, advances in technology that allow humans to travel at ever greater speeds—whether in a chariot or an automobile or an airplane—have facilitated human movement around the world, making the world appear “smaller” all the time. Technological improvements also make these modes of transportation safer, but the increased speed also means that when there are accidents, the effects are more devastating.

In the early twentieth century, physicist Albert Einstein developed his special theory of relativity, which established, among other things, that the speed of light is constant for all frames of reference—in other words, traveling toward a light source does not make the light coming from it seem to move faster; similarly, the headlights on a moving car do not shine light at c plus the speed of the car—the light still travels at c. Understanding this has led to a number of seeming paradoxes in physics related to the behavior of space and time, or space-time as it is more properly called. One example is time dilation, which refers to the phenomenon of time moving more slowly on a moving object from the perspective of an object at rest; as an object approaches the speed of light, this phenomenon increases. Time dilation has been demonstrated a number of times in experiments where extremely accurate atomic clocks have been synchronized with one another and then sent into space or flown around the world and then found afterward to be out of synch by a few nanoseconds in a way that was predicted mathematically.

This is just one example of the significance of speed in high-level theoretical physics, although this does not really affect the average person’s daily life. In contrast, the ability to travel at high speeds in a car, train, or airplane has had a huge impact on people’s daily lives. Just a few centuries ago, traveling great distances was a tremendous undertaking that took a long time and involved high risk. Traveling from Europe to America, or from coast to coast of the North American continent, could take weeks or months and involved exposure to the risk of accident, disease, or starvation. Today, those same trips are completely routine and take a matter of hours with substantially lower risks. The effect on people’s lives and the global economy of this ability to travel at great speed cannot be overstated.

Having said that, it is also a physical reality that the faster an object is moving when it collides with another object, the greater is the force that is exerted. Thus, an automobile accident on the highway involving heavy vehicles traveling at 65 miles per hour or more is far more devastating than a wagon rollover at 20 miles per hour. Although automobile safety is constantly improving, and traffic fatalities decline almost every year, it remains the case that more than thirty thousand Americans die every year in traffic accidents, a reminder that in daily life, speed still carries risks as well as rewards.

Bibliography

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