Introduction and Overview
Speed is the rate of change of position of an object with respect to a fixed point, or the change in distance per unit time an object has moved. The formula for speed is length/time. Thus, speed can be represented by any length unit divided by any time unit. Inches per year may seem a bit ridiculous for most uses, but is common for measurements of tectonic movement, or the movement of continental plates. Inches per day might be appropriate for a snailís pace, but is hardly useful for supersonic jets.
Mach is the ratio between the speed of projectiles and powered craft through the atmosphere, and the speed of sound through that atmosphere. Since the speed of sound through air varies with the density of the air, the standard mach number is defined for air at 32 degrees F at sea level. It is named for Austrian physicist Ernst Mach. It is very important because the airflow around such a moving object chnages radically as the object passes through Mach 1. Chuck Yeager was the first person to power an aircraft through that magical barrier. Flying faster than Mach 1 is called supersonic flight. Flying faster than Mach 5 is called hypersonic flight.
This megaConverter converts between the most commonly used speed units of today. For explanations of the various units see megaConverters Length #2 and Time #4.
Speed is a scalar unit, which means it is independent of direction. Up, down, east, west, north or south, speeds are the same in all directions. Velocity is not a scalar unit, but is called a vector unit. Velocity changes as direction or speed changes so it must have a reference direction as well as a fixed reference point. While you might say the speed of the earth in its orbit around the sun is relatively constant (it varies slightly depending on the distance from the sun), the velocity of the earth changes constantly because the direction is always changing. A changing velocity means that a force is acting on the object (see the Force megaConverter #8). In the case of the earth, the force that is acting on it is the gravity of the sun. You can feel the force of changing velocity when you round a curve in the road at a constant speed. The force is created by the friction between the tires and the pavement. If the road is wet and slippery, there isnít enough force to make the car go along the curve and you wind up in the ditch. Speed is also affected by force. If you crash head on into a brick wall, your speed goes down to zero but your direction does not change much.
For a more comprehensive treatment of measurements, find "NTCís Encyclopedia of International Weights & Measures" by William D. Johnstone at your local library. For a better discussion of speed, velocity, or force, see any college physics textbook.
Much of our written history still refers to things in common units. The Bible does not refer to meters or kilograms, but to cubits and stadia, or shekels and drachma. Wouldn't it be nice to know what they were talking about way back then?
Now you can use megaConverter! For a more complete listing of ancient, foreign, and obsolete measures, download our 'megaSpreadsheet' of conversions in MS Excel format.
Note: Because of round-off errors, converting from very large units to very small units or vice-versa may not be accurate (or practical). Conversion factors can be found by converting a quantity of 1 unit to another unit several steps above or below the first. You may need to string several conversion factors together to find the factor from a very large unit to a very small unit, and then you can use a calculator with sufficient digits to find your answer.