Metric Mass and Weight 

Since we all live at the surface of the earth where the acceleration of gravity is relatively constant, it has been natural for us to confuse the concepts of mass and weight. To study the difference, we need to separate these two "parameters." We could either leave the earth's gravitational field so that we would have no weight, or create an experimental situation where weight is not a factor. An easy way to do this is to use floating objects. Floating objects still have all their mass, and move almost friction-free in still water, but we don't notice their weight since the water supports (opposes) it. A small block of wood floating in water can be pushed back and forth very easily by hand. Pushing a large motorboat requires either a much greater push or a longer time to have the same effect. The effort required to change the motion of an object is a measure of its mass. The motorboat has a lot more mass than the block of wood.

The metric unit of mass is the kilogram. The kilogram was intended to be the mass of a cube of water one-tenth of a meter on a side. In actual practice the official standard of the kilogram is a cylindrical piece of platinum-iridium alloy kept in a secure location in France. A megagram (metric ton) is the mass of a cubic meter of water. A kilogram is about 2.2 pounds. A metric ton is about 2200 pounds.

The metric unit of weight (or any other force - such as that of a stretched spring, or a bat striking a ball, or a locomotive pulling a train) is called the newton. One newton is defined as the amount of force required to cause one kilogram to accelerate one meter per second every second. A newton is roughly the weight of four ounces, or a pound weighs about four newtons. A kilogram at the earth's surface has a weight of about 9.80 newtons. Above the earth's surface it weighs less and less. Below the earth's surface (in a mine or borehole) it also weighs less. Can you figure out why?

In orbit around the earth astronauts feel no weight at all, even though they are still as massive as they were on the ground. This is not due to the distance they are from the earth, which may be only 200 kilometers or so. They are weightless because they are in free fall, as you would be if you fell off a cliff. They don't fall straight down, however, because they are traveling forward at about 30 times the speed of sound (mach 30), and keep missing the earth! The sailors who refused to travel with Columbus were right. You can fall off the earth (if you are moving fast enough).


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Updated July 15, 2009
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