Gravity: A force that pulls objects toward the center of earth and gives weight to objects.
Kilogram: The Standard Unit of Mass.
A gram is a very small unit of mass. About 28 grams equal one ounce. A larger unit of mass is the kilogram (kg). A kilogram equals about two and one-fifth pounds and is the standard mass unit in the international system. One gram (g) is one-thousandth (0.001) of a kilogram (kg). A kilogram contains one thousand (1,000) grams.
Once the unit of mass is defined, samples of matter can be compared to the unit and thus measured. Comparing a sample of matter to the standard unit of mass, or to its subdivision the gram, can be done on an equal-arm balance. When making this comparison, a sample of matter is placed on one pan of a balance. On the other pan are placed as many gram units or kilogram units as are necessary to create a balance. The mass of the sample is then the number of grams required to balance the sample on the equal-arm balance. This number would be the same for that sample at sea level, at three kilometers above sea level, or even at three thousand kilometers above sea level. This illustration shows an extremely important characteristic of mass. For a particular sample of matter, the mass does not vary.
Stanley the Scientist Talks About Mass
Why are we using the term mass instead of weight? We are careful to use the correct term because mass is not the same as weight. To understand the difference picture an elephant. Picture an elephant here where you are. Next, picture the elephant in a satellite orbiting the earth.
Then picture the elephant in space out beyond the moon. In each case--on the earth, above the earth, and in space--has the elephant changed? Aside from small changes produced by bodily functions, has the elephant gained or lost matter? If the elephant neither gains nor loses matter, then the elephant's mass is the same wherever he happens to be.
Mass is a property of matter. Any object--any piece of matter--has mass. The mass of an object remains the same.
How about the elephant's weight? Does that change? You have seen enough television and magazine pictures of men in space to know that as objects leave the earth they weigh less. You have seen men hopping over the surface of the moon like beach balls. They were able to move so easily because on the moon they weighed less than on the earth.
An object's weight, then, depends on where the object happens to be. Our elephant weighs less in a satellite than on the earth, and is almost weightless in space. Weight is the measure of the pull of gravity on an object. The force (pull) of gravity can change with location on the earth and with distance above or below the ground.
If mass and weight are different, are they measured differently? Yes: mass is measured on a balance and weight is measured on a scale. A scale is an instrument which can measure the force (pull) of gravity on a given object (mass).
Mass of water. The metric units of volume and mass are related to water. One cubic centimeter of water equals one milliliter of water, and the mass of each volume is one gram. A small box that is one cubic centimeter in size holds exactly one milliliter of water, and that one milliliter of water has a mass of one gram. A liter of water contains 1,000 cm³ or 1,000 ml of water, therefore, a liter of water contains 1,000 grams. One thousand grams (g) equals one kilogram.
Remember, mass is the measure of matter. Mass does not depend upon distance from the earth, moon, or sun as weight does. It depends only upon the make-up of the object, and it is a property of that object. We measure samples of matter by comparing them with standard units of mass.
