Overview
Mass and weight define distinct but related concepts. Mass refers to the quantity of matter an object contains, is measured by a balance, and is invariant no matter where it is located. In contrast, weight depends on the pull of gravity on the object, is measured on a scale, and really depends on where the object is located.
Mass or Weight?
In science, the mass of an object is often more relevant than its weight. Suppose one has a kilogram of cotton candy and a kilogram of gold. They would both weigh the same (1000 g), but the volume of space the kilogram of cotton candy would take up would be very large, sweet, and sickening, made of spun sugar and air. In contrast, a kilogram of gold would take up much less volume, and be much more valuable at the current rate of prices.
Figure 1: Balancing cotton candy and gold.
Quantity of Matter
Mass refers to the quantity of matter an object contains, while weight depends on the pull of gravity on an object. The mass doesn’t change, but its weight does. Most of the time this doesn’t matter on earth, but it can in scientific measurements. Suppose an engineer, Dr. Xenia Xavier, in Research and Development is testing the effects of mechanical stress on a new metal alloy. She devises an experiment before using the alloy Aluminex in the jet engine, before going to the expense of having machinists fabricate Aluminex parts. Several ingots are made of Aluminex, and each ingot has a mass of one kilogram. In every condition of the experiment, the ingot is measured and weighed. The weight of the ingot is one kilogram when the jet is sitting on the runway. That ingot is taken out and replaced with another. In the next condition, the weight of the ingot is measured at 5kg while the jet is climbing. Tests are done on the metal to see how it performs under that weight. The ingot is replaced with another for the third condition, where the jet climbs to enter free flight. The 1-kg mass now weighs nothing.
Figure 2: Mass is a measure of the quantity of matter, while weight is a measure of the pull of gravity.
Balance or Spring Scale
In the previous set of experiments, a known mass was compared under different conditions where its weight changed. Suppose the tests of Aluminex have gone far enough that pieces of Aluminex have been machined into parts. Now that the ingots have been shaped into parts, the mass of each part is unknown. Another member of the R and D team, a chemist, Dr. Ying Yee, measures the mass of each part using a balance. He puts the metal part in one pan of the balance and adds the exact amount of weights of known mass to the other pan. After the mass of a part is measured, he measures its weight using a spring-type scale.
Figure 3: Scientists use special equipment to measure mass and weight with precision.
Invariance or Location?
Now for the last situation comparing mass and weight. Another member of the R and D team is going to test Aluminex, a computer specialist named Dr. Zelda Zygonowitz. Will Aluminex be a good metal to use in the rockets that the company has under development? Dr. X measured Aluminex using the SST to show that the weight of a known mass varied at different points in the flight, when the gravity changed, and Dr. Y measured the mass and weight of parts made of Aluminex. Dr. Z does many of her experiments using a computer, because the expense of traveling into space is too great. She performs calculations showing although the mass of Dr. Y’s parts do not change, the weight of that same part is different on the moon, where gravity is lighter, on Jupiter where the gravity is heavier, and near the event horizon of a black hole, where gravity is much heavier. It remains for Dr. Z’s descendant in the 24th century, an astrochemist named Dr. Zena Zane, to test those parameters on a five-year deep space mission.
Figure 4: Presenting test results during a deep space mission.
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