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You may have heard that people are weightless in outer space before and wondered how that could be possible. In daily life on Earth, we tend to think of mass and weight as synonyms. However, weight is actually a description of the pull of gravity on an object, while mass is the actual value of how much "stuff" it contains, which doesn't change in different locations.

Specifically, weight is defined as the product of mass and gravitational acceleration, or W = m*g, with g representing the gravitational acceleration, or force of gravity pulling on the object. On Earth, g = 9.81 m/s$^{2}$.

Since the moon is less massive than the Earth, it has a smaller gravitational acceleration. Because of this, your weight on the moon would be 1/6th of your weight on Earth!

In chemistry, we use mass to determine the amount of substance (which is a fancy word for "chemical") present. Grams, abbreviated as g, are the units used in the International System of Measurements to measure mass.

Mass is the measure of the amount of matter in a substance, and weight is a measure of how gravity acts upon that mass. Thus, your mass is the same no matter where you go, but your weight changes depending on where you are (Earth or another planet). The base unit for measuring mass is grams, but modified units, like kilograms or milligrams, can be used for heavier or lighter substances. Weight is a force, so its unit is the Newton. Here is the formula for weight:

Here are some more differences between mass and weight:

• Mass can never be 0, but the weight of an object can be depending on where it is! (In space, when there is no gravity acting upon you, you technically weigh nothing!)

• Mass is a scalar quantity (can be described with numbers alone) but weight is a vector quantity (need direction and magnitude) that is always directed at the center of gravity.

Need help remembering? Listen to this handy dandy song!

Mass vs. Weight

Mass and weight are often confused for each other. Mass is how much matter there is. It is usually measured in kilograms. Weight depends on the gravitational force acting on the mass, and is usually measured in newtons or pounds. This might be a little confusing, but remember that the mass of an object never changes, while the weight does.

Converting Between Mass and Weight

One of the reasons you may be confused by the distinction between mass and weight is that they are often used interchangeably. Moreover, it is incredibly simple to convert between the two. In order to convert mass ($m$) into weight ($W$), all we have to do is multiply the mass expressed in kilograms by gravitational acceleration ($g$), $$W=mg$$ On Earth, $g \approx 9.8 \frac{m}{s^{2}}$. $\\$ But this formula will give us the weight in newtons, which is a unit most people are not accustomed to using. Since the pound is part of the imperial system of units, and the kilogram is part of the metric system of units, the conversion between the two is a little messy. Luckily, the approximation below is close enough for everyday use. $$2.2\text{ lbs}\approx \text{kg}$$