Wednesday, December 19, 2012

Confusing pounds (force) with kilograms (mass)

An article I read today in the Atlantic stated that man has left 400,000 lb. of material on the moon. This is incorrect in a way; that's 400,000 lb. on Earth. Due to the reduced gravitational force on the Moon, the weight on the moon would only be 66,200 lb. If you convert the 400,000 lb. to 181 metric tons (181,000 kg.), which is a unit of mass, then you can use that for the mass on the Moon as well as mass does not change with gravity.

There are two ways to rewrite the sentence that bothered me: "The moon, in particular, currently hosts nearly 400,000 pounds on Earth or 66,200 pounds on the Moon of man-made material." If you want the sentence to be short but correct: "The moon, in particular, currently hosts nearly 181 metric tons of man-made material."

In researching this point, I ran across two other sites that don't understand the difference between force and mass. This excerpt from the National Geographic (who should know better) is wrong: "Even though your mass would be the same on Earth and the moon, if you weigh 132 pounds (60 kilograms) on Earth, you would weigh about 22 pounds (10 kilograms) on the moon." No, your mass would be the same; if your mass is 60 kilograms on Earth, it would be 60 kilograms on the Moon as well.

The Moon Connection Web site has it wrong as well. I have sent correction requests to both sites.

By the way, there are many moons in our solar system but only one Moon. If you are referring to Earth's Moon, it should be capitalized.

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