Resources
Outside the Site
Multimedia, Etc
Web Shortcuts
Q&A: Miscellaneous

Q:
I need to know if it is wrong to say that a person's "mass" is 350 pounds. Is it wrong to call "pounds" mass? So if I said, "The mass of one wrestler: 350 pounds = 158 kilograms" would I be wrong? Also, I have been taught that weight can only be in newtons and never in pounds. Is that correct? Do you ever call pounds weight?

A:
Mass tells how many particles an object, such as a wrestler, contains. It is measured in kilograms (metric) , or slugs (English). Weight tells what the pull of gravity on that mass is. It is measured in newtons (metric) and pounds (English). Mass is the same wherever you are, as long as you are moving slowly compared to the speed of light. Weight varies depending on where you are. The wrestler could weigh 158 kilos on Earth, but he would weigh only 60 kilos on Mars. Astronauts in orbit are in a state of apparent weightlessness, because their spacecraft is accelerating toward Earth at the same rate they are, so they feel as if there is no gravitational force acting on them, that is, they feel weightless. To be truly weightless, they would have to be far away from the Earth, Sun, moon, or any other bodies with mass, or else the pull of all the bodies would have to exactly cancel.

Now, to your questions: Strictly speaking, in the metric system, mass is in kilograms and weight is in newtons, and in the English system, mass is in slugs, and weight is in pounds. So, it would be wrong to say that the mass of a wrestler is 350 pounds, or that his weight is 158 kilograms, but as you have probably noticed, it is common usage to use pounds as a mass and kilograms as a weight.