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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.
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