# Homework Help: Gravitational Weight into kilograms

1. Jun 26, 2013

Isnt Weight = mass * gravity of earth

so 1 kilogram = 1 lb? Or am i missing something

2. Jun 26, 2013

### gary32

Mass is measured in Kilograms (SI unit)
Weight is measured in Newtons (SI unit)

1kilogram is 2.2lbs,
Lbs (pounds) is another unit of measuring weight

3. Jun 26, 2013

### PeterO

And those pounds are a unit of mass also. The American method of calling pounds a force, with slug as a mass confuse this comparison further.

As for the weight of a person - my own is approx 1000N - we don't like number values to be too large so my weight is generally stated as:
"I weigh (as much as a mass of) 102 kg"
But those words in brackets are never stated, merely implied.
Thus you hear people say "I weigh 102 kg"

Oh, and before Australia went metric, I had a weight of 224 lb which meant "I weigh (as much as a mass of) 224 lb"

Last edited: Jun 26, 2013
4. Jun 26, 2013

### D H

Staff Emeritus
Pounds aren't metric units. Assuming Earth standard gravity, one kilogram has a weight of 9.80665 newtons. On the Moon it's about 1/6 that.

Using equality (1 kg = 9.80665 newtons) is not a good idea. A kilogram is not equal to any number of newtons. The units aren't even equal.

Strictly speaking, the correct US unit is the pound force, not pound. (And being even more strict, the unit of mass is the Avoirdupois pound, but *nobody* uses that qualifier.) The pound is a unit of mass. If you mean force, use "pounds force".

Or just use metric and get rid of the confusion.

You weigh 102 kg, and a one pound can of beans weighs one pound here, on the Moon, and elsewhere.

This is a case where scientists are, IMHO, overly pedantic about a distinction invented by science. Weight as a synonym for mass has a much longer history than physicists redefinition of weight as a force. Up until 100 years or so ago, even technical people were split on what weight represented, force or mass. Legally and colloquially, weight remains a synonym for mass.

When I'm working technically I try to avoid the word "weight". It has rather limited technical meaning. If weight is the product of mass and gravitational acceleration then (1) It's not constant. It varies place to place on the Earth, and going into space, it varies, oh, a whole lot. (2) You can't measure it. Gravitational force is not measurable. The same applies to any other fictitious force. Weight defined that way is a useful fiction, but a fiction nonetheless. (3) What most people mean by weight is what they measure when they step on a scale. That force isn't gravity. How can it be? It's pointing in almost exactly the opposite direction as gravitational force!

5. Jun 26, 2013

### PhanthomJay

I agree
Agree
we just call it pounds, both in layman and engineering use
The unit for mass is the slug in USA, but nobody uses that term.
Won't happen in the USA for decades
This contradicts your opening sttatement. A person with a mass of 102 kg weighs 1000N N on earth. and about1/6 of that on the moon
Nah, a pound of beans here on earth has a mass of 1/32 slug and weighs 1 pound here on earth, per W =mg. On the moon, it weighss about 1/6 of that. In free space, it weighs nothing.

6. Jun 27, 2013

### D H

Staff Emeritus
Who's this "we" you are talking about?

The official unit of mass in the US is the kilogram. This has been the case for over 100 years. Before that it was the pound (avoirdupois). In 1893, the pound was relegated to the status of a derived customary unit of mass. As far as the slug is concerned, it didn't even enter into the official lexicon as a derived customary unit of mass until sometime after 1975. *The* unit of mass in the US is not and never has been the slug.

Not really. The lay term "weight" has multiple meanings. Sometimes "weight" means mass, other times, force. That's no different from many other words. Words can have multiple meanings. There's nothing wrong with that. In the lay sense, weight oftentimes is a synonym for mass. When someone says "I weigh 102 kg" the meaning is clear: that person is obviously talking about mass.

The legal usage is not overloaded. In the USA and in some other countries, "weight" legally is a synonym for mass, and never force. NIST Handbook 130 repeatedly says: "NOTE 1: When used in this Law, the term “weight” means “mass.”"

7. Jun 27, 2013

### PhanthomJay

"We" are the engineers of the USA , most of whom have no idea what a Newton is.
"We" are the non-technical majority of the US, none of whom know what a Newton is and most of whom have no idea of what a kilogram is. We are the construction guys and gals who, if we were told to erect a couple of columns 21.6 meters tall spaced 85 cm apart on concrete pads utilizing 10 cubic meters of concrete would tell you to go jump in the (expletive deleted) lake (feet, inches, pounds, yards only, please).
i guess most of us were never told that. The term mass is used only by the tech folks, never by others. Everything is in weight units.
I used it in high school Physics back in 1963
in Physics 101, if you want to use Newtons laws F=ma and W=mg, and you want Your force or weight in pounds, you had best use mass in slugs and acceleration in feet per second per second, if you want to pass.
lawyers and engineers never agree on anything. In fairness, I Asked a few engineers at work today what the unit of mass is. They had no idea, except that you divide weight on earth by 32 and magically you get the mass unit .

8. Jun 28, 2013

### D H

Staff Emeritus
Unless they work for NIST, engineers are not the ones who get to decide what the mass unit is in the US. Yes, engineers used the slug long before the term entered the official lexicon as a derived customary unit. The slug was invented in 1900. What that means is that you engineers were using an unofficial unit. The official mass unit in the US was the pound, up until the 1893 Mendenhall order. After that, it was the kilogram. The Mendenhall order relegated the pound (along with all of our other customary units) to the status of units derived from their corresponding metric counterparts. The pound is still used as a unit of mass, but officially, we've officially been metric since 1893.

We've had this fight before, Jay. It's a stupid fight. The pound is a unit of money, a unit of force, and a unit of mass. Multiple units of mass; a pound of feathers weighs more than does a pound of silver or gold.

9. Jun 28, 2013

### PhanthomJay

Hah, indeed! I tend to be repetitive.
Clever!