Mass of a person

Joe436

Hi,

Can someone tell me how to find out the mass of a person. If someone weighs 150 lbs (on Earth) what is their mass? Any chance someone can point me to a web page that will convert weight to mass for me.

Thanks,
Joe

Pupil

Weight is a force. f = ma. a on earth is g = -9.8m/s^2. m = F/g.

mathman

In the usual units, the mass of something on earth has the same numerical value as its weight on earth. Anything which weighs 150 lbs. on earth will have a mass of 150 lbs. If you ever take a physics course they will try to set up separate units for mass and weight, but it can be confusing.

mgb_phys

It's a little simple in metric. In imperial units pounds are used for force and mass.

A 150lb (mass) person is about 68kg (mass)
On earth they have a weight (force) of f = mg = 68 * 9.8 = 666N

Joe436

Can I use lbs for F or do I have to use kgs?

Thanks

Joe436

But your weight will change if you're on the moon, mars, etc... Your mass is always the same regardles of where you are.

Winter Flower

http://en.wikipedia.org/wiki/Force#Units_of_measurement

(force) = (mass) (accelleration due to gravity)

Like mathman said a pound is actually also unit a for mass but the SI unit which is usually used is kg.

If you want to use pounds for force you're actually using lbf (pound-force) units.

In physics courses they typically use Newtons (= kg x m/s^2) for force.

btw Technically "weight" is the magnitude of the force vector, which changes depending on the local gravity, but people sometimes use the word interchangeably with mass since with the lbf units they are the same on Earth.

HallsofIvy

You can't use kgs, that measures mass not force. If you are working in "mks" (the "meters, kilograms, seconds" version of the metric system), force is measured in "Newtons" as mgb_phys said. In the English system "pounds" is a force, in the laboratory, the "slug" (the mass that a force of 1 pound would accelerate at 1 foot per second squared) is used for mass.

Joe436

OK, I think there is some confusion in what I am asking. I step on a scale in my bathroom and it says I weigh 175 lbs or using the metric system 79.5 kg. If I'm on mars, venus, pluto, etc... my weight will be different. But my mass, which I believe is measured in KG is always the same no matter where I weigh myself. What I want to know is the formula for finding out my mass given my weight on Earth. Now most formulas use the metric system so I'm assuming I can't input my weight in pounds, I have to use KG also.

So what is the forumal for getting my mass given my weight on Earth and what unit of measure do I have to use for weight?

Thanks,
Joe

xxChrisxx

Those scales factor in the 9.81 m/s of gravity, they are measuring weight but giving a mass output.

If the scales say 79.5 kg, then you technically 'weigh' 779 Newtons.

Basically earth scales are onlyvalid on earth, as they mesure weight then divide by 9.81 to give the reasong in SI units.


However you can convert the units to imperial or english if you want, the relationship holds.

F = mg thats all it is.

Pounds are (stupidly) used for both mass and force, they are NOT the same value however.

1 lbf (force) = 1lb (mass) * 32.17405ft/s^s (gravity)

so 1lbf = 32.17 lb.ft/s^2.

What your scales weigh if the give a reading of 175 lbs is actually 5629 lbf(force) and then divide by the value of gravity to give your mass of 175 lb(mass).



So this is the point, you dont 'weigh' 79 kg or 175 lb. That is your mass. The scales have done the calculation for you.

Buckleymanor

So the Earth's gravity varies at different locations and if I record my mass using the the scales at these locations.
Won't the scales give a different value for my mass depending on location.
If the scales had done the calculation the value for the mass would not vary.

mgb_phys

Yes for accuracy better than 1% you need to recalibrate the scales depending on your location.
From the equator to the poles your weight varies by about 0.5% and local geology also has a slightly smaller effect.

xxChrisxx

The scales will give a different value for mass becuase they are calibrated to a specific gravity. In localised regions of higher/lower gravity the reading will be off because your weight will change but the scales dont 'know' the gravity has changed.

The weight to mass calcualtion is just a constant multiplier that either has to be programmed in on a digital scale or the wheel has to be calibrated on an analogue scale.

Buckleymanor

So where do you decide is the correct location to give the accurate reading for both mass and weight.
In other words where is the place it is all nailed down.
Who decided and why.

mgb_phys

Weight and mass aren't really defined like that.
Mass is fixed an independant of position, so if you measure the weight (ie force) at a particular location - that gives you the local value of 'g' = the acceleration due to gravity.

xxChrisxx

Mass never changes.

The value of 'g' used to calculate weight is taken from an average of different 'g' values from different points.


This is the reason we assign a standard value for gravity.

Its also the same reason we assign a standard value for atmospheric pressure, that vaires but for the sake of ease of calulation a value is chosen that minimises error at all locations.

Buckleymanor

So if mass never changes how is it fixed independently of position.
Presumably it has to be calculated at some place or every place using fixed instruments which have been calibrated with different 'g' values.