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Mass of a person

by Joe436
Tags: mass, weight
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Joe436
#1
Aug30-09, 08:13 PM
P: 10
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
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Pupil
#2
Aug30-09, 08:24 PM
P: 136
Weight is a force. f = ma. a on earth is g = -9.8m/s^2. m = F/g.
mathman
#3
Aug31-09, 06:54 PM
Sci Advisor
P: 6,076
Quote Quote by Joe436 View Post
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
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
#4
Aug31-09, 07:52 PM
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Mass of a person

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
#5
Aug31-09, 08:24 PM
P: 10
Quote Quote by Pupil View Post
Weight is a force. f = ma. a on earth is g = -9.8m/s^2. m = F/g.
Can I use lbs for F or do I have to use kgs?

Thanks
Joe436
#6
Aug31-09, 08:27 PM
P: 10
Quote Quote by mathman View Post
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.
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
#7
Aug31-09, 11:17 PM
P: 18
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
#8
Sep1-09, 05:57 AM
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Thanks
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Quote Quote by Joe436 View Post
Can I use lbs for F or do I have to use kgs?

Thanks
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
#9
Sep1-09, 11:10 PM
P: 10
Quote Quote by HallsofIvy View Post
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.
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
#10
Sep2-09, 04:18 AM
P: 2,048
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
#11
Sep2-09, 01:37 PM
P: 490
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.
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
#12
Sep2-09, 01:39 PM
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Quote Quote by Buckleymanor View Post
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.
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
#13
Sep2-09, 02:17 PM
P: 2,048
Quote Quote by Buckleymanor View Post
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.
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
#14
Sep5-09, 06:54 PM
P: 490
Quote Quote by mgb_phys View Post
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.
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
#15
Sep5-09, 07:05 PM
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P: 8,953
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
#16
Sep6-09, 03:46 AM
P: 2,048
Quote Quote by Buckleymanor View Post
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.
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
#17
Sep6-09, 05:51 PM
P: 490
Quote Quote by xxChrisxx View Post
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.
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.
PhaseShifter
#18
Sep6-09, 06:32 PM
P: 272
Mass is independent of g, it is an inherent property of the object in question.

There are numerous ways to determine mass without having to adjust calculations for local values of g. Here are a few:
  • Apply a known force and measure the resulting acceleration
  • Attach the object to a spring and measure its frequency of vibration
  • Attach the object to a rotor (or string) and measure the amount of centripetal force required to swing it in a circle at a given velocity


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