
#1
Aug3009, 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 



#2
Aug3009, 08:24 PM

P: 136

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




#3
Aug3109, 06:54 PM

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#4
Aug3109, 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 



#5
Aug3109, 08:24 PM

P: 10

Thanks 



#6
Aug3109, 08:27 PM

P: 10





#7
Aug3109, 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 (poundforce) 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. 



#8
Sep109, 05:57 AM

Math
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Thanks
PF Gold
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#9
Sep109, 11:10 PM

P: 10

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 



#10
Sep209, 04:18 AM

P: 2,032

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. 



#11
Sep209, 01:37 PM

P: 475

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. 



#12
Sep209, 01:39 PM

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From the equator to the poles your weight varies by about 0.5% and local geology also has a slightly smaller effect. 



#13
Sep209, 02:17 PM

P: 2,032

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. 



#14
Sep509, 06:54 PM

P: 475

In other words where is the place it is all nailed down. Who decided and why. 



#15
Sep509, 07:05 PM

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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. 



#16
Sep609, 03:46 AM

P: 2,032

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. 



#17
Sep609, 05:51 PM

P: 475

Presumably it has to be calculated at some place or every place using fixed instruments which have been calibrated with different 'g' values. 



#18
Sep609, 06:32 PM

P: 274

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:



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