Calculate Mass From Weight: Easy Conversion Guide

[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]

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]

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]

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]

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]

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]

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]

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]

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 that's 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 don't 'weigh' 79 kg or 175 lb. That is your mass. The scales have done the calculation for you.

 

[Buckleymanor]

So this is the point, you don't '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]

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]

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 because 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 don't '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]

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]

Weight and mass aren't really defined like that.
Mass is fixed an independent 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]

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]

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]

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

 

[mgb_phys]

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.

The mass is fixed it's a block of platinium in Paris that is (roughly) the same mass as 1000cc of water (the original definition of the Kg)
This block would weigh different amounts in different places - but if you compare it's weight with another identical block (with just a simple see-saw balance) that would also be the same anywhere and wouldn't depend on the weight (or the details of the balance)

If you used one of these blocks to calibrate a weighing scale you would have to do that where you are using the scale - by putting a 1kg block on the scale and marking where it registered 1kg (or 981N). In practice for a high accuracy digital lab scale you might just calibrate it as the factory (in known g) and supply a table of adjusted g for each city.

 

[tony873004]

You can't use kgs, that measures mass not force.

I was overseas recently, and noticed on a packet of sugar, it said Net Weight 5g. I commented to the person with me that this was technically wrong. It should say net mass. But I kept seeing this convention over and over again, using kg and g as units of weight. This was true in different English-speaking countries outside the US.

So I looked it up on the internet. Wikipedia has an article that says that kg and g not only represent kilograms and grams, but they also represents kilogram-force and gram-force, the force a mass of 1 kg or 1 g exerts under an acceleration of Earth's surface gravity.

Is anyone familiar with this? It makes sense, since people all over the world speak of their weight, not mass, and most of the English-speaking world outside the US and UK don't use pounds for anything.

 

[xxChrisxx]

I use kg and g when referring to weight (apart from myself because I acutally don't know my height and weight in metric units, only imperial). And I was aware of kg-force (its just the same as lds and lbf) but that's because I remember my year 8 or 9 physics lessons.

People use kg when referring to weight for the same reason people in the US and use pounds and pounds its just easier to do that and its clear what is meant by context. The only problem with this convention is that laypeople (who don't remembe rtheir early physics lessons) get confused and think weight acutally is measured in kg.

I can never imagine walking up to someone at the deli counter in Morrisons and asking them for 5N of sliced honey roast ham. They just wouldn't know what the hell you were on about.

 

[f95toli]

Is anyone familiar with this? It makes sense, since people all over the world speak of their weight, not mass, and most of the English-speaking world outside the US and UK don't use pounds for anything.

Yes, we use kg (or g) when we talking about weight. When people (including physicists) talk about "weight" in countries that use the SI they are generally NOT talking about the force, but the mass.
In some languages it would nowadays even be wrong to use the word for "weight" for a force; over the years the meaning has changed to mean just mass.

Also, just a comment about the value of "g". Although I am sure some books use an average value (=standard gravity) that is not always true; you can quite easily look up the value for g at your location in a table.
I went to high school in the north of Sweden and then we used g=9.82 m/s^2; but at university (in the south of Sweden ) we used g=9.81 m/s^2; this was just due to the difference in latitude.

 

[M Grandin]

None appears having mentioned compensation for Arkimedean displacement lifting force = (density of air) x (volume of person), that should be added to showed weight on scale.
If you want mass and not just weight.

If air density is 1/1000 of body (=water) density, it makes weight on scale should be multiplied by 1.001 to get mass. (Or more correctly divided by 0.999).

It may be easier realized If you imagine scale and person in water instead: The scale had shown around zero weight, and corresponding displaced volume times water density must be added.