Question on value of force in structural calculation 2

[indimech]

Hi everybody

I have a basic question.

Suppose I need to provide a support for a section of piping which have valve and some instruments on that. The weight of valve and instruments in catalog are given as 75 kg. So do I need to consider it as weight of valve and in calculation use 75 N for force or should I multiply by 9.81 to convert it in weight (W=mg) and then use as force in calculation?

Similarly if the fluid density is given as 1200 kg/cubic meter is it mass density or weight density for the purpose of calculations?

Thanks in advance

 

[drawoh]

indimech,

You sound vague and confused. "Weighing scale" does not mean anything to me.

A spring scale measures the force due to gravity. This is weight, in units of Newtons and pounds.

A balance compares your mass with that of some standard masses. The units are kilograms and slugs.

When you do your calculations, try doing a unit balance. If you are doing this wrong, your units will not make sense, and you will know you have to fix something.

JHG

 

[slickdeals]

When you stand on a weighing scale and measure your weight, its unit is not kg-m but kg-f. It is the measure of the earth's pull towards the core. Your true mass = weight in kg / 9.81. Your mass is always a constant, but your weight will change depending on where you are (earth/moon etc)

 

[indimech]

After getting so many responses I think its good to conclude that if an object is weighed on spring scale, the weighing scale reading will show its weight and to include that weight (force) in calculation as static load I need not to convert that weight into force by multiplying with acceleration due to gravity.

On the other hand if the object is weighed on balance the value of the mass against which it has been balanced will give me the mass of that object and to include that in calculation as static load I need to convert that mass into weight (force).

And obviously I will have to contact the vendor to know if the shown value on the catalog is measured by spring scale or balance. I would consider myself lucky if he knows that.

Otherwise better to consider the value on catalog as mass convert to weight and do the calculation. Some conservatism may come in the results but in view of all uncertainties it would be acceptable.

Any other thoughts are welcome.
Thanks all for the discussion.

 

[rb1957]

uh ? ... if you weigh something (spring scale, balance beam) you get it's weight.

i think i was correct in pointing out that a weight of 75kg is equivalent to a mass of 75kg (pretty obvious when you express it that way !), the force is 736N.

 

[nutte]

Amazing that this issue has gone on so long without resolution.

If the "weight" is in kilograms, it is mass, and should be multiplied by "g" to get to force.

If the "weight" is in pounds, it is a force, and no multiplication needs to be done.

It doesn't matter what type of scale was used. If you put a 5 pound bag of sugar on a balance scale, the corresponding object that balances it will have a weight of 5 pounds. They also have equal masses, but nobody wants to think about slugs.

 

[corus]

I find this discussion quite astonishing, but I'm glad that nutte has brought up 'pounds' to confuse the issue.

Going back to my schooldays I seem to remember that a pound is a mass but a poundal is the force, but nobody says poundal and confusingly calls the force pound, the same as the mass. Numerically though, one is different to the other. I'm just glad that here in the UK we switched to metric where mass and weight have conveniently different names and nobody gets confused.... or do they?

More slugs please.

corus

 

[BAretired]

rb1957,
You stated:

i think i was correct in pointing out that a weight of 75kg is equivalent to a mass of 75kg (pretty obvious when you express it that way !), the force is 736N.

That statement may be true at sea level, but not on a mountain top, not in a valley and certainly not on the moon.

BA

 

[rb1957]

BA,

you're quite right, but i'm pretty sure that 99.99% of engineers would use 32.174ft/sec2 (or 9.807m/sec2) for "g".

and if the application was for extraterrestal then the OP would state so (probably not at first, but usually in a later post "oh, btw, this is on the moon")

 

[indimech]

nutte,
I talked about spring weigh and balance because when I go to purchase any commodity it is measured in kg. The ref weight they keep on one side of balance say 1 kg and the commodity on other side say sugar.
I wanted to know whether that 1 kg ref weight represents mass and gives mass value or it is weight. and I will not get same "numerical value" if i compare an object with a "mass" and then weigh on spring scale unless spring scale is calibrated to show mass reading.

It is correct for lbm and lbf on earth since coincidentally both are numerically equal on earth.

 

[nutte]

A kilogram is a unit of mass. Period. End of discussion.

 

[indimech]

I guess I got the concept.

When weight is given in pounds there is no need to think about converting to force because pound (mass) and pound (force) both are numerically equal on earth. Hence same numerical value shall be used as static load.

But when weight is given as Kg it implies Kgf and we need to multiply by g to convert to force to use as static load.

The confusion arouse due to dropping of 'f' from Kg due to convention. Weight 27 Kg means 27 Kgf.

The following vendor site gives weight in pounds (without mentioning lbf or lbm)and Kg.

http://www.fisherregulators.com/products/literature/sr8/

If I compare both values then it comes that the value in Kg is weight in kgf and value in pounds is also weight.

This lets me deduce that both values are in force units lb in lbf and kg in kgf but f is omitted for simplicity.
thats why
27 kgf becomes 27x9.81=264.87N
and 60 lbf equals 60x4.45=267N

 

[drawoh]

indimech,

Your grocer is not doing acceleration calculations, so it is harmless for them to confuse units.

When I was in college, we learned English units and metric. We learned one set of equations for acceleration equations using force units, and another set of equations using mass units. You can analyze force using lbm and kgf with gay abandon if you select the correct set of equations every time.

There is a simpler way, less likely to result in mistakes.

m = w/g

f = ma = (w/g)a

m is in units of kg or slugs. In English calculations, if you insert (w/g) for mass, you never have to see a slug.

w is in units of N or lb.

JHG

 

[Ussuri]

I'm with corus on this. I have never heard of lb as a unit of force so I guess this must be an American convention. To me 1kg = 2.2lbs, both units of mass.

 

[rb1957]

i'm with corus, this had gone on way too long, and not gotten far, based on the last couple of posts.

"when weight is given as Kg it implies Kgf and we need to multiply by g to convert to force to use as static load" ... no, no, no. the "f" in kgf stand for force; when weight is given in kg (or kgf) it is a force and we Don't have to multiply it be anything to get force.


ussuri, "To me 1kg = 2.2lbs, both units of mass" ... absolutely correct except for the last word, which makes it absolutely wrong. a 1kg bag of sugar is equivalent to a 2.2lb bag of sugar ... it's weight not mass. i'm guessing you're in europe or some other place that hasn't seen the imperial system ... pounds have been used for weight for an awful long time, longer than N (or kg).

the problem is the metric system introduced the kg as a force (i guess the prols couldn't handle a Newton) and messed up a very nice system. however, the saving grace is that the weight of a 1kg mass is 1kgf (so long as your g = 9.807m/sec2). the only time to be careful is when you need the mass of something, like for momentum calcs, or mass flow rate.

 

[indimech]

Sorry I made wrong statement.

"when weight is given as Kg it implies Kgf and we need to multiply by g to convert to force to use as static load"

It should be:

"when weight is given as Kg it implies Kgf and we need to multiply by g to convert in 'Newton' to use as static load"

 

[rb1957]

geez, no, no, no ... kgf is a force. the correct statement is ... "when weight is given as Kg it implies Kgf and we use as static load"

sometimes people are confused (lazy?) and say "weight" when they mean "mass" (and vis-a-versa).

 

[nutte]

I'm flabbergasted.

I have never heard of lb as a unit of force so I guess this must be an American convention. To me 1kg = 2.2lbs, both units of mass.

That is wrong wrong wrong. Pound is a unit of force. The corresponding mass has the unit "slug."

a 1kg bag of sugar is equivalent to a 2.2lb bag of sugar ... it's weight not mass.

This is also wrong. Kilograms are a unit of mass. Pounds are a unit of force (weight is a force). A 1kg bag of sugar has the same mass as a 1lb bag of sugar. They also have the same weight. But that doesn't mean the two numbers with different units are the same. The weight in force units of a 1kg bag of sugar is 9.81 N (Newtons). 9.81N and 2.2lb are the same, and both are units of force.

Back to the original question:
Kilograms measure mass.
Pounds measure force.
To get the force induced by a 75kg mass (on earth, at sea level, when you hold your nose just right), you multiply it by g. The force is 75*9.81=736N.

This should have been resolved with the second post.

 

[indimech]

Since I am more comfortable with working in Newton as force than Kgf as force that's why I wrote to convert it into Newton.

But Kg implies force when given as weight that's the crux of discussion.

 

[Ussuri]

Off topic.

rb1957, pound is a unit of both mass and force. I disagree that pound as a mass is 'absolutely wrong'.

Thankfully, being British, and not old enough to remember imperial I do not have the problem of sorting out pound-force and pound-mass.

 

[nutte]

But Kg implies force when given as weight that's the crux of discussion.

It may imply force, but it's giving mass. Kilograms are always mass.