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

 

[BAretired]

A kilogram is a unit of mass. A 1 Kg bag of sugar has a mass of 1 Kg on the earth, on a spaceship or on the moon. But its weight varies according to the acceleration of gravity in its particular locale. I am amazed that we are arguing about this.

BA

 

[rb1957]

nutte, BA

when was the last time you were in a store ? buying a kg of sugar ??

 

[indimech]

Guys,
I started this discussion to clear my doubt why a vendor gives a mass unit in the heading of weight and whether he means that given value represents the mass or weight?

Please see the website for my doubt:

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

Now after reading the point of views of so many persons and probably from different geographies of earth where different system of units are followed I got more insight in this topic which I want to outline:

1. Before this discussion I used to think pound is unit of mass.
2. In USCS (United States Customary System) pound is unit of "Force" and Slug is unit of "Mass". Where as in FPS unit system pound is unit of "Mass"
So depending upon system of units it represents both.
But on earth co-incidentally pound force are pound mass are equal.
2. We get groceries or commodities on weight basis so if i purchase 1 pound sugar it means that sugar "weighs" 1 pound (lbf) similary where things are sold in Kg it means the weight of sugar I purchased will be 1 Kgf and for convenience they omit 'f' and if I convert this in newton it will become 9.807 N

1 lbf sugar = 4.45N
1 lbm sugar = .453 Kg of sugar (mass basis)
weight of .453 Kg = .453x9.807=4.4425 N

On earth due to the value of proportionality constant of Newtons second law equation F= Kma, lbf and lbm are equal.

3 The direct equivalent of Kg (mass) is NOT Pound (mass). It is Slug.

4. If you are using mass in Kg or Slug then you can directly use the equation F=ma (K is equal to one in these cases) but if you are using mass in pound you have to use F=kma.

5. If a vendor writes weight as 27 Kg he means 27 Kgf to get the value in newton it should be multiplied by 9.807 for those who work in N. For those who work in Kgf as force no need to multiply by 9.807.

If weight is given in pound it is basically pound force.
If anybody uses USCS there is no confusion since he is incorporating force in his calculation.

Those who work in FPS taking pound as mass if they convert it in force say pound force they will get the same value.
For example 60 pound mass on earth will be 60 pound force.

 

[BAretired]

rb1957,

I don't buy sugar. It's not on my diet.

BA

 

[imcjoek]

1 lbm * 32.2 ft/s^2 * 1/32.2 lbf*s^2/lbm ft = 1 lbf

1kg * 9.8 m/s^2 * 1 N*s^2/kg*m = 9.8 N

(Mass * g)/gc = Force

Going against the grain, I'm going to have to say I like the FPS system better... 1 = 1! (as long as you are religious about indicating lbm vs lbf it works very nice).

On the subject of vendor data-sheets claiming "weight" in kg. They do that. Sometimes I think the datasheets are produced by the sales department. I would hope you can figure out by the factor of 10 what is meant.