Loaded test question!!!

[Besoeker]

Tweeet! Flag on the play (card if you are a soccer fan--not sure if there is a whistle because I can't hear over the vuvuzelas).

Both countries have adopted the pound as a unit of mass. That said, it is also customary to use pound to indicate force as well. We often see specs where the force is labeled with "lb" instead of "lbf". It is just the way it is and while the use of pound-mass and pound-force is encouraged, tradition continues.

I'm an old fellow - the Victor Meldrew of real life.
Yes, UK did use the pound and other Imperial measures and many older people still think in those units.
I'm so old that I learned both systems and I can readily convert from one to the other.
But I have to say I prefer SI.
The arithmetic is so much simpler.

 

[Smart $]

No mention of the gravitational constant in that definition.
Do you have one that does?

I can find several which, in essence state, "any of various units of mass and weight". The preceding quote is from Webster's Collegiate Dictionary. However, with metrification of weights and measures in the US over the last century, it is becoming increasing difficult to find governmental authorities on the matter which will outright state the pound is a unit of mass and weight.

That said, there are several publications, or sections thereof, by the authorities which are titled or headed with "Weights and Measures", and within give equivalents in the same measurement system then give conversion factors only to mass and not force. This was intentionally effected under federal directives given throughout the metrification process. As such, formal documents will not state the pound as a unit of weight (i.e. force). The intent of the directives are to deter not only use of the term "weight" to mean "mass", but to disassociatate the term "weight" from any formal measurement system.

The following excerpt is the only formal statement I could find via the internet...

NIST Handbook 130 states:

V. "Mass" and "Weight." [NOTE 1, See page 6]
The mass of an object is a measure of the object?s inertial property, or the amount of matter it contains. The weight of an object is a measure of the force exerted on the object by gravity, or the force needed to support it. The pull of gravity on the earth gives an object a downward acceleration of about 9.8 m/s2. In trade and commerce and everyday use, the term "weight" is often used as a synonym for "mass." The "net mass" or "net weight" declared on a label indicates that the package contains a specific amount of commodity exclusive of wrapping materials. The use of the term "mass" is predominant throughout the world, and is becoming increasingly common in the United States. (Added 1993)

 

[Besoeker]

The following excerpt is the only formal statement I could find via the internet...

The mass of an object is a measure of the object?s inertial property, or the amount of matter it contains. The weight of an object is a measure of the force exerted on the object by gravity

That nicely distinguishes the difference between mass and weight.

 

[Smart $]

That nicely distinguishes the difference between mass and weight.

Just to give you an idea of how metrification is proceeding over here, here's the same section in the 2008 publication...

8.3 Weight
In science and technology, the weight of a body in a particular reference frame is defined as the force that gives the body an acceleration equal to the local acceleration of free fall in that reference frame [4: ISO 80000-4]. Thus the SI unit of the quantity weight defined in this way is the newton (N). When the reference frame is a celestial object, Earth for example, the weight of a body is commonly called the local force of gravity on the body.
Example: The local force of gravity on a copper sphere of mass 10 kg located on the surface of the Earth, which is its weight at that location, is approximately 98 N.
Note: The local force of gravity on a body, that is, its weight, consists of the resultant of all the gravitational forces acting on the body and the local centrifugal force due to the rotation of the celestial object. The effect of atmospheric buoyancy is usually excluded, and thus the weight of a body is generally the local force of gravity on the body in vacuum.
In commercial and everyday use, and especially in common parlance, weight is usually used as a synonym for mass. Thus the SI unit of the quantity weight used in this sense is the kilogram (kg) and the verb ?to weigh? means ?to determine the mass of? or ?to have a mass of.?
Examples: the child?s weight is 23 kg the briefcase weighs 6 kg Net wt. 227 g
Inasmuch as NIST is a scientific and technical organization, the word ?weight? used in the everyday sense (that is, to mean mass) should appear only occasionally in NIST publications; the word ?mass? should be used instead. In any case, in order to avoid confusion, whenever the word ?weight? is used, it should be made clear which meaning is intended.

 

[gadfly56]

It is a prep test for instrumentation calibration.

Ahhhhh! Context is everything. The smart money is on "A", since you have to pick some answer. Although it's possible an instrumentation tech would be interested in "force", he/she would be very unlikely to be interested in volume or density since instrumentation doesn't measure these items directly. Well, density maybe, but it's not common in an industrial setting outside a lab.

 

[Lady Engineer]

Not a clue....I'm late!

 

[gadfly56]

In real life I get the Specific Gravity of the liquid, and use the Formula
P = H x (SG). As an example: for a product of a SG of .5 with a pressure reading of 100 inches of water gives a product level of 50 inches.

Ummmm, wouldn't that be 200 inches?

 

[gadfly56]

David,

I believe that you made a small error when you said 'the pound is not a unit of force, it is a unit of mass'. I don't believe that you can ignore the fact that pound has been used with both meanings.

The term 'pound' has historically been used both as a unit of force or as a unit of mass. As you have made clear, 1 pound force a different unit than 1 pound mass, so the term pound itself is ambiguous.

The most common usage that I encounter is to use pound to mean force, as in pounds per square inch, or as a unit of mass for commerce, with a 1g field assumed.

Just to be clear, I disagree only with the way you've assigned the word pound to have a specific meaning, not the physics of lbf and lbm.

-Jon

After a careless error in a sophmore physics exam, I have for the last 33 years always includes the subscript "m" or "f" when writing "lb"; always "lbm" or "lbf". If I weigh 200 lbm (I wish :grin:!) I have a mass of 6.22 slugs, or 200 poundals.

 

[Smart $]

...he/she would be very unlikely to be interested in volume or density since instrumentation doesn't measure these items directly. Well, density maybe, but it's not common in an industrial setting outside a lab.

Perhaps not common but possibly more than you think.

Consider a scenario where a mixing tank has inlets for two or more liquids having different densities. One liquid is pumped in until the bottom pressure indicates a preset level (Pressure = Level ? Specific Gravity), and thus volume knowing the base area. Now the process must pump in an amount of another liquid of a different density until a predetermined ratio of the two liquids is achieved. With another pressure sensor installed at some level above the bottom, we would have two pressure readings with a known vertical distance between them. Both transmitters are calibrated for inH2O and zeroed to their level. We can then subtract the pressure reading of the upper transmitter from that of the bottom then divide by the distance between them to get a Specific Gravity reading of the mixture. Knowing the SG of the mixture, we can determine the mix ratio and when to stop pumping in the second. We can also correlate the pressure reading of the bottom sensor to determine the volume of liquid. Comparing with the volume data of the first liquid we can calculate the mix ratio by volume for added assurance.

 

[BigDon]

Interestingly, I had the same question on my instrumentation certification test yesterday. I'm pretty sure I put "A".

 

[markstg]

Ummmm, wouldn't that be 200 inches?

nope, its weights less than water, therefore excerts less force than water therefore less pressure.

 

[gadfly56]

nope, its weights less than water, therefore excerts less force than water therefore less pressure.

Precisely, so if the reading is "100 inches of water" then the column itself must be 200 inches tall if it's sg is 0.5 that of water.

 

[gadfly56]

Perhaps not common but possibly more than you think.

Consider a scenario where a mixing tank has inlets for two or more liquids having different densities. One liquid is pumped in until the bottom pressure indicates a preset level (Pressure = Level ? Specific Gravity), and thus volume knowing the base area. Now the process must pump in an amount of another liquid of a different density until a predetermined ratio of the two liquids is achieved. With another pressure sensor installed at some level above the bottom, we would have two pressure readings with a known vertical distance between them. Both transmitters are calibrated for inH2O and zeroed to their level. We can then subtract the pressure reading of the upper transmitter from that of the bottom then divide by the distance between them to get a Specific Gravity reading of the mixture. Knowing the SG of the mixture, we can determine the mix ratio and when to stop pumping in the second. We can also correlate the pressure reading of the bottom sensor to determine the volume of liquid. Comparing with the volume data of the first liquid we can calculate the mix ratio by volume for added assurance.

Waaaay too complicated. How do you ensure that you have "a well mixed volume" without local high/low concentrations in the area of your transmitters? And how do you compensate for the volume change of mixing? Take two perfectly miscible liquids and 100 gallons of each, put them together and you may wind up with 190 gallons. Either put the tank on a scale and fill by weight, or if impracticable, install a couple of coriolis flow meters (measures mass flow directly) with flow integration and call it a day.

 

[markstg]

Precisely, so if the reading is "100 inches of water" then the column itself must be 200 inches tall if it's sg is 0.5 that of water.

Yes you are correct. So used to multiplying the H * SG to get the range of the level transmitters, I went backwards.

 

[david luchini]

And my point is that pound started out and still is a unit of weight. As such, an object weighing one pound by definition has one pound of mass and exerts one pound of gravitational force. The gravitational constant is already included by definition. It really is that simple.

Yes it is that simple. Using the gravitational constant, 100 pounds of mass weighs 100 pounds (or exerts 100 pounds-force) AT sea level. The same 100 pounds of mass weight 99.7 pounds on the top of mount everest.

Close enough? Maybe. The same? No.

 

[Smart $]

Yes it is that simple. Using the gravitational constant, 100 pounds of mass weighs 100 pounds (or exerts 100 pounds-force) AT sea level. The same 100 pounds of mass weight 99.7 pounds on the top of mount everest.

Close enough? Maybe. The same? No.

I never said they are exactly the same.

Close enough? Well taken that the all time percentage of objects weighed in places much, much closer to Earth's mean radius versus atop Mt. Everest, I'd guess the ratio to be astronomical. So the "down to earth" discrepancy is much less than you make it seem

So do you also account for g-force deviation due to lattitude and the angle of g-force?

In a weighing of product, do you continually compensate for air bouyancy? You do know the federal trade definition of a pound weight is within a vacuum, right?

When is the last time you checked the gravitational force at your location?

 

[Besoeker]

I never said they are exactly the same.

From this, it seems like you did.

As such, an object weighing one pound by definition has one pound of mass and exerts one pound of gravitational force

My physics teacher (from what you'd consider high school) explained the distinction between mass and weight thus:
A force of one Newton will give a mass of 1kg and acceleration of 1 m/s^2. On earth, on the moon, or in space. Local gravity determines it's weight.

 

[david luchini]

I never said they are exactly the same.

No you didn't say they were exactly the same, you said:

1 lb(weight) ≈ 1 lb(mass)

which allowed you to make the leap that P = H * D (as a complete equation) because ft x lb(mass)/ft? becomes "pounds per square feet."

Well imagine, instead of a column of water, you had a lead cylinder which is 2" dia and 12" long. And at sea level (so that g=32.147ft/s?) the cylinder was sitting on end with a magnetic field above the cylinder that put a force on the cylinder that produces an upward acceleration of 5ft/s? on the cylinder (opposite to gravity.)

Would the pressure at the base of the cylinder still be calculated by P = H * D? Of course not. You'd be off approx. 18%. Add "acceleration" back into the equation: P = H * D * a, and you'd have a complete equation.

 

[Smart $]

...

Well imagine, instead of a column of water, you had a lead cylinder which is 2" dia and 12" long. And at sea level (so that g=32.147ft/s?) the cylinder was sitting on end with a magnetic field above the cylinder that put a force on the cylinder that produces an upward acceleration of 5ft/s? on the cylinder (opposite to gravity.)

Would the pressure at the base of the cylinder still be calculated by P = H * D? Of course not. You'd be off approx. 18%. Add "acceleration" back into the equation: P = H * D * a, and you'd have a complete equation.

So I'm supposed to comment on your posts while you ignore the better part of mine... :roll:

 

[Smart $]

From this, it seems like you did.

Taken out of context, it sure seems that way

My physics teacher (from what you'd consider high school) explained the distinction between mass and weight thus:
A force of one Newton will give a mass of 1kg and acceleration of 1 m/s^2. On earth, on the moon, or in space. Local gravity determines it's weight.

Within the context of my comment and the discussion on my part, places on earth where gravitational force is at its extreme, on the moon, or in space are most certainly are excluded from the other side of a leapt chasm.