Pound Force to Pound Mass Conversion : Physics & Math

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Pound force to pound mass conversion only requires that you use a specific equation designed for that purpose. Find out about pound force to pound mass conversion with help from a warm, engaging and experienced science enthusiast in this free video clip.

Expert: Kristin Oviedo
Filmmaker: Blake Higgins

Series Description: Physics and mathematics are directly related in a number of very important and very interesting ways. Get tips on physics and math with help from a warm, engaging and experienced science enthusiast in this free video series.

[jinnaminapple]

I'm here to shed some light, as I see there being A LOT of confusion from this video. It's also scary to see how so many seasoned engineers and scientists seem to get this wrong in the US...
First off, please DON'T consider it a "conversion". The concept of lbm to lbf is like kg to newton. So when you go from lbm to lbf or kg to newton, a question you may be trying to answer is "how heavy am I on Earth?" So bottomline: an object with mass 1 lbm weighs 1lbf on Earth, and an object of 1 kg weighs 9.81N on Earth. Why? Because F=ma. Just take a look at the units and this will make a lot of sense.
In terms of Newtons, 9.81 N = 1 kg * (9.81 m/s^2). So Newton has a unit of kg*m*s^-2. SIMPLE.
In terms of lbf, it is 1 lbf = 1 lbm * (32.2 ft/s^2) * a constant. 32.2 ft/s^2 is the acceleration in terms of ft. Notice that because Americans wanted to keep it so that it is an easy "1 lbm weighs 1 lbf on Earth", we need to multiple by a constant. The constant has a value of K = 1/(32.2 (lbm·ft)/(lbf·s2)). To make matters even more confusing, we decided to ALSO term the reciprocal of K. That is, g_c = 32.2 (lbm·ft)/(lbf·s2). So if you calculated F=ma in America, 1 lbf = 1 lbm * (32.2 ft/s^2)/(32.2 (lbm·ft)/(lbf·s2)). Remember, that is NOT to say 1 lbm equals 1 lbf. It means an object of 1 lbm weighs 1lbf on Earth.
Let's take an international example: I'm in a European country. I hop on a scale and figured I "weigh" 65 kg. Technically speaking, even this is wrong to say... 65 kg is a measure of mass. Your scale is able to convert the force you exerted into mass by dividing by 9.81 m/s^2. Remember, (m=F/a). "a" is pretty much the same in most places around Earth, so you won't be called stupid - it's a simple conversion.
Next, I speak to my buddy in the US. He wants my weight in terms of "pounds". What he actually meant is, what is your weight in terms of "lbf". You probably would have thought "lbm" because the magnitude works out to be the same. But if you are talking about weight (which is a measure of force a mass exerts given acceleration) you should need to say "lbf". The "conversion" factor you know to date for kg to lbf is 2.205 (with a BUNCH of units). Keep in mind it is NOT a conversion. It is you asking how much a mass of X kg weighs in terms of lbf on Earth.

[commanderstarstrider]

This is why we should switch to metric.

[jski]

So am I understanding this correctly? A 4" bore pneumatic cylinder can put out around 1130 lbs of force at 90 psi, so does that mean it could lift about 1130 pounds of weight?

[acferrad]

this is how Martian landers crash into the surface, this is incorrect: 1 lbm exerts a force of 1bf (on Earth)

[husnainhyder]

So cleared
You've made it very simple

[whirltech]

From a soon-to-be Aerospace Engineering grad: THIS IS CORRECT
While THIS IS CORRECT; pounds-mass are an inconsistent unit (requires a correction factor). This video does not discuss the consistent unit (unit which does not require a correction factor) of mass in the Imperial unit system: the slug. A discussion of the slug helps, because it highlights why a conversion factor is used in the Imperial system and is not needed in the SI system.

Why this film is right:
In the Imperial system, one pound-force is *defined* as 1 pound-mass undergoing an acceleration of 32.17 ft/s^2 (one "g"), as shown in this film (though the film does not explicitly state this as a definition). From this definition the slug is *derived* as 32.17 pounds-mass. In this manner, 1 slug undergoing an acceleration of 1 ft/s^2 equals 1 pound-force - a consistent system. In the SI system 1 newton (force) is *defined* as 1 kg undergoing an acceleration of 1 m/s^2. Thus, the units are consistent, by definition.

[stephenphilbrook]

This is false. 1 lbf equals 1 lbm on Earth but if you go some place where the acceleration due to gravity is different from 32.1740 ft/s^2, this will not work. The correct form of Newtons 2nd Law that she should be using is F=(1/g)ma where g is standard acceleration due to Earths gravity, m is the mass, and a is the local acceleration. Another way to say it is 1 lbf ONLY equals 1 lbm when the mass is accelerated at EXACTLY 32.1740 ft/s^2. If it is accelerated at any other value, they are not equal. I hope the person who created this video either takes it down or corrects it because this is wrong.

[HALEdigitalARTS]

This is the most efficient clarification on this point I've seen to date.
Thanks for the great vid.

[ianlorber]

I don't think anyone can answer the following question
What are the US customary equivalent for Newtons (N)?
What are the US customary equivalent for Kilograms (kg)?

[Dodgybob]

Sorry but I think this is wrong! I would hate for students to get confused

[SuperFredAZ]

The unit of mass in the Imperial system is the slug.

[robertingersoll]

This is totally wrong -- remove this video - you are misguiding young kids.

[johngaver]

This is wrong. Please delete this video