But I'm sure Speed and velocity are the same thing

[Jack21222]

In what instance would someone misuse the words? I'm clear in the distinction, but am having trouble thinking of an everyday scenario where someone would misuse either term

One example: somebody telling me they were driving on a road with curves at a constant velocity. They mean speed, but they're trying to sound pretentious.

 

[russ_watters]

Not per the US standard bearers, the National Institute of Science and Technology. The word "pound" sans any qualifier designates the avoirdupois pound, a unit of mass. If you mean force it is best to say "pounds force", or lbf for short.

I think this is bad advice. We just had a thread where someone asked why pressure is psi and not pounds. Someone else - instead of answering the actual question - gave a criticism on using pounds force instead of pounds, which was very unhelpful.

Most of the time it is better to answer a question according to the meaning that is clearly understood rather than not answer it because a word was very slightly improperly used.

https://www.google.com/search?q="pounds+force"+site:nasa.gov

NASA doesn't always get it right either: https://www.google.com/search?q="po...a0003da4073894&bpcl=40096503&biw=1184&bih=655

NASA sites use both "pounds per square inch" and "pounds force per square inch", as well as "weight" in "pounds". Worse, the second link says "pounds of pressure per square inch"!

 

[russ_watters]

One example: somebody telling me they were driving on a road with curves at a constant velocity. They mean speed, but they're trying to sound pretentious.

Some people do actually use the words interchangeably, so I don't think it is likely the person intended it to be pretentious. Correcting the usage, however, would be.

In the case of "pounds"... I would go so far as to say even in many technical fields it is most often used - unqualified - to indicate force, not mass.

 

[BobG]

I think this is bad advice. We just had a thread where someone asked why pressure is psi and not pounds. Someone else - instead of answering the actual question - gave a criticism on using pounds force instead of pounds, which was very unhelpful.

Most of the time it is better to answer a question according to the meaning that is clearly understood rather than not answer it because a word was very slightly improperly used. NASA doesn't always get it right either:

Good point. It's common (and common sense) to use context as a clue to what's actually meant by a word. When it comes to celestial mechanics/orbital maechanics, etc, how many times do you really want to type out 'specific energy per unit of mass' or 'specific angular momentum per unit of mass'? It doesn't take very long to learn that when a book about orbital mechanics talks about angular momentum, they mean specific angular momentum per unit of mass.

But, it is common to see equations that use mass measured in pounds - uncommon enough that it's kind of annoying when it is expressed in pounds, but common enough that one should know that pounds is a perfectly legitimate unit of measure for mass. (It can also be confusing for students when 'g' pops up in an equation used to calculate the amount of thrust and fuel to escape the Moon, for example - "Why the heck are we using Earth's gravitational acceleration when we're on the Moon?!") In fact, I'd say it's annoying enough to give a person a very, very strong preference for the metric system.

Well, at least until you use the exact same equation in the metric system and divide one term by 'g' to get a special unit (specific impulse) and multiply one term by 'g' to calculate fuel flow rate and start to wonder why 'g' is inserted into the calculation in the first place because all sense of context has been lost.