I think like many engineers, one of the reasons that I liked the idea of going into engineering was the mistaken belief that a career in engineering meant working with numbers and not having to write. I have never been very good with words. I love numbers. Numbers make sense to me. Words confuse me at times. Also, the English language in general makes no logical sense to me, and I am a native English speaker. [Well, I’m a Southern, so you can go ahead and make an argument against the native English speaker part.] I can’t spell worth crap because of the aforementioned English language illogical thing. This is actually fairly common among engineers. Had my family known the warnings signs, it would have been obvious since I was about 5 years old that I was destined to be an engineer. I have always been good with numbers, and I can’t spell worth crap. Take note parents, as these are the warning signs your child may be an engineer. Also, an early love of duct tape.
I am fairly good with grammar though probably because there are more rules and less exceptions. Partially because of this and for other reasons, at my current job, I often edit other people’s documents not just for science and engineering accuracy but also for grammar, readability, and clarity. I have also been a peer reviewer for a few manuscripts submitted to scientific journals, and previously, I used to edit manuscripts that were about to be submitted to peer review journals. I keep coming across certain words and phrases that are scientifically and grammatically incorrect. There are many grammar style manuals that exist, but I have yet to come across a science grammar style manual. If one exists, I would love for someone to point me to it. So I’ve decided to start writing about some of the most common and inappropriate phrases in the hope that maybe it will stop at least one one person from using these incorrect phrases.
The most common and completely wrong phrase I see is stating that something or someone weighs a certain number of kilograms. Ironically, non-US citizens, i.e. people who live in a country where they use the metric system, are just as guilty of this phrase as US citizens who sort of have the excuse that they live in a country that refuses to stop using the completely archaic and impossible to use if you are a scientist or engineer, English imperial or US customary units. For the benefit of everyone who doesn’t understand why this is wrong, let me explain why it is.
A kilogram is a unit of mass. Mass is the amount of stuff that an object has.
To say something weighs something, you are saying it has a certain amount of weight. A weight is a specific type of force, and because it is a force, weight, like all other forces, is measured in Newtons (N) in the metric system. Weight is the amount of force on an object due to gravity. Therefore, weight is the mass (the amount of stuff) multiplied by the acceleration due to gravity. If you happen to be a person on Earth with a mass of 70 kg, then since the gravitational acceleration on Earth is 9.81 m/s2, you have a weight of 687 Newtons (N). Let’s say you happen to be an astronaut, and the Moon program gets revived, and you go to the moon. Your mass will not change. Your mass will still be 70 kg, but once you arrive at the Moon, your weight will be about 114 N because the Moon’s gravitational acceleration is 1.63 m/s2. While you are on your journey to the Moon in space, there will be no gravity, so you will have no weight. You will be weightless, hence the fun videos of astronauts floating, but you will still have the same 70 kg mass.
To review, a person or an object has a mass that can be measured in kg. A person or an object that is on any celestial body with gravity has a weight that can be measured in N. It is completely incorrect to say that a person or an object has a weight in kg. It is also confusing. Does it mean you have a mass in that number of kg? Does it mean you have a weight in that number of N, not kg? Please, don’t use kg and say weight. Mass and weight are not interchangeable. They do not mean the same thing.
Using KG for wieght makes more sense than using amp-hours for energy. Essentially all batteries do this. 9.8 m/s^2 is much more precise and common than figuring out the voltage for a battery. AA batteries are 1.5 volts, right? Nope. Disposable AA batteries are often 1.6 volts when new. Rechargable AAs claim 1.3 volts but are often 1.56 volts. And, the rechargables hold their voltage better under load than disposables. So how does one compute energy? It drives me nuts. In the case of batteries, amps is the thing that’s measured, not watts. So, amps is what is reported. In the case of mass, weight is what is measured, not mass. But the units reported are mass. Bathroom scales should report Newtons.
By the time the average American child is 10, she knows 10,000 words of vocabulary, and 10,000 rules of grammar. That’s an exception rule of grammar for every word, on average. My math SAT scores were better than my english scores. Yet, most of my engineering peers’ grammar was terrible, even for native Americans. I could type, and we had access to a very good spelling checker. So, our reports came out OK.
In the 90’s, i met a non-native English speaker who was new to the US. He asked me if Americans really have a 40,000 word vocabulary. In particular, did i have a 40,000 word vocabulary? I said no. I’m highly educated. So, here’s how you estimate your vocabulary size. Get a dictionary that boasts how many words it has. Open it to a random page, and jam your finger down on a random word. Move your finger up to the first dictionary word. Do you know it? Could you use it in a sentence? Now check the definition. If you really know it, mark one in a “known” column. If you don’t, mark one in an “unknown” column. Repeat maybe 30 times. Multiply the number of words in your dictionary by the “known” amount, and divide by the number of trials (eg: 30). That’s your vocabulary size estimate. These days, i argue with spelling checkers.
Cool idea with the dictionary. But, random word selection like that will be biased towards words with long definitions. Just to pick a nit…
Using kilograms for weight/force/tension makes no sense at all. No school of Engineering or Science in the world teaches their students that mass and weight/force have the same numerical value and the same units, period! The SI metric system clearly defines these things. And to calculate reaction loads, etc, in Newtonian mechanics using kilograms as weight/force will give you both the wrong answer and the wrong resulting units. In short, your answers will be off by an order of magnitude and you will eat up your designed-in safety factor and put the lives of people at risk. So it is a good thing that people who design bridges, airplanes, etc know better and the layman can go on thinking there is no difference and keep calling weight/force kilograms. This clearly shows the failure of the education system to teach people the difference.
You must be fun at parties, Frank. 🙂
Let me ask you a question… The last time your wife, brother, cousin or friend asked you, “Hey Frank, how much do you weigh?”, what did you say in response? You either responded, “Well, I’m currently at X pounds (or kg if you’re not in a silly non-Metric country like I am – ‘Murica!)”. There is no way that you responded with “Well, I’m currently at X newtons…”.
Oops! I meant to also say that I wasn’t advocating for the incorrect usage in any of the sciences. Rather, I was just pointing out that we ALL use mass to indicate our own “weight” or the “weight” of an object, like my television which I say, “…weighs 47 pounds”.
Should we be advocating for correcting the bad habits were use every day of our lives? Probably. However, we can’t even get people to use the Metric System which is a million times easier to use once
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Well, of course when we weigh things we are usually interested in their mass. Usually this is found by measuring the weight and assuming gravity (much like an airspeed indicator actually measures dynamic pressure, but displays an inferred airspeed based on assumptions. And an altimeter is really a barometer). So calibrating a scale in newtons is more correct but less useful. If you have a balance scale, such as used in doctor’s office, you’re more truly measuring mass by compensating for local gravity. Main point though – I totally agree it’s incorrect in a technical context to talk about ‘weight’ when you mean mass (even when the mass was obtained by weighing). For the same reason you give — using the proper term is not just correct, but it serves to assure the reader that you really understand the measurement you are describing (and, to help the reader understand when they aren’t so sure themselves).
When someone talks about putting ’55 pounds of air’ in a bicycle tire, you may know what they really mean, but it certainly doesn’t give the impression they understand the measurement.
And not only does the use of ‘pound’ for ‘pound mass’ and ‘pound force’ cause confusion, it may also make it harder for people used to ‘pounds’ to understand the difference between the two; leading in some cases to confusing and just wrong application of metric units. I used to have a Japanese car in which the oil pressure gauge was calibrated in ‘kg/cm2’, which of course is a unit of fertilizer application, not pressure. I don’t know if that was Mazda Japan (who should know better), or Mazda USA specifying how the car should be adapted for the Canadian market.
So when I see that, I think, “ok, probably they really mean kg/cm2 *(force of gravity) as converted from psi”. But if they get the units that wrong, why would I trust that they converted from psi properly? So to me it was in “pressure things”.
My favorite – I once had a torque wrench that had a scale ‘foot-pounds’ (ie.. foot-pound_force) and another scale labelled ‘Metric Kilograms’. I never even bothered trying to figure that out. I just read it as “Derp Metric! who knows probably kilograms I guess; use the foot-pounds”.
According to physics we measure mass of a body in kg and weight of a body in Newton but we measure weight of a body in kg in every day life. Why? It’s really confusing……