﻿ Mr. Dawson, why is gravity different in math than in physics? | ZDNet

# Mr. Dawson, why is gravity different in math than in physics?

Summary: I was actually asked this question yesterday by one of my physics students. He was surprised to see a figure of 32 in his pre-calculus book associated with the acceleration due to gravity on earth, rather than the 9.

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TOPICS: Nasa / Space
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I was actually asked this question yesterday by one of my physics students. He was surprised to see a figure of 32 in his pre-calculus book associated with the acceleration due to gravity on earth, rather than the 9.8 we had been discussing in class. Of course, he missed the units (feet/second^2 for the first, versus meters/second^2 in our physics class). However, it begs the question, why would we possibly be talking about scientific applications of math in English units of measure?

Want to know what happens when scientists use US customary measures? They blow up multi-million dollar spacecraft.

I know that here in the States, we're the last holdouts in the world still using units derived from Imperial measures. To the rest of the world, I apologize for our arrogance. However, I just can't understand why a math textbook would still be talking about trajectories and freefall would fail to use the metric system. We're not talking about a problem where little Johnny needs to find the area of his 3 foot by 4 foot sandbox. We're talking about the acceleration of gravity in a high school mathematics textbook. It's not even that old (although since the rest of the world had largely adopted the metric system by the 70's, it would have to be really old for this to be excusable).

My students know that the metric system (or its lack of use here in the States) is a remarkable pet peeve of mine. However, I'm a physics and math teacher; I think I'm allowed to get irritated about this. I'm educating future scientists, engineers, astronauts, and citizens in a global economy. If one of them blows up another multi-million dollar spacecraft because he or she is thinking in pounds, I'm moving to Canada, where they have the sense to think in kilograms like the rest of the world.

[poll id=41]

Topic: Nasa / Space

Chris Dawson is a freelance writer, consultant, and policy advocate with 20 years of experience in education, technology, and the intersection of the two.

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## Talkback

• ### kepp it all

I'm speeking as a pure product of France scientific education, where we do not even realise some people might not be using metric system before the time we see there is a real world out there with people having habits different from ours.

AT first I just could understand we people would even be willing to use imperial unit system. Each time i encounter a basket ball player profile, I have to jump to a calculator to convert his weird height in something that make sense to me, i-e meters and centimeters. Hopefully I found out google convert units pretty well.

From my experience as a pupil then student, and then occasionnal home teacher, I can assure you that one of the most important problem facing children, especially in pysics, maths being a fabulous unit less world, is the hability to cope with different units. I m not talking of magnitude here, nm and km are the same units to me. I'm talking real unit, m vs. light year, Pa vs. bar vs. Psi.

working with different unit systems is a training of its own. Had the people you talked about with the Mars mission be more aware of the possibility that they might encounter different unit systems, rather than assume only one world exist, they would probably not have had the problems they had.

Keep your textbook, and use it to stress th existence of different system, and teach them that knowing a figrue without knowing the units it is expressed on is not only useless but dangerous
• ### The USA does not use the imperial system

It makes me laugh when you guys from the USA talk about yourselves using the imperial system, when you don't.
The imperial system was first defined in the British Weights and Measures Act of 1824.
The USA use standard or English units, note that both names do not contain the word "imperial" and the correct term for the USA system is "US customary measurement systems"
Both the USA and the UK used the English system from the 1700's and in 1824 the UK changed their system to the imperial system, while the USA kept the old English system.
In 1958, the U.S. and the British yard were defined identically, this does not mean the USA can claim the rights to the use of the imperial system.
Your gallon is still 20% smaller than the imperial Gallon at
1 imperial gallon = 4.546 09 litres (covers both liquid and dry)
1 liquid US gallon = 3.785 411 784 litres
1 dry US gallon = 4.404 842 803 2 litres

So leave us British with something of our own and use the correct term for your measurements the "US customary measurement system"
Sammie
• ### This brings up

This brings up the "We're the last holdout" comment from the story. I'm an American with a scientific background whose been living abroad for about eight years. I use the metric system every day.

Still, I'm often hassled by Brits and Canadians for being part of the holdout group. Ironically, when we discuss stuff, these people will invariably stop me and ask something like "What's that in inches?" or "What's that in stone?"

My point is that many countries which officially use the metric system have populations which don't.
• ### Canada is still also a hold out

Canada actually has three units of weights and measures under its weights and measures act. Firstly, because the ton is 2000lb and not 2240lb (the "Imperial ton") Canada uses US customary, not "Imperial" except for the size of the gallon.
However, Canada also supports a French-derived system in parts of Quebec, where the French Foot is a little bit larger than the English foot, and a type of equivalent to the "rod", the "arpent" is used for both linear measure and measure of area.

When one goes to the supermarket, prices are quoted in \$/lb, but the receipt is always in \$/kg. The move towards metrication in Canada came to a complete halt in about 1981.
Being both Australian and Canadian, I saw that Australia completed its path to metrication, completely, in 1984. Canada has been frozen in the same state that Australia was since 1981. Truly amazing.
In Australia, customary lengths were rounded, to avoid "peculiar" measures - so, the standard house frame size of 3 feet was converted to exactly 900mm, not 915mm, so that things were easy to remember.
The Canadian and US house framing size, 4 feet, would convert to 1200mm, if things were done properly. Repairs of old houses do not pose too much difficulty in Australia.
I notice that in the CSA for electrical standards, imperial distances are converted exactly, rather than rounded, so we see 610mm for 2 feet, rather than a rounded 600mm. Whilst ever this persists, metric will be seen as an inferior standard.

In Australia, with US Free Trade, there has been "reverse metrication". For a while, containers were metric, e.g. 125g, 250g, 500g, 1kg. However, with the "one size fits all" approach, and "free trade", imperial sizes converted to metric are appearing, so now we get margarine in 908g tubs (2lb) and so on.
Also, at Starbucks, when you ask for a "Venti" (20 fluid oz coffee) is that a Canadian Fluid Oz, or US? (Also, many pubs are advertising "pints", but a check showed that this was a US pint of 16 US fluid oz in Canada - I am seeking legal advice as to whether there is a breach of the Canadian weights and measures act here)
• ### Your Venti is 20 US fluid ozs.

The imperial measure for fluid ounces was dropped in Canada years before the switch to metric. Quarts became 40 oz. and gallons 160 oz. This was done so consumer products could be labeled with the same ounces for both countries.
• ### Canada is still also a hold out - NOT

I don't know what part of Canada you live in but nobody except old people (over 60) use imperial measurements. Last month at a super market some old geezer asked the deli kid for a half-pound of cheese and it caused all kinds of confusion until I leaned over and told the kid that 450 grams would be close enough. With regards to tons, no business worth its salt would order a ton on anything without knowing what kind of ton was being quoted. So its usually metric tons (1000 Kg) when dealing with Europe and 2200 pounds when dealing with the yanks. But it is always better to ask ahead of time.

n/t
• ### Finally - voice of reason

There's not a single rational reason to use US/Imperial units today. Even the lenght of yard is officially defined in meters, pound is defined in grams.

Put the pathetic emotions aside and go metric - think of it, folks, there are just three non-metric countries - USA, Liberia and Myanmar. Excellent!
Britain may display miles on roadsigns but their placement at the junctions is planned in metric units.
• ### UK is still Imperial at heart.

In the UK, there is broad, yet totally irrational, indignation that the EU would dare to undermine "britishness" by imposing forced use of the metric system. If the metric system was seen as ruthlessly efficient product of the German engineering mind, it might be more acceptable, but, it turns out its comes from that most unholy place, France. The Brits didn't kick the cr*p out of Napoleon and rescue the Frogs in two world wars just to have their pints taken off them!

The fact that nations of people place a unit system within their concept of self identity is amazing. Thankfully, Darwin wasn't french.
• ### Indeed and...

...to borrow one of your American expressions "We don't want no stinkin' Eurolooney numbers here!"

A. (in UK)
• ### UK is still Imperial at heart.

2 pints = 1 quart;
4 quarts = 1 gallon;
2 gallons = 1 peck;
4 pecks = 1 bushell.
You've got to be kidding me. And let's remember that the foot really was the size of some king's foot. Religious fundamentalism aside, I thought we were living in an age of logic and reason. Time for all wacky measuring systems to go.
• ### 3 Non-Metric Countries

Now there's a group we Americans should be proud to be a member of!!

I am american, but not what you meant by 'americans', I am from south America. America is a big continent, not 'your' country.
• ### Good point!

A friend of mine who lives in Canada corrected me on that once, and I realized how arrogant we UnitedStatians (how's that sound?) really are. Canadians are Americans and Mexicans are Americans. It just goes to show how pompous we are that we think we own the whole continent and can take it's name for our own heritage.

• ### ZDNet Censorship

words whiny and grow up are now considered horrible vicious personal attacks.
• ### Excellent point...

Thanks for the reminder! I did say we were arrogant, didn't I?

No offense intended to any non-US Americans :)

Cheers,
Chris
• ### No, it's a stupid point.

American is short for United States of America. It's an abbreviation of the country.