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Changing units

Page history last edited by Joe Redish 7 years, 7 months ago

Class content >Modeling with mathematics > Using math in science > Dimensions and units


Even if you have made sense of measurement -- how we assign numbers to physical quantities, and to dimension -- how we describe the different kinds of measurements we make, you still have to figure out how to attach units to your measurements. This sounds kind of trivial.  If I measure something in centimeters and find that it is 100 of them, I just write "100 cm."  If I measure it in meters, I'll just get 1 and will write "1 m".  What's the big deal?


Well, it might seem trivial, but there are some subtleties.  I know that I can represent the length of a meterstick by reporting its length either as 100 cm and 1 m.  So - since 100cm and 1m have the same dimension, and represent the same physical system (a meterstick) I can equate them


100 cm = 1 m


This is a bit strange if you haven't learned to pay attention to the units. It sort of looks like you are saying "100 = 1" but you're not.  You're saying "100 of these things (called cm) is equal to 1 of these other things (called m)." Not the same.  You have to keep in mind that you're putting an equal sign between "things" (in this case, lengths) not simply numbers.


An equation says that the two sides are the same, so I know that if we divide both sides by the same thing I should still get the same thing.

That still seems OK.  What if I divide each side by "1 m"?  Then I get


(100 cm) / (1 m) = 1.


This turns out to be immensely useful. I know I can multiply anything by 1 and not change it.  So can I just put in factors of 100 anywhere?  The answer is, "Yes, I can -- as long as I also keep the cm/m part."  That might make things more complicated, but it also might make things easier. For example, suppose you're given a problem that has mixed units. Here's the key principle.


     To change units, multiply by 1 in the appropriate form -- taking the ratio of the same thing expressed in different ways.


As an example, to change 60 miles/hour to meters/second we can write the chain:


Note what we've done.  In the first line, each set of parentheses represents "1" -- the top and the bottom are the same physical quantity represented in different ways. And we've chosen to write them so that the unit in the top is cancelled by the one at the bottom until we get to the quantity we want. We then put all the numbers together and all the units together. We multiply the numbers on our calculator to get a final number and we cross out all the top/bottom places where the units are the same to get a final unit.


(Note the grammatical peculiarity: Because I've written the unit out instead of just using the symbol, I have to write "5280 feet" but "1 foot".  The unit is the same and we can cancel them despite the fact that they look different when written out. That's a good reason for using an abbreviation -- "s" for either "second" or "seconds". They mean the same thing here.)


Expressing things in different units is valuable because it helps us think about the same thing in different ways. For the example above, thinking that your car is going 60 miles/hour is useful if you're thinking about how long it's going to take you to drive 120 miles on the highway -- or whether you might get a speeding ticket in a 35 mile/hour zone! But the other way is important if you're thinking about how far you travel in one second -- if you're figuring out how far you might travel before you start to brake (in order to figure out when you're too close to another car on the highway).


Joe Redish 7/13/11

Wolfgang Losert 8/29/12


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