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Physics problems and units


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I just gave my 16yo son the lecture of his life about always including the units when doing physics problems. I don't just mean including the units in the answer, I mean carefully carrying them through all the intermediate steps. If nothing else, it seems to me that this is good housekeeping.

 

So, physics gurus, am I being unreasonable?

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In my chemistry class, I encourage my students to write out their problems with the units and to work them out that way.

 

It's a little more difficult to do that with physics. If you are doing problems with power and work, the symbol for power is P, the SI unit for power is W (watts), and the symbol for work is W. It's awfully easy to get confused between the symbols and the units because you are often working problems that have units that look identical to a symbol.

 

For my physics classes, I have my students write down everything they start with and then convert everything to compatible units. THEN I have them write out the problem in symbols and THEN I have them plug in their values.

 

If your student can manage to keep his symbols and units straight, then putting units in the problem is a good idea.

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Carrying the units through the calculation is absolutely vital!

1. Unit analysis can help identify errors. If you calculate a force and your unit comes out in Nm instead of N, you know that there has to have been a mistake and can go back and search for it.

2. Units with prefixes, which occur especially in electricity and magnetism, pose a big danger for a student who is dropping units. If your charge is 1nC that means it is 10^-9 Coulombs - a student in the habit of dropping units is often not only losing the Coulombs, but the nano along with it - and the answer is off by 9 orders of magnitude: a factor of 1000 million.

 

If you feel able to stretch your student a bit further: teach him to solve problems in symbols completely before putting numbers in. It is the cleanest technique, causes the least errors and enables a better analysis of the answer. Solve for the desired quantity in symbols, and only in the last step put in numbers for quantities - with their units.

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So, physics gurus, am I being unreasonable?

 

 

Absolutely not! It's all rather meaningless without the units...you might as well just be doing math.

 

Tell him to be thankful that his parents aren't physicists. I've been insisting on dimensional analysis with every word problem since about the 5th grade :-)

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