Unit Conversions

In chemistry today we started our first day of chapter three. In this section we learned about unit conversions. D Liebs gave us notes and lectured us for the majority of the class. You can find the notes on Moodle. Anyway first we discussed unit factors. A unit factor is basically a ratio. For example, if you want to say, there are 5280 feet in 1 mile, you would write it as: 5280 ft/1 mile. This is an example of a unit factor.

Here is an example of a unit conversion problem:

Convert 45 miles per hour to feet per second. Well to solve this problem first set up this fraction:

45 mi/1 hr

Now we have to multiply this unit factor by many others so that we can get the units to be feet per second. To do this, first think how many seconds are in one hour. Well there are 3600 seconds in one hour, so set up the fractions next to each other like this:

45 mi/1 hr * 1 hr/3600 sec

Note that in the first unit factor the hours is in the denominator. But in the second unit factor the hours is in the numerator. You must set up the units like this so that when you multiply across, the units will cancel out.

Now you have to cancel out the miles. But we need to think about how many feet are in one mile because we are trying to get feet/second. So now we multiply the fractions like this:

45 mi/1 hr * 1 hr/3600 sec * 5280 ft/1 mi

Remember to put the miles in the denominator, because in the first fraction the miles are in the numerator. Okay the last step is to multiply all of the numerators together and then divide that number by all of the denominators. So you do:

45 x 1 x 5280=237600
237600 / 3600= 66

Now you cancel out the units and you are left with feet in the numerator and seconds in the denominator. So the answer is 66 ft/1 sec.
After D Liebs explained the notes and such he gave us the rest of the class period to work on the worksheets with solving unit conversions. I know I was supposed to post this a little while ago but Blogger was not working. I also wanted to put some real fractions in the blog but copy and paste does not work well in Blogger. Well that's all for today, the homework was the WebAssign and complete the unit conversions worksheets.

Next scribe is................................................ Robert S.