we need a little help figuring out conversion factors in pre-algebra

[jenniferlee]

Hi there,

My oldest is going through life of fred pre algebra.

She's doing great with it over all. However, neither of us understand how to do problems that require using conversion factors. Like how to set up the problem properly. She just usually figures it out in her head, as do I.

So we're wondering if there is a resource that would explain this in more detail. and maybe a bunch of practice problems to nail it down.

 

thanks again

 

Jen

[regentrude]

I do not have a source for you, but will try to give a very simple explanation - because it really is not difficult once you understood what you are doing.

 

A conversion factor is basically a big fat factor of ONE with which you multiply your quantity to obtain the SAME quantity, just with a different unit. You need to set up your factor as a ratio, involving the unit you have, and the unit you wish to convert to

So to give a simple example: you want to convert 2 hours into minutes.

You know that 1 hour=60 minutes, so your factor of one is the ratio 60minutes/1 hour. Make sure you use the ration the right way round, so that your wanted unit ends up on top and your unwanted unit on the bottom:

 

2 hours = 2 hours * (60 minutes/1hour) the 60minutes/1hour is your factor of one - 60 minutes and 1 hour is exactly the same.

 

Now you have

2 hour * 60 minutes / 1 hour

 

You see that hours in the denominator and numerator cancel, but minutes remain:

 

2*60 minutes

120 minutes

 

Of course this simple example you can do in your head, but now lets to something a bit more complicated:

 

Convert 2 days into seconds

We have several conversion factors, from days to hours, hours to minutes, minutes to seconds: EACH of which is a ratio equivalent to ONE:

 

2 days = 2 days * ( 24 hours/1day) *(60 minutes/1 hour) *(60 seconds/1 minute)

 

Make sure you have written the conversion factors the right way round, so that all the unwanted units cancel. if you cancel the days, hours, minutes, only seconds remain and you obtain

 

2*24*60*60 seconds (and can calculate a number - i don't have my calculator here)

 

Does that help?

[jenniferlee]

yes, actually that does help. I was not undertanding where to put the different items so that the cancelling worked out. Also the factor of one thing...

thanks! we'll work on it.

 

I'd still love a place where we could get some practice problems for this.

[Capt_Uhura]

I think the CPO Investigations manual which is free online had practice problems as well as an explanation for doing dimensional analysis.

[Brenda in FL]

yes, actually that does help. I was not undertanding where to put the different items so that the cancelling worked out. Also the factor of one thing...

thanks! we'll work on it.

 

I'd still love a place where we could get some practice problems for this.

 

Have you checked out Khan Academy? Just google for the link. And search the boards for more info.

[jenniferlee]

We tried khan academy and couldn't find what we were looking for. Maybe we didn't know how to word it?

I showed my daughter the explanation that regentrude posted and she had a big aha moment! So thanks again.

I'll check the cpo.

thanks again everyone

[Dana]

You can also look under "dimensional analysis".

[kiana]

Search Khan Academy for unit conversion for more explanations.

 

When you search for practice sheets, dimensional analysis or unit conversion are both useful.

[Mom22ns]

What about this one at Khan academy?

http://www.khanacademy.org/video/unit-conversion-with-fractions?playlist=Developmental%20Math

[jenniferlee]

that's a great video! thanks

I just didn't know what to search for.

[jenniferlee]

Ok, I searched for dimensional analysis practice problems and came up with this awesome site:

http://www.alysion.org/dimensional/fun.htm

 

thanks again everyone!