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March 1, 2015

Posted by **Lena** on Saturday, August 25, 2007 at 5:31pm.

2. How many minutes does it take light from the sun to reach Earth? (Distance from sun to Earth is 93 million mi, speed of light= 3 x 10^8 m/s) Use dimensional analysis.

Please, and thank you!

- Conversions! -
**DrBob222**, Saturday, August 25, 2007 at 5:56pmThese are easy to do if you follow through and check the units. All of them are as follows:

something x factor = another unit.

For #1.

We have gold at 200 dollars per oz. We want to convert that to dollars per gram. The problem tells us that the factor is 1 oz = 28.4 and we can write that as 1 oz/28.4 g OR 28.4 g/1 oz.

cost per gram = cost per oz x factor

cost per g = $200/oz x (1 oz/28.4 g) =??.

Note that oz in the denominator of the first term will cancel with the oz in the numerator of the second term and we are left with $/g for the cost which is what we want. We COULD have written $200/oz x (28.4 g/1 oz) but then the final answer would be in units of $*g/oz^{2}and that is NOT the unit we want. You will ALWAYS know which way to place the factor if you follow the units and make sure the final unit is the one you want. I hope this helps. I will leave #2 for you for practice.

- Conversions! -
**Anonymous**, Monday, August 27, 2012 at 10:15pm8 minutes

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