Conversions
Science brain teasers require understanding of the physical or biological world and the laws that govern it.
When you convert from an English measurement to a Metric measurement, you multiply by a number. For example, to find out how many kilograms are in 10 pounds, you multiply 10 by .454. To find out how many meters are in 10 yards, you multiply 10 by .914. This is the procedure for every conversion except for one. When you convert from Fahrenheit to Celsius, you subtract 32 and multiply by 1/9.
Why does the temperature conversion require a subtraction and a multiplication, while all the other conversions require just a multiplication?
HintCompare the zeros.
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Answer
For distance, weight, and every other measure, a result of zero is the same no matter what the units are. The temperature scales, however, have arbitrary zeros. This means that 0 degrees Celsius and 0 degrees Fahrenheit are not the same temperature.
(As an interesting side note, the Kelvin and Rankine scales both have their 0 degrees set to Absolute Zero, so you can convert from one to the other with a simple multiplication.)
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Comments
unklemyke
Oct 11, 2010
 Well, actually, you subtract because Celsius sets the freezing point of water as 0º and Fahrenheit sets it at 32º. likewise, the boiling point of water is 100º C, 212º F.
The 180 point difference in F as opposed to the 100 point difference in C yeilds the constant  and it's 5/9 from F to C, not 1/9. Conversely, you multiply by 9/5 going form C to F. 
Nerine
Jan 06, 2011
 Huh, it's quite obvious when you knoew the answer 
AndrewWalker
Jan 31, 2012
 I didn't get the exact answer, but I do know, if you graph the resulting conversions as linear functions, each line has a unique slope. This means there is a nonlinear relationship between the two conversions. Also the two lines intersect at the point (40,40) so 40 degrees F= 40 degrees C 
eighsse
Jul 28, 2013
 If you have a decent understanding of the temperature scales, this isn't much of a teaser, just asking an everyday question. But hey, not everyone does have a good understanding of it, so I'm sure some people learned something here 
Dezaxa
Dec 23, 2014
 An interesting corollary is to ask, Why did Dr Fahrenheit set the zero on his scale where he did? Several answers have been suggested, but the most plausible seems to be that he was an experimental chemist and he set his zero to be the coldest termperature that he could conveniently make in his laboratory. Since he was working before modern methods of refrigeration, this was achieved by mixing ice and salt. 
The_God_of_Pies
Jan 08, 2015
 I asked my physics teacher a teaser like this before, it was amusing to see him getting frustrated after I told him that the answer was simple, it still took him ten minutes to figure out and then he was embarrassed he didn't get it sooner. Good times. 
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