Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Radians - why do we use these instead of decimal?

  1. Aug 2, 2006 #1
    I'm basically homeschooled in math so please forgive me if this question seems simple, but I have never been able to find a book that shows how radians are calculated. Through inspection I've sorta determined 45 degrees = pie/4, 60 degrees = pie/3, 90 degrees = pie/2, 180 = pie and so forth but I input pie into my calculator and change to degrees and it returns 200?

    Is there a formula I can use to calculate radians? There is no way I can commit the tables to memory... :frown:

    I can understand why before calculators, radians were easier to use, is there and reason why we continue to use radians over decimal today? Maybe if I understood them their continued use would make more sense to me.

    Many thanks!
     
  2. jcsd
  3. Aug 2, 2006 #2
    degree*PI/180,higher calculus they use radians...and trig functions in programming use it.
     
  4. Aug 2, 2006 #3
    The formulas you worked out are correct, and can be generalized as mentioned to be radians = degrees * pi/180.

    There is a very good reason to use radians instead of any other system you can think of. For one thing, all other systems are arbitrary, radians is based on mathematical fact. The length of a curve traced along a circle of radius R for an angle theta = R*theta, if and only if theta is measured in radians.

    Mathematical functions like sine and cosine for angles have series expansions. sin x = 0 where x = n*pi, not n*180. Those expansions are Taylor expansions, mathematically solid, not arbitrary.

    Personally, I hate any system other than radians. Radians are natural units, degrees are man-made.
     
  5. Aug 2, 2006 #4
    Really you shouldn't even think of a "radian" as a unit. When you say that an angle is [itex]\theta[/itex] (in radians), you just mean that if you rotate a unit vector by that much, the length of the arc its tip traces out is [itex]\theta[/itex]. So it's really dimensionless. We call them "angles" because we want to identify them as something special having to do with rotations. You'll notice that this is really the only place people use "synthetic" units in mathematics, and it's annoying!

    Degrees are an arbitrary unit. I am not sure why people insist on continuing to use them. Radian measure arises naturally.

    As BoTemp mentioned,

    [tex]\theta = \left(\frac{180\theta}{\pi}\right)^\circ[/tex]

    (where [itex]\theta[/itex] is the natural angle measure in radians).

    If you calculator told you that [itex]\pi = 200^\circ[/itex], then your calculator is broken :tongue2:. It is more likely, however, that you actually accidentally converted to "gradians," for which there are 400 in a circle. This is just another arbitrary unit. The conversion there is

    [tex] \theta = \frac{200 \theta}{\pi} \mbox{grad},[/tex]

    or from degrees to gradians,

    [tex]x^\circ = \frac{200 x}{180}\mbox{grad}.[/tex]


    Actually it's quite the opposite, the degree measure was first used by the Babylonians thousands of years ago. From Wikipedia:

    On the other hand, radians (or, at least, the natural angle measure, if not by that name) have only been used for a few hundred years. All mathematicians use them now, because they arise so naturally and make a lot of things more convenient.
     
    Last edited: Aug 2, 2006
  6. Aug 2, 2006 #5
    Oh, and by the way, [itex]\pi[/itex] is "pi." :rolleyes:
     
  7. Aug 2, 2006 #6

    Office_Shredder

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    360 was probably also used because of how divisible it is: into halves, thirds, fourths, fifths, sixths, etc.
     
  8. Aug 2, 2006 #7

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Another point is this: if x is in radians, then the derivative of sin(x) is cos(x). If in degrees (I don't understand why you refer to degrees as "decimal") then the derivative of sin(x) is [itex]\frac{180}{\pi}cos(x)[/itex].
     
  9. Aug 2, 2006 #8
    Maybe he doesn't realize that radians don't have to be written out as multiples of [itex]\pi[/itex] (so for example [itex]1 = \left(\frac{180}{\pi}\right)^\circ[/itex], or [itex]60^\circ \approx 1.0472.[/itex]).
     
    Last edited: Aug 2, 2006
  10. Aug 3, 2006 #9

    mathwonk

    User Avatar
    Science Advisor
    Homework Helper
    2015 Award

    we use radians in calculus to simplify the answers, i.e. in radians, sin' = cos, but not in degrees. (there would be a constant factor in there, maybe of 2pi/360?.)


    this is the same reason we use e^x instead of 10^x as basic exponential function.

    i.e. d/dx e^x = e^x but d/dx 10^x = 10^x ln(10), where ln is inverse to the function e^x.

    this is also the same reason physicists use consistent units, i.e. one unit of mass is the mass of one unit of volume of pure water at sea level at a standard

    temperature? etc...

    one unit of volume is a cube with sides one unit of length,......

    a radian is the angle subtended by an arc whose length is one standard unit of length where the standard unit of length on a circle is of course the radius.
     
    Last edited: Aug 3, 2006
  11. Aug 3, 2006 #10

    mathwonk

    User Avatar
    Science Advisor
    Homework Helper
    2015 Award

    all those who think lecturers are useless and books hold all knowledge pay attention. of course this answer is in some books, but which ones?

    i do know some cocky students who used to go to class only once, to get the lecturers reading list.
     
  12. Aug 3, 2006 #11
    Many thanks, that helps a great deal (and for the spelling correction too). I've been up 4 days (2 days straight) learning complex numbers and polar coordinates - needless to say I'm radian-mad. I've been approaching this chapter by trying to equate the radians to decimal, I guess becuase they make me uneasy. I understand now I'm looking at them the wrong way.

    Agreed, lectures are invaluable. I'm completing this course through correspondance so I can be accepted into uni and WOW will I ever appreciate having a math prof who I can speak with. Until then this forum is a wealth of knowledge and much appreciated....
     
  13. Aug 3, 2006 #12

    mathwonk

    User Avatar
    Science Advisor
    Homework Helper
    2015 Award

    i am a math prof at uni. and we are speaking. best wishes.
     
  14. Aug 3, 2006 #13

    mathwonk

    User Avatar
    Science Advisor
    Homework Helper
    2015 Award

    by the way, simple appreciation for what you are taught, such as you have shown here, will get you plenty of willing teachers.
     
  15. Aug 4, 2006 #14
    Actually, the degree measure of angles dates back to the ancient Babylonians, who had a roughly 360-day calendar.
     
  16. Aug 4, 2006 #15

    Office_Shredder

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    http://www.physlink.com/education/askexperts/ae373.cfm

    That wasn't the sole and overbearing reason that smites all others of course :tongue2:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Radians - why do we use these instead of decimal?
Loading...