1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does this pic makes sense? (Trigonometry)

  1. Sep 24, 2010 #1

    Femme_physics

    User Avatar
    Gold Member

    Does this pic make sense? If pi is equal to 3.14, how come it seems drawn at where 1 supposed to be?

    [PLAIN]http://img210.imageshack.us/img210/6190/sine.jpg [Broken]
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 24, 2010 #2

    CRGreathouse

    User Avatar
    Science Advisor
    Homework Helper

    The 1 is on the y-axis, where the value is... umm... 1. The multiples of pi are on the x-axis.
     
  4. Sep 24, 2010 #3

    Femme_physics

    User Avatar
    Gold Member


    I know, but if you look at it carefully you can see that the "1" on the y-axis is way longer than the pi on the "x-axis"...how does that make sense if pi is equal to 3.14?
     
  5. Sep 24, 2010 #4

    Pengwuino

    User Avatar
    Gold Member

    You can scale a graph physically however you want. Your "ticks", as I call them, can be spaced out however you want, as long as it is uniform on the axis. For example, lets say instead of graphing y = sin(x), you graphed y = 10000sin(x). If you actually drew your graph so that the length along the x-axis from 0 to pi matched the y-axis as you want, your graph would be so tall as to lose all usefulness.

    edit: I say they have to be uniform on the axis... but that's not entirely true but for your purposes they do.
     
    Last edited: Sep 24, 2010
  6. Sep 24, 2010 #5

    Femme_physics

    User Avatar
    Gold Member

    I think you're wrong, Peng.

    Are you saying that the X-plane and the Y-plane are simply not symmetrically aligned in this case?
     
  7. Sep 24, 2010 #6

    Pengwuino

    User Avatar
    Gold Member

    If you mean they are not similar in the length scale, obvious they are not. And there is no need for them to be. Like I said, if you plotted something like 1000sin(x), would it make sense to allocate the same lengths per tick for the x and y axis?
     
  8. Sep 24, 2010 #7
    Dory the quantities on the x axis are different to those on the y axis.The x axis is giving angles in radians and the y axis is giving the sines of those angles.They are two different things so how can you compare the numerical values of them?It's rather like me saying that four seconds is bigger than three metres.It's a meaningless comparison.
     
  9. Sep 24, 2010 #8

    Femme_physics

    User Avatar
    Gold Member

    Ah...getting the hang of it. Thanks.
     
  10. Sep 24, 2010 #9

    Mentallic

    User Avatar
    Homework Helper

    We shouldn't be worrying ourselves about the units that correspond with the graph. We could just as well be plotting something hypothetically numerical that has no physical significance. Pengwuino has already answered the question appropriately.

    Another example, if we plotted y=x it will only look like there is a 45o angle between the line and the x-axis if the unit lengths on the x and y axis are spaced equally apart. If we had 1cm=1 on the x axis but 1cm=10 on the y axis then it will look like y=x/10 instead of y=x.
     
  11. Sep 24, 2010 #10
    Yes of course you can choose any scale you wish but also it is meaningless to make numerical comparisons between quantities that are different.The graph in question is physically significant,both quantities are unitless but they are different things.
     
  12. Sep 25, 2010 #11

    Mentallic

    User Avatar
    Homework Helper

    So you're saying that every function we plot needs to have some physical significance? We can't just find the relationship between two numbers by plotting an input and its appropriate output defined by some function?
     
  13. Sep 25, 2010 #12
    No I wasn't referring to "every function" I was referring to the function introduced by the op which is physically significant.
     
  14. Sep 25, 2010 #13

    Mentallic

    User Avatar
    Homework Helper

    Right, so if I showed you a quadratic then you would agree that the numbers can just be a relationship between each other. If I then changed my story around and told you this quadratic describes the motion of a projectile then everything suddenly changes? No, it has no reason to.

    We are describing the relationship between the quantities on the x and y axis as nothing more than numerical values. No matter what the units are, we are only comparing the relationship of the numbers. This isn't the reason why we change the scale of each axis, pengwuino has given the correct answer.
     
  15. Sep 25, 2010 #14
    You "are only comparing the relationship of the numbers"? .It is not valid to change the original question and consider the numbers only and ignore what those numbers stand for?The answers we give here should refer to the graph that has been presented which has one set of numbers referring to angles and the second set referring to sines of those angles,two different things.
     
    Last edited: Sep 25, 2010
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook