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!

How to know cos sin tan

Tags:
  1. Oct 19, 2015 #1
    hello everyone! I wanna know how to verify cos sin tan
    I always feel confused when i am doing the physics exercises.
    are we always use cos when it is x-axis and use sin when it is y-axis??
    I feel so confused.
     
  2. jcsd
  3. Oct 19, 2015 #2

    jedishrfu

    Staff: Mentor

  4. Oct 19, 2015 #3

    tony873004

    User Avatar
    Science Advisor
    Gold Member

    It's almost always cos for x and sin for y.
    Snell's Law is an exception. You measure your angles off the normal rather than the x-axis. In either case, its pure soh cah toh. Don't memorize it. Like jedishrfu says, "just draw it out".
     
  5. Oct 20, 2015 #4

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Except when it isn't. That's why you should memorize the definitions of sine, cosine, and tangent using a typical right triangle:

    trig_defs_1.gif
     
  6. Oct 20, 2015 #5

    rcgldr

    User Avatar
    Homework Helper

    The old way "sohcahtoa" == sin => opposite over hypotenuse, cos = adjacent over hypotenuse, tan = opposite over adjacent.
     
  7. Oct 20, 2015 #6

    davenn

    User Avatar
    Science Advisor
    Gold Member

    haven't heard that one before
     
  8. Oct 20, 2015 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Since I was at school, I have been using a good 'cheat' method to check whether I should be using sin or cos. It avoids the 'x-axis / y-axis' question. What you do is to imagine that the angle in question is nearly zero (just imagine lowering the slope or altering the direction of the string etc.). Then decide whether the effect of the force would be to be small and increasing, as you increase the angle (in which case you choose sine) OR whether the effect of the force will be big and decreasing as you increase the angle (in which case, choose cosine). Try it for a simple example where you already know the answer and you will see what I mean. I think that advice helped a number of my students, who were confused in the same way.
    Of course, there's no real substitute for drawing the proper vector diagram and really sticking to the rules you've been taught but the above method should give you confidence. There is also the problem of getting the right angled triangle the right way round (with the 90° in the right place). Just practice, is all I can tell you.
     
  9. Oct 20, 2015 #8

    Mark44

    Staff: Mentor

    It's been around as long as I can remember.
     
  10. Oct 20, 2015 #9

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    MR Worthington taught us: Some Old Hulks Carry A Huge Tub Of Ale. Could I ever forget that?
     
  11. Oct 20, 2015 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    MR Worthington taught us: Some Old Hulks Carry A Huge Tub Of Ale. Could I ever forget that?
    [Edit: Sorry about the repeat. My domestic wifi plays up sometimes.]
     
    Last edited: Oct 20, 2015
  12. Oct 20, 2015 #11
    I have very poor memory, I remember concepts / connection... so what worked for me was Remembering Sin (0) = 0 and Tan(0)=0

    So Knowing Sin 0 = 0 means it must include the Opp in the Numerator ( as the angle goes to 0 - the Opp does, and you know nothing about the Adj and Hyp) - same for Tan (0) = Sin /Cos = again Opposite must be in the numerator, and thus Sin must be in the Numerator.. I know this is weird but there is no other solution. For example if Tan (0) = Cos/ Sin - and Sin(0) ... this would be undefined, etc...

    This basic fact helped me test into Calc 1 - not having taken any Trig in HS.
     
  13. Oct 20, 2015 #12
    We learned "Oscar Had A Heap Of Apples".
    But then you have to remember that the order is sin, cos, tan.
     
  14. Oct 20, 2015 #13

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Hi Mark

    an American thing maybe ?

    like that one too :smile:


    with this topic and other basic stuff I have seen on PF over the years
    I find it a bit bewildering thinking back and realising all these things I was never taught at school :frown:

    Dave
     
  15. Oct 20, 2015 #14

    Mark44

    Staff: Mentor

    In reference to "soh cah toa"...
    It might be, but I don't know.
     
  16. Oct 22, 2015 #15
  17. Oct 26, 2015 #16
    Some Old Hags Can't Hide There Old Age.


    That's how my math teacher taught it, schools were less than PC then.
     
  18. Oct 27, 2015 #17
    At school I was taught "Peter has been here playing billiards"
     
  19. Oct 27, 2015 #18

    NascentOxygen

    User Avatar

    Staff: Mentor

    You'll ALWAYS remember that ditty?
     
  20. Oct 27, 2015 #19

    NascentOxygen

    User Avatar

    Staff: Mentor

    How does that help?
     
  21. Oct 28, 2015 #20

    epenguin

    User Avatar
    Homework Helper
    Gold Member

    How do any of them help? Mystified.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: How to know cos sin tan
  1. Sin cos tan (Replies: 16)

  2. Sin/cos/tan of theta (Replies: 8)

  3. Sin, Cos, Tan (Replies: 2)

  4. Sin/Cos/Tan question (Replies: 4)

  5. Sin/cos/tan by hand? (Replies: 2)

Loading...