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Homework Help: Approximation of small angles

  1. Aug 27, 2010 #1
    Hi! I have a question about approximation of functions with small angles. I was looking through some notes from my teacher and didnt understand why the following approximation is valid. We have a system which is at equlibrium at an angle, say a. Now we wanted to se what happens with the equilibrim if we deviate from a, with a small angle say, n. All the calculations isnt necessary here so i cut them out.

    He chooses to write cos(a+n) = cosa * cos n - sina * sin n, just by expanding.

    However for sin(a + n) he just writes sin(a+n) = sin(a)

    Can you please validate this and tell me why this is ok to do? I mean why we dont expand sin(a+n) in the same manner?
  2. jcsd
  3. Aug 27, 2010 #2
    If you check it and expand it yourself you get:


    if N is small, Sin(n) is insignificant. lets just say close enough to 0. and using the same principle, if N is small, Cos(n) is close enough to 1. Think of the sin and cos curves for small values. This leaves you with;

    sin(a+n)=sin(a)1+0cos(a) which therefore approximates to Sin(a) as stated.

    All he has done is missed out the calculation because he knows something about the values of N for sin and cos when they are small.

    Btw this should be in the maths section not the physics.

    Hope this helped.

  4. Aug 27, 2010 #3
    I'm not sure this expansion is correct. If you want to expand for small n you have to keep all term that are linear in n. In the case of the sine this would be
    [tex]\sin(a+n)=\sin(a)\cos(n)+\sin(n)\cos(a)\approx \sin(a) + n \cos(a)+ \mathcal{O}(n^2)[/tex]

    for the cosine it would be
    [tex]\cos(a+n)=\cos(a)\cos(n)-\sin(a)\sin(n) \approx \cos(a) - n \sin(a)+ \mathcal{O}(n^2)[/tex]

    It is inconsistent to keep the linear term for the epxansion of the cosine but not the sine.
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