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

Sin(theta) = dy/dx ?

  1. Jun 28, 2013 #1
    In preparing for an acoustics course, I ran across the following sentence which confused me:

    "If (theta) is small, sin(theta) may be replaced by [partial]dy/dx."

    I expected to see sin(theta) = (theta) so this threw me off. This came up in the derevation of the one dimensional wave equation after approximating (by Taylor series) the transverse force on a mass element of a tensioned string with [partial]d(Tsin(theta))/dx. The approximation in question thus gave T*([partial]d2y/dx2)*dx.

    In the original setup, x and y are cartesian axis in physical 2D space and (theta) is the angle the string (with tension T) makes from the x-axis after displacement from equalibrium.

    I've never seen sine approximated by dy/dx before and was hoping somebody might shed some light for me :)
     
  2. jcsd
  3. Jun 28, 2013 #2

    LCKurtz

    User Avatar
    Homework Helper
    Gold Member

    When ##\theta## is small, ##sin\theta \approx \tan\theta = \frac{dy}{dx}##. In the derivation for the vibrating string, ##\theta## is the slope angle of the string.
     
  4. Jun 29, 2013 #3
    Thank you! :)
     
  5. Jun 29, 2013 #4

    rcgldr

    User Avatar
    Homework Helper

    sin(θ) = Δy / sqrt(Δy^2 + Δx^2). For small θ, Δy is small compared to Δx, so

    sin(θ) ≈ Δy / sqrt(0 + Δx^2) = Δy / Δx
     
    Last edited: Jun 29, 2013
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted
Similar Discussions: Sin(theta) = dy/dx ?
  1. A question about dy/dx (Replies: 9)

  2. Dx/dy vs dy/dx (Replies: 8)

  3. Question about dy/dx (Replies: 4)

Loading...