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

Why does dθ = ds/r

  1. Nov 26, 2013 #1
    I know how this formula is made, but surely Δs becomes highly inaccurate for large values of Δθ and r?

    In fact when Δθ is ∏ surely Δθ ≠ Δs/r.

    Elucidation would very appreciated.
  2. jcsd
  3. Nov 26, 2013 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    No, if "s" is the arc length on a circle, and the angle is measured in radians, then the relations holds exactly for ALL values.
  4. Nov 26, 2013 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It holds even for θ = 2∏!
  5. Nov 26, 2013 #4
    I know it holds for arc length but it's presenting that 's' as displacement, it's even linked linear velocity to this rule.

    v = rω.....how???

    v = rω can't hold for 2∏ as displacement is 0...
  6. Nov 26, 2013 #5


    User Avatar
    Science Advisor
    Gold Member

    That's why the correct formula uses "d" not "Δ".

    This formula doesn't have displacement in it.
    Last edited: Nov 26, 2013
  7. Nov 26, 2013 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    [itex]Because \ v = lim_{t \rightarrow 0}\frac{ds}{dt} = lim_{t \rightarrow 0}r\frac{dθ}{dt}[/itex]
  8. Nov 26, 2013 #7


    User Avatar

    Staff: Mentor

    Even if ##s## is not the arc length, but the straight-line displacement between two points, ##ds=\frac{d\theta}{r}## is still exact because it's stated in terms of infinitesimals. If you do the line integral along the arc, you'll get ##\Delta{S}=\frac{\Delta{\theta}}{r}## where ##\Delta{S}## is the length along the arc.
  9. Nov 26, 2013 #8
    I just used "d" because i didn't know how to use the symbol Δ in the title.

    v = rω is concluded from avg(v)= r(avg)ω which is derived from using Δs/Δt = rΔθ/Δt

    Yes to all posters talking about t approaching 0 and infinitesimals I agree it works there, but when t is large doesn't the equation become inaccurate?
  10. Nov 26, 2013 #9
    Ah, I said when ds is very big and dθ when i should have said dt.
  11. Nov 26, 2013 #10


    Staff: Mentor

    Well, the formula in the title is incomplete.

    The exact formula for distance in a plane is given by the Pathagorean theorem:

    The exact transformation to polar coordinates is
    x = r cos(θ)
    y = r sin(θ)

    Substituting the exact transformation into the exact distance you get exactly
    ds² = dr² + r² dθ²

    Which is the complete formula. Of course, if you want to calculate a large distance then you have to integrate.

    In the special case that dr=0 then you get your formula
    dθ = ds/r
  12. Nov 26, 2013 #11
    If Δs is a displacement vector (i.e., relative position vector) between two points on a circle, then the equation only applies infinitecimally. If Δs is the arc length between two points on the circle, then it works for all values of Δθ. Did you mean to imply that Δs is supposed to be a relative position vector?
  13. Nov 26, 2013 #12


    Staff: Mentor

    FieldvForce, if Chestermiller is correct then you should be aware that a position vector is a path where dr≠0, so you cannot use your incomplete formula. You would need to use the full formula instead. In that sense, it would be inaccurate since you are using a simplified formula in a circumstance which violates the assumption on which the simplification is based.
  14. Nov 26, 2013 #13
    Thank you very much for clearing things up.
  15. Nov 26, 2013 #14


    Staff: Mentor

    You are welcome! :smile:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Why does dθ = ds/r
  1. Torque: Why F*r? (Replies: 8)

  2. Why r^2? (Replies: 16)