Results 1 to 6 of 6

Math Help - long compass (arc segment drawing)

  1. #1
    Newbie
    Joined
    Oct 2008
    Posts
    2

    long compass (arc segment drawing)

    Hello
    i was wondering if you geometry gurus can help me out

    if you look at this
    Long compass

    can you tell me if there is name for this
    and can you also help me to formulate a formula to allow me to generate the radius from the angle of the two sticks and the distance of the pins

    i am hoping to use this type of movement to cut large radius in plywood
    say from 8ft to 75 ft radius over the long side of a sheet of ply

    we build concrete skateparks and use large radiuses all over the place

    thanks
    Bryant
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Super Member
    Joined
    Nov 2007
    From
    Trumbull Ct
    Posts
    910
    Thanks
    27

    long compass

    posted by junkdog

    I have never heard of this but in reading the description it is a method of drawing arcs of large dia circles given three points on the arc.A large arc can also be drawn using a long string of desired radius. Given an arc of unknown radius mark three points equidistant apart and connect the points. Then construct the perpendicular bisectors of each chord. these two lines meet at the center of the circle. Measure radius. Or if the angle between the chords can be determined then the radius can be calculated


    bj
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Super Member
    earboth's Avatar
    Joined
    Jan 2006
    From
    Germany
    Posts
    5,830
    Thanks
    123
    Quote Originally Posted by junkdog View Post
    Hello
    i was wondering if you geometry gurus can help me out

    if you look at this
    Long compass

    can you tell me if there is name for this

    ...
    Have a look here: Circle - Wikipedia, the free encyclopedia and then scroll down the page until inscribed angles.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Super Member
    earboth's Avatar
    Joined
    Jan 2006
    From
    Germany
    Posts
    5,830
    Thanks
    123
    Quote Originally Posted by junkdog View Post
    Hello
    ...
    and can you also help me to formulate a formula to allow me to generate the radius from the angle of the two sticks and the distance of the pins

    ...
    I've attached a sketch of the situation: The 2 sticks are drawn in green, the angle formed by the 2 sticks is called \alpha, the distance between the pins is d and the radius is called r and is drawn in red.

    The perpendicular bisector of d, half of d and r form a right triangle.

    Since \sin(\tau)=\sin(180^\circ - \tau) you'll get

    \dfrac{\dfrac12 d}{r} =  \sin(\alpha)~\implies~\boxed{r = \dfrac{\dfrac12 d}{\sin(\alpha)}}
    Attached Thumbnails Attached Thumbnails long compass  (arc segment drawing)-longcirc_rad.png  
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Newbie
    Joined
    Oct 2008
    Posts
    2
    thats exactly what im looking for


    if i rearrange your formula to

    angle = asin*(1/2d)/r

    would that be correct according to "math rules"
    this works for me if i do the calculations im just not sure its correct
    been a long time in doing this kind of stuff for me

    this gives me a couple of ways of checking my geometry in a spread sheet so i can see if my machine will fit inside my shop as well possible work in a portable unit

    also what program did you use to create that formula and nice line drawings from not saying the program did the work it just looks nice


    thanks for your time eathboth

    Bryant
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Super Member
    earboth's Avatar
    Joined
    Jan 2006
    From
    Germany
    Posts
    5,830
    Thanks
    123
    Quote Originally Posted by junkdog View Post
    thats exactly what im looking for


    if i rearrange your formula to

    angle = asin((1/2d)/r) You have to use brackets!

    ...

    also what program did you use to create that formula

    have a look here: http://www.mathhelpforum.com/math-he...-tutorial.html

    and nice line drawings from not saying the program did the work it just looks nice


    thanks for your time eathboth

    Bryant

    ...
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. compass question
    Posted in the Geometry Forum
    Replies: 2
    Last Post: February 12th 2010, 10:45 AM
  2. Suspended compass
    Posted in the Geometry Forum
    Replies: 1
    Last Post: January 1st 2010, 03:45 AM
  3. compass directions
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: September 3rd 2008, 09:31 AM
  4. COMPASS trigonometry question
    Posted in the Trigonometry Forum
    Replies: 3
    Last Post: July 31st 2008, 08:12 AM
  5. COMPASS trigonometry question
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: July 31st 2008, 06:35 AM

Search Tags


/mathhelpforum @mathhelpforum