The stuff physics textbooks leave out...

Discussion in 'Physics' started by nsaspook, Sep 8, 2016.

  1. nsaspook

    Thread Starter AAC Fanatic!

    Aug 27, 2009
  2. crutschow


    Mar 14, 2008
    A video of a shell flying through the air??
  3. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    I like the shock wave.
  4. wayneh


    Sep 9, 2010
    How do you keep the camera trained on a projectile like that? Some of the railgun videos show the lens effect of the shock wave. Very cool.
  5. nsaspook

    Thread Starter AAC Fanatic!

    Aug 27, 2009
    Ok, something for you.
    Nihal goel and joeyd999 like this.
  6. MrAl

    Distinguished Member

    Jun 17, 2014

    Somebody playing with their new high speed camera? :)

    You can stop the string in mid action with a white LED and microcontroller. It looks like it is stopped dead even when vibrating rapidly. You can see the effect of the often neglected slight stretch of the string when it has reached it's outer most position because the vibration frequency is slightly higher due to slightly more stretching.
    The uC pulse is set at the same frequency as the open string should be, then the string tension adjusted as usual when tuning. As the frequency is adjusted closer and closer, the string looks like it comes to a dead stop even though still vibrating. So it's just a basic strobe light tuned in time with the string. It can be used as a precise tuner for an instrument because the strobe frequency can be set very precisely.
    As the string is plucked hard however, the string still appears to vibrate a little until the frequency comes back down to the exact tuning frequency of the strobe. In most simple problems this effect is neglected for simplicity.
    The code for the uC is very very simple because all it has to do is fire the LED once per half cycle, with the frequency set to the desired frequency of the open string. If you have a development platform set up it takes about 5 to 10 minutes to program and 5 minutes or less to wire up with jumpers.

    Also interesting, in that video we can see the exponential part of the response which is also usually neglected in simple problems with vibrating strings. The more accurate description would come from a more complicated PDE than the usual two term wave equation.
    Last edited: Sep 11, 2016