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Marking system for ballistics pendulum

  1. Apr 9, 2007 #1
    Marking system for ballistics pendulum?

    Does anyone have any ideas for a low resistance marking system for a ballistics pendulum?

    The professor I am working for wants me to build a ballistic pendulum (as a demonstration apparatus) for a very weak airsoft gun. He said there must be a marking system (can't just eyeball it). Even marking with charcoal or felt tip pens drastically skews the results (pendulum indicates lower velocity than it should).

    I had some crazy idea that there might be some type of exposure paper that is sensitive to red laser (laser pointer) light, which I could use for this application (resistance-free marking). But then I realized that if this were feasible, people would already be walking around with this paper taking notes with their laser pens, no? Plus I googled it (in many formats) and didn't return any such products (doesn't mean they don't exist, but...).

    I suppose the actual problem is not so much classical physics specific, but since the ballistics pendulum is a classical physics device... (let me know if I posted this in the wrong place!) Thanks!
    Last edited: Apr 9, 2007
  2. jcsd
  3. Apr 10, 2007 #2
    The classical system used in seismographs was a metallic needle over a soot covered glass plate (cylinder for seismographs). The friction force of the needle over the glass is really negligible and the soot is easy made and easily scratched.
    Less performing would be using a needle over a sand box. But Galileo used it.

    Another approach is to accept a known friction force. Your pendulum pushes a small block over a surface. When it retreats, the block rests at the furthest point. You can easily measure the coefficient of friction (dynamic) of the block over le support surface, and the mass of the block. To measure the dynamic coefficient if friction it suffices to measure the angle at which the block slides when pushed downwards.
    Of course, you must include this friction in your calculus.
    Last edited: Apr 10, 2007
  4. Apr 10, 2007 #3
    I don't really know how well this would work, but I'll put it out there anyway.

    What if you had a light powder that was visible in the air (such as baby powder), and put it in a small cavity on the opposite end that the projectile is hitting it. Hopefully The powder would not come out when it first got hit, but would be visible in the air only when it changed directions. Then, of course, you would need to eyeball where the dust cloud was (and might be hard to determine)... blah nevermind, the idea sucks.
  5. Apr 11, 2007 #4
    Thanks for the suggestions. This ballistic pendulum is going to be used for a physics 1 demonstration, so I don't think I'm allowed to factor in friction.

    Right now I am experimenting with lightwieght clock hands and getting better results. I think I might be able to tune it up to standards. I might also try the needle from a multimeter.

    As for a non marking or "eye-balling" system (even though the professor said he prefers a marking system), I was thinking a laser attached to the pendulum pointed at frosted glass with an appropriately scaled protractor drawn on. If we had a replay camera we could even be pretty accurate in finding the apogee angle (if that's even correct termonology...)
  6. Apr 11, 2007 #5
    Your idea of lightweight clock hands is very good. A light clock hand well equilibrated and pivoting at the same level as the strings supporting the pendulum can be pushed by it. If you add a little friction on the pivot it will stay at the maximum deviation. I think that the friction needed to hold a well balanced clock hand will not impair the measure.
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