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Bob Howes
Bob Howes
Inspiring
July 7, 2017
Question

An Experiment

  • July 7, 2017
  • 3 replies
  • 1258 views

I have a twelve year old son (an afterthought--his brothers and sisters are in their early 30s) who's into science.

Today he came to me with an idea for an experiment.  He's built a sort of sling shot device that involved a bottle top and part of a balloon and has been shooting those hard silver balls you can use to decorate cakes.  He gets enough power that he can penetrate a pop can.

Anyway, he wanted to know how fast he was shooting in feet per second and worked out that if used a couple of mics recording him firing at a target over a known distance, we could calculate the velocity.

So, a couple of mics and a six foot range and over 4 shots we got a variance between 172 and 222 feet per second...I'd call it an average of 200fps.  I wouldn't want to be hit!

I certainly proved useful to be able to zoom in to ten thousandths of a second on Audition though. 

Frankly, I'm more proud that he worked out the method and did the maths than I am with the shooter thingie!

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    3 replies

    davescm
    Community Expert
    davescmCommunity Expert
    Community Expert
    July 7, 2017

    I love the ingenuity of this experiment.

    Dave

    Luke Jennings
    Luke Jennings
    Inspiring
    July 7, 2017

    A bit off topic, but true story, my wife once used those little silver balls to decorate a cake and they were terrible. Upon examining the bottle, the label said they were "not for human consumption" It turns out they were craft silver balls that accidentally found their way to the kitchen.

    Trevor.Dennis
    Community Expert
    Trevor.DennisCommunity Expert
    Community Expert
    July 7, 2017

    Way to go (as we say over here).  Sounds like a lad with proper values, and I suspect he will go far.  I am wondering what the microphones are picking up though?  Are you using the time between sound of the release, and the sound of the soda can being hit?  Or how does this work?

    If you could drill a tiny hole through one of the projectiles and make it whistle, you could maybe work out the speed with Doppler?  That was how the Brit MOD measured artillery shell speed at Foulness — my last boss before I retired from Ford Research used to work there doing exactly that, and it was fascinating hearing about it.  Luckily he didn't have to kill me after telling us how it was done.

    At one time, they looked at building a third London airport off Foulness, but one of the problems was clearing the thousands of shells they'd fired out onto the MOD owned mudflats (Maplin Sands)

    Bob Howes
    Bob HowesAuthor
    Inspiring
    July 7, 2017

    Two SDCs...in this case AKG C451s as that's what I had handy. On was a

    few inch from the target (a piece of wood leaned against a blanket to

    avoid too much reverb) and the other at where he was shooting (resting

    on the edge of a chair a measured distance from the target. Then I just

    hit record and told him to "Go". We did four takes to get a good average.

    The measurement was just zooming tightly in on the wave forms, far

    enough to get 4 decimal places. Then a little math got us the time for

    how long it took from shooting until hitting the target then more math

    to get the feet per second number.

    You've got me interested in trying a doppler test though, if we can find

    a way to make the "ammo" whistle.

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