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Homework Help: Coursework Experiment involving rotational mechanics

  1. Dec 10, 2007 #1
    The experiment for my A level coursework is as follows:
    Place a pencil so that it is balancing with one end hanging off a table (and is perpendicular to the table edge). Apply a force (eg by hitting it with the side of the hand) to the end hanging off, so that the pencil flies across the room. (It makes sense once you try it!)

    I am not looking for specific equations (although if you can think of any directly relevant I would be grateful), more some other thoughts on how this might work. I have carried out various experiments- my set-up involves placing the pencil so that a recorded distance hangs off the table, and then dropping a mass through a tube onto the end of the pencil. I have tried changing the mass and also the distance the pencil hangs off and got some very interesting (but not so conclusive!) results.

    I am getting a variation of approximately 80cm between my results for each mass (eg a 20g mass can produce any distance from 1.6m to 2.4m)- this may just be down to the inaccuracy of my experimental set-up, as I think the mass may hit the pencil in a very slightly different place each time.

    Thinking about the mechanics of the situation- the centre of mass of the pencil should travel with parabolic motion, although I'm not sure how to work out the angle the pencil sets off at as the rotation makes this hard to find. I should be able to measure it from the videos I have made.

    One thing that is currently baffling me (and my teachers!) is that; when the pencil is placed so only a small length hangs off the end of the table (eg 3-4cm), the pencil sometimes jumps upwards and backwards (towards the centre of the table) instead of the usual forwards motion. (This does not seem to happen EVERY time- I have video shots if you don't believe me!) I have no idea how this could happen as I cannot find any backwards component to the forces I am considering.

    I realise this is an extremely long post so thank you for reading it! I have more information on the situation if needed. Thanks in advance for any replies.
  2. jcsd
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