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Homework Help: Flying pencil

  1. Mar 16, 2008 #1
    1. The problem statement, all variables and given/known data

    Place a pencil on the edge of a table and I hit it. How shall it fly? When will it fly the highest distance?

    2. Relevant equations

    Definetely F=m*a and for the flight x = 1/2 a t^2

    3. The attempt at a solution

    Well there are several hidden factors that are making the problem difficult. One is that when I hit the pencil, I continue on my path, so it is just not an impulse. Another is the width of the table - the pencil jumps of it.

    Basically, the only thing I know for sure is that if I place the pencil on a table that has no width and in a way that the center of mass is directly above edge of the table, then if I hit it very near of the center of gravity, it will just rotate and will not fly at all.

    I know that the length of the flight depends on how far is the center of mass from the edge of the table and also on where and how I hit it. Hitting it further from the center of mass should give it bigger impulse, but I think it turns into more rotation and not so much into tangential velocity. But on the other hand hitting it nearly the center of mass gives it greater rotation too, because the hitting object will change the pencil's angle more quickly. I am getting really confused... Is there anyone who can help me?
  2. jcsd
  3. Mar 17, 2008 #2
    I did the very same experiment for coursework a while back =] although I'm afraid I can't be a great deal of help since I didn't manage to get to grips with the maths properly.

    I looked at it from an experimental point of view- I did some experiments where the pencil was on a fixed axis, and measured the angular velocity of the pencil after it was hit, in an attempt to work out the amount of energy given to the pencil... If you then measured the angular velocity when the pencil was in flight presumably you could work out how much of the energy is transferred for parabolic motion? I don't know.

    Another thing to look at is maybe the motion of whatever you use to hit it- for example in my experiment a mass dropped vertically moved sideways after hitting the pencil indicating that horizontal momentum had been transferred to the pencil..

    Experimentally, the pencil also sometimes flew backwards, (when its centre of mass was a long way onto the table).. think of hitting a metre ruler when only a few cm hang over the edge- here it would definetely go backwards and not forwards.

    So basically I cannot help you with the theory behind it but I do have some pretty screenshots of the experiment itself, which may or may not be of use.

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