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Question about bouncing pens

  1. Nov 4, 2012 #1
    Hi, i am studying collisions at school and have a simple question about something i observed while absentmindedly dropping pens.

    1)First find a a useless pen (or any other similarly shaped object) and hold it still such that it is parallel to the horizontal about 20 centimeters above your desk and carefully let it fall. If you did it accurately then the whole side of the pen will touch the desk at the same time.
    2)Now do the same thing again but this time do it keeping the pen completely vertical witheither side facing down.

    You probably got what I am getting to-
    "Why does the center of mass of pen in the second case bounce so high when compared to the first where the pen barely bounces?"
    Seems strange that velocity of pen in case 2 as it rises is more than that in case 1.(We cant consider it to be point particle???)

    Possible explanations I thought of-
    1)Air resistance in first case when the pen rises is much more than that in the second case.
    (True but i doubt if that would make such a big difference)
    2)I somehow feel that in the 2nd case all the force passes through the center of mass and that somehow makes a difference. (not able to properly explain)
    3)Some ideas related to torque which I cant explain as i haven't studied that topic.

    All ideas and help is appreciated!
  2. jcsd
  3. Nov 4, 2012 #2


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    On vertical impact the pen compresses more, and stores more in energy in that deformation.
  4. Nov 4, 2012 #3
    So does the desk.There is more force per square inch when the verticle or tip of the pen hits.
  5. Nov 4, 2012 #4


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    Don't forget moving parts, such as the spring!
  6. Nov 4, 2012 #5
    I agree but on horizontal impact more points on the pen and the desk compress. Does that make a difference? If we consider the pen to be made of same material throughout then coefficient of restitution would be same in both cases.

    I dont completely understand.....regular pens with no spring also show this effect.
  7. Nov 4, 2012 #6
    The answer is a pretty simple impulse momentum case. It's just as the others have said, Let's say that the force that the pen applies to the table is F=ma. In the first case you're distributing the force over a much larger area, thus the force pushing it back up is significantly smaller.
  8. Nov 5, 2012 #7


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    Structural properties like coefficient of restitution of a macroscopic object are not just a function of the material. The geometry also plays a role in determining how much energy is returned, and how much goes to heat. Otherwise springs would not be winded helices, but just straight rods. And if you drop a spring sideways it will also jump less than when dropped vertically.
  9. Nov 5, 2012 #8
    examine your pen is it not semi hollow the impact when droped on its side is obsorbed by the air behind it think a pillow would you rather be hit with that or a solid hunk of wood (even with equal mass) it becousse the air between the fiber in the pillow obsorbes the potential energy
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