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Table tennis problem!

  1. May 18, 2008 #1
    I'm not quite sure where to post this but it certainly isn't homework/coursework so I suppose here is appropriate.
    The situation is that a friend claims something which I believe to be physically impossible happened, and though it is a minute and pointless point to argue about, I want to explain it to him in a way in which he will understand. The scenario is this:

    A friend and I were playing table tennis at school, using wooden paddles (this is important as they cannot impart any significant amount of spin onto the ball).

    During the match, I hit the ball from the back of the table in the centre, give or take a foot, and the ball went back over the net, towards the side which is to "my right". However it clipped the edge the smallest possible amount, and made a tiny sound, whilst continuing its journey down virtually unchecked.

    Now in table tennis, the rules state that if the ball clips an 'edge', it counts as in, but if it hits the 'side' of the table, it counts as out. (www.allabouttabletennis.com/table-tennis-rules.html for clarification).
    Whilst my friend acknowledged that it clipped something, he argues that it clipped the 'side' of the table as opposed to the 'edge'.

    I am trying to explain to him that it isn't possible for a ball which has come from OVER THE SURFACE the table to clip the 'side' without hitting the 'edge': unless of course I put so much sidespin on the ball, that it first traveled outside the constraints of the table and then back in towards it again, clipping the side on the way down... but with a wooden bat, this isn't even conceivable.

    I was wondering if anyone here would be able to put it in a succinct and convincing way so that I can explain it to him, as it's bugging me that he really doesn't get it. Feel free to use any method of explanation.

    I have a diagram which can help clarify the situation:

    img261.imageshack.us/img261/1448/conanni4.jpg (This shows how the ball was struck and explains the problem).

    www.allabouttabletennis.com/images/table-tennis-table-playing-surface.jpg (This shows the difference between a 'side' and an 'edge').
    Last edited by a moderator: May 18, 2008
  2. jcsd
  3. May 18, 2008 #2


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    Welcome to PF, Stefan.
    I couldn't access your pictures, but it doesn't matter because your description is very clear. So is your explanation of the facts. Frankly, I think that you don't need any help on this; you've done great on your own.
  4. May 18, 2008 #3
    Physically take the ball in your hand and ask him to show the motion required to clip the side without the edge. Obviously he will have quite a difficult time doing this.

    Sounds to me like your friend is just a sore looser! :rolleyes:
  5. May 18, 2008 #4


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    This is exactly the reason I only use 2-dimensional tables now! Sure, their lack of literal existance can be a drawback; but you never have to wonder if the ball hit the side (and besides, they're so easy to store).
  6. May 18, 2008 #5


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    Quite true, but you have to be very careful to not cut yourself. Those non-edges are sharp.
  7. May 18, 2008 #6
    Technically it is possible, but the ball would have to have been hit quite high in the air for it to be possible.
  8. May 18, 2008 #7
    If gravity had been the only force acting on the ball, and gravity was acting perfect vertically downwards (relative to the table as horizontal), then what your friend claimed happened would be impossible.

    But in the real world, as a table tennis player, I am not confident enough that I would tell the friend "it's impossible." First and foremost, wooden paddles definitely can put some spin on the ball (I played for a long time before getting a paddle with rubber on it). Discounting that (after all, you hit the ball and you report no spin) my next thought is that the ball was effected by air currents, which could even have been from the movements of your opponent.
  9. May 18, 2008 #8
    Crosson, if you modelled the ball then no it would not be possible. But surely in the real world when there are alot more variables it would be possible.
  10. May 18, 2008 #9
    Well, yeah, wooden bats can put some spin on the ball but even if you were to slice the ball as much as possible with a wooden bat, the ball's path through the air is not changed very much; I have tried it.
    I have thought about the air current movement myself, as this is how the wobble from 'long pimples' bats works, however realising that I hit the ball from the middle of the table, the angle of change would have had to be something akin to if a hairdryer was pushing the ball the opposite way.

    I am sure that the ball did indeed hit the edge, and whilst it is not impossible for a ball to clip the side when coming from over the table, it is without a significant amount of spin. If the ball were hit from the very right hand side of the table and hit the right hand 'side', then perhaps it would be considerable in this circumstance. As it is, the ball was hit from around the middle and that leaves a path through the air which isn't really possible.

    Besides, my friend has already accepted that
    a) the ball came from over the table
    b) the ball never went out of the constraints of the table until it had already clipped the edge.

    I just can't explain to him that unless the ball did indeed go outside the table at some point, it would not have hit the edge.

    Wait, if the ball was moving directly down, then would it not hit the edge before the side as they are in the same vertical plane?
    Otherwise if the ball went really high and managed to hit the sides, it seems it would actually be moving backwards on impact.


    Ah, I see. Well the ball's downward movement was nothing compared to its forward movement anyway, so this wasn't the case.
    Last edited: May 18, 2008
  11. May 18, 2008 #10


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    Generally, the only time a ball hits the outside edge of a table is if the ball was previously struct from outside the sides of the table back towards the table. If far enough to the side, the ball could be hit around the supports for the net. Note that hitting the ball around the next (or even around and below the net from the side) counts as legal hit if the ball first's contact afterwards with the table is on the top of the table. With sticky rubber it would be possible for a ball to have enough sidespin to go beyond the side of the table, and curve back inwards.

    In practice if the ball was within the boundaries of the table just before hitting the edge of the table, then it wouldn't be possible for the ball to have hit the side of the table.
  12. May 18, 2008 #11
    These are my sentiments exactly. Moreover, The ball was struck from well within the middle of the table, which means this is not even a borderline case.
    However, I am looking for ways to explain this to my friend which will make him believe it to be so: if anybody has any more ideas, please contribute.
  13. May 18, 2008 #12
    If he can't understand something this obvious just by picturing the situation, then your are going to have to show him. And conversely ask him to show you how his situation could ever possibly be true.

    It would be quite a show to physically hit the side of a table with an object falling from above, from inside the area of the table, and falling without any spin and without crossing the border of the table until shortly before impact.

    Like I said before, it seems to me like you have a stubborn sore loser on your hands... :grumpy:
  14. May 18, 2008 #13
    Thanks, I shall try a 'physical' demonstration.
  15. May 18, 2008 #14
    Hmmm, if the ball was spinning whilst moving, this will give it an added sideways force (I can go into why, if you're interested), so I'm not sure if it is altogether impossible tbh.
  16. May 19, 2008 #15
    Yes vertices, you are correct, and I understand how spin works. However, I have already stated that the bats used were not capable of putting much sidespin on, not to mention the fact that even with spin, the ball would have to first travel outside the constraints of the table, then travel back inwards again to hit the side.
    It is impossible if the moment at which the ball strikes the edge, the ball is coming from over the surface of the table towards the outside of the table.
  17. May 19, 2008 #16
    And how do you know it didn't go outside the constraints of the table? I mean it would only have to do so marginally.
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