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Something I have always wondered.

  1. Jul 20, 2009 #1
    Hello all, I joined this form to ask you all this question.

    If you could make a funnel that has no friction on the inner surface, what would happen if you dropped an object into this funnel?

    The ideal funnel: A standard 6 inch long automotive funnel.

    The ideal object: A tennis ball.

    I have several ideas as to what would happen, but I wanted to see what you all have to say about it.
  2. jcsd
  3. Jul 20, 2009 #2
    It would drop into the funnel........................ why do you think friction is necessary here? If anything, friction hinders the object from going through the funnel.
  4. Jul 20, 2009 #3


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    The tennis ball would simply slide down the side of the funnel, hit the bottom and come back up again (without spinning). It would repeat this several times before coming to rest at the bottom of the funnel.
  5. Jul 20, 2009 #4
    I beat you, I win.
  6. Jul 20, 2009 #5


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    I challenge you to pass a tennis ball through a standard automotive funnel :wink:
  7. Jul 20, 2009 #6
    LOTS of oil.
  8. Jul 20, 2009 #7
    The funnel is large enough at the opening to allow the tennis ball in, however, it quickly becomes smaller at the outlet, much smaller than the tennis ball.

    This is why I ask what would happen if there were no friction. The tennis ball wouldn't stop, right?
  9. Jul 20, 2009 #8
    Ok, so how exactly does a solid object pass through a hole smaller than itself.......it doesn't.
  10. Jul 20, 2009 #9


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    Yes it would, due to the energy lost deforming the ball.
  11. Jul 20, 2009 #10
    Why would the ball deform?

    If there is no friction on the walls of this funnel, how it is able to push on the ball?
  12. Jul 20, 2009 #11
    I think you don't understand exactly what friction is. It's a resistive force to motion. It's not a compressive force. You should read up on the fundamentals of what you are trying to describe before you use it in an argument. This is whats causing you to stumble.
  13. Jul 20, 2009 #12


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    If there is no friction on the walls of this funnel, how it is able to push on the ball?[/QUOTE]
    Whilst the reaction force from the funnel may not act parallel to the ball's radius, there will certainly be a radial component and therefore although the ball will not spin it will still 'bounce'.
  14. Jul 20, 2009 #13
    I understand that. But, I don't understand how the funnel is able to compress the ball.
  15. Jul 20, 2009 #14
    It's physically smaller than the ball. It has to compress the ball. Said differently, because the ball is falling, it will wedge itself tightly against the sides of the funnel and stop. The energy contained in the balls velocity as it falls will be converted to deforming the ball as it wedges itself upon stopping.

    Really, its the ball wedging itself into the funnel, not the funnel wedging the ball into it. This can easily get into cause/effect semantics, but I think it illustrates to you the physics of what's happening and why.
  16. Jul 20, 2009 #15
    If you wanted to, you could calculate exactly how far it could wedge itself. I just though of a simple way how to:

    Assume the tennis ball is a linear spring, with some spring constant k and that the slope of the walls is dy/dx. Find the normal direction to the walls and do the dot product in the direction of gravity. This is the direction of the upwards force on the ball. The magnitude of the force is simply F=k (dx), where dx is how much the ball is physically pushed into itself because of the slope of the walls. When the dot product of the force is equal and opposite to gravity, thats how far down the ball will go into the funnel before stopping.

    http://img241.imageshack.us/img241/580/tennisball.png [Broken]

    Notice, no where did I draw friction in this picture.
    Last edited by a moderator: May 4, 2017
  17. Jul 20, 2009 #16
    Surface roughness is not the only source of friction.

    A funnel that has a "frictionless" surface will still cause friction if it gets too small for the ball.

    Therefore the design of your experiment is a complete contradiction. It is like asking, "If you get shot in the head but the gun wasn't loaded, does the bullet kill you?"
  18. Jul 20, 2009 #17
    That's an improper use of the word friction in your description. There is a resistance normal force, it is not due to friction. As per the problem, it is a frictionless funnel.

    .................what........................... :uhh:
  19. Jul 20, 2009 #18
    There will be some energy loss associated with air resistance. So put the whole shebang in a vacuum. There might be some energy loss due to deformation as gravity pushes the ball against the funnel, so get rid of the funnel. Now you have a planet orbiting a star. The system goes for billions of years without running down.
  20. Jul 20, 2009 #19
    Enter black hole.
  21. Jul 20, 2009 #20
    *Cue dramatic music*

    Anyhow... To the OP: If you drop a tennisball onto a horizontal frictionless surface, would it then still pass through because there is no friction? Of course not, it would bounce just the same as if there was friction (assuming perfectly perpendicular).

    If you drop a tennisball on a sloped frictionless surface, would that change things? Would the ball pass through now?

    The funnel is no different, except for the fact that it has a slope at all sides.
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