Can a ball roll fast enough that it becomes airbourne while rolling?

  1. For ex, say you were rolling a ball down a steep ramp (about 85-90 degrees to horizontal, so almost straight). The ramp is say at least 100m long so there's enough time for the ball to build up velocity, etc.

    Is it possible for any circumstances to happen where the ball would temporarily become airbourne as its rolling?

    I couldn't reason out any way for this to happen, (I'm not very knowledgeable in concepts/theory though..)
    Is there a way for this happen? If so, could someone explain / link me to a page that talks about it?

    Thanks!
     
  2. jcsd
  3. mfb

    Staff: Mentor

    If the ramp (or your ball) is not perfect, the ball can bounce at small bumps in the ramp, and become airborne for a while.
    If the ramp is perfect, but you have air, this can lift the ball.
     
  4. Or if the ramp is curved it can happen. Imagine a ball rolling down the outside of a sphere. At some point the ball will have enough horizontal velocity that it will leave the surface of the sphere.
     
  5. The steeper the angle of the ramp the less gravitational force there is holding the ball to it.
    As the ball pushes air out of its way some of the air will get compressed between the ball and the ramp creating pressure which will act to push the ball away from the ramp.
    At any given ramp angle there will be a certain velocity where the pressure is greater then the gravitational force and the ball will break contact with the ramp. At a certain angle or steeper gravitation will provide enough force to accelerate the ball to that velocity.
    So the answer to your question is yes, though I do not know how to calculate the required velocity or angle.
     
  6. Baluncore

    Baluncore 2,862
    Science Advisor

    There are several effects:
    The ball has “top spin” so it will fall due to the Magnus effect.
    The ball will “stick” to the surface due to the Coanda effect.
    When above the speed of sound in air, the ball will lift on compressed air trapped between the ball and the surface.
     
  7. rcgldr

    rcgldr 7,534
    Homework Helper

    The flow on the bottom half of the ball is restricted, and this would interfere with Magnus effect.

    Seems the issue is if the ball reaches terminal velocity due to drag before it's moving fast enough to "plane" off the surface that it is rolling on.
     
  8. Baluncore

    Baluncore 2,862
    Science Advisor

    Obstruction of the Magnus effect by the presence of the ramp will only occur on the ball's narrow equatorial zone where the friction of contact causes the ball to rotate. The majority of the ball will retain a boundary layer. The boundary layer at the point on the ball furthest from the ramp will be travelling at twice the speed of the ball's centre.

    In reality, for a close to vertical ramp, the ball will bounce down the surface. Any imperfection in the ramp, (or ball), will impulse the ball away, then the ramp slope, the Coanda and the Magnus effects will bring it back to bounce again. On average the ball will not be in contact with the ramp. For a steep ramp the ball may be in contact for less than 1% of the time. It could not really be called “flying”, nor does the term “airborne” describe it as it is not actually being carried or supported by the air. Indeed, the air is acting to hold it to the ramp. It is more like skipping a stone on a pond.
     
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