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Ping-pong balls, Bernoulli, and water

  1. Jul 5, 2016 #1
    So I saw the post with the Bernoulli math, but playing with my grandsons in the leisure pool we have observed a ping pong ball released from about 12 inches below clear the surface of the water by an unmeasured inch and a half. And when the twelve year old swims to the bottom of the four foot deep section and releases the ball it appears to clear by the same inch and a half. We were wondering why we didn't see a change. Of course three feet in depth may not be enough to make a difference in casual observation, but the lifeguards don't like us playing with ping pong balls in the deeper pool.
    Second thing we have noticed is that no matter how carefully we release it the ball seems to spin just below the surface of the water and only creates a splash on one side and always the same side.
    Help a Grandma out here, these boys are budding scientists.
     
  2. jcsd
  3. Jul 5, 2016 #2

    johnnyrev

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    Gold Member

    Hi Susan, welcome!

    It's fun experimenting with your kids. Mine are grown now, except for one, so we don't get to do many projects anymore. We built a Rube Goldberg machine for part of her Physics final recently.

    There's an Education forum on the site, and the web has tons of experiments for kids with expected outcomes so you can easily relate what you did in an experiment to what happened and why it happened.

    For your question, lemme toss a couple things out there (I'm no fluid dynamics person), and see if someone will jump into the discussion:

    You're dealing with water, which has high cohesive properties, and also adheres to the ball some, so it doesn't really want to let go of the ball when it leaves the pool's surface, making the "air time" similar in both instances.

    Since it's a ball, there is a fluid dynamic that will make it spin, but I can't recall it at the moment. Help, please?

    It's not the Coriolis effect, where fluid pressures on a spinning ball make it curve. It's why a pitcher can make a baseball curve, and I've heard of at least one experimental sail that was a vertical spinning column that actually propelled the boat.
     
  4. Jul 5, 2016 #3

    fresh_42

    Staff: Mentor

    I think it is mainly the water's resistance that results in a force in the opposite direction to buoyancy, similar to the resistance in the air but much stronger. E.g. if an object drops from high altitude, it will not accelerate the whole fall but gain a final constant speed due to air resistance. (I remember school days here, so correct me if this is wrong.)
    So if this final speed is attained within a few inches in the water, it will make no difference when diving the ball deeper.
     
  5. Jul 5, 2016 #4

    Nugatory

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    Staff: Mentor

    Yep - and OP might want to google for "terminal velocity" for details.
     
  6. Jul 5, 2016 #5

    rcgldr

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    Homework Helper

    If I remember correctly, you can get a bit more height by starting with the ping pong ball only partially submerged, so that it doesn't have to displace any water as it travels upwards.
     
  7. Jul 6, 2016 #6
    Thank you. This has been a great help. I think you are right about getting a little more distance when not submerging it totally. I noticed this and had wondered if it was because it wasn't using any of its energy breaking the surface of the water. I remember my grandmother teaching me that the surface of the water was stronger than we think by observing the skater bugs with me. But the boys hadn't commented on it so I hadn't pushed in that direction. And I will challenge the boys to research both terminal velocity and curve balls.
    Thanks again for your help. I will remember this website the next time they toss one of their questions my way. I love kids, they love to learn but not always while sitting at a desk.
     
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