Ping-pong balls, Bernoulli, and water

In summary, the children observed that a ping pong ball released from about 12 inches below the surface of the water by an unmeasured inch and a half. It also appeared to clear by the same inch and a half. They were wondering why they didn't see a change in depth with casual observation, but the lifeguards don't like them playing with ping pong balls in the deeper pool. They also observed that the ball only creates a splash on one side and always the same side. The children were experimenting with their grandsons and built a Rube Goldberg machine for part of their Physics final. There is an Education forum on the website where the child can find experiments that are related to what they did and what happened and why.
  • #1
Grandma Susan
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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.
 
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  • #2
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, let me 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.
 
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  • #3
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.
 
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  • #4
fresh_42 said:
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.
Yep - and OP might want to google for "terminal velocity" for details.
 
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  • #5
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.
 
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  • #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.
 

Related to Ping-pong balls, Bernoulli, and water

1. How does Bernoulli's principle apply to ping-pong balls?

Bernoulli's principle states that as the speed of a fluid increases, its pressure decreases. In the case of ping-pong balls, when air moves around the ball at high speeds, the pressure on the top of the ball decreases, causing it to rise.

2. Can a ping-pong ball float on water?

Yes, a ping-pong ball can float on water because of its low density compared to water. The air inside the ball also adds buoyancy, allowing it to stay afloat on the surface of the water.

3. How does the shape of a ping-pong ball affect its flight?

The shape of a ping-pong ball, specifically its roundness, allows it to have a smooth and streamlined flight through the air. This minimizes air resistance and allows the ball to travel further and faster.

4. Why does a ping-pong ball spin when hit with a paddle?

When a paddle hits a ping-pong ball, it creates friction on the surface of the ball. This friction causes the air around the ball to move at different speeds, creating areas of high and low pressure. This pressure difference results in the ball spinning as it moves through the air.

5. How does the size of a ping-pong ball affect its behavior in water?

The size of a ping-pong ball affects its buoyancy in water. A larger ball will displace more water, making it more buoyant and easier to float on the surface. On the other hand, a smaller ball may sink due to its lower buoyancy force.

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