# How Can Bernoulli's Principle Explain a Hovering Ping Pong Ball?

• Tugberk
In summary, Tugberk is trying to find out how to find the dynamic pressure, but is having trouble because the density changes with velocity. He is assuming constant density, but finds that it is a tiny fraction of an atmosphere.
Tugberk
Hey everyone, this is my first post here, I hope this is a good place to put it. I take HL physics for my IB program and thinking of doing the EE on physics. I looked into Bernoulli's principle and thought this would be interesting. A good experiment is having a ping pong ball hover over a stream of air and see at what angle the ball falls off the stream. I then can change the velocity of the air which should change the pressure, ∴ changing the angle. My problem is that I don't know how to find the dynamic pressure because to find that, you need density, but density also changes with velocity and to find density you need pressure. So I'm stuck at this point and don't know what to do!

Welcome to PF!

Hey Tugberk! Welcome to PF!
Tugberk said:
… I don't know how to find the dynamic pressure because to find that, you need density, but density also changes with velocity and to find density you need pressure.

No, water is pretty nearly incompressible (at fixed temperature), so its density stays the same.

tiny-tim said:
Hey Tugberk! Welcome to PF!

No, water is pretty nearly incompressible (at fixed temperature), so its density stays the same.

That's good to know, but I'm using air and not water :/

oooh, sorry …

i saw the word "stream", and somehow it lodged in my mind as water

Under the conditions you are working with, the change in density should be very little. Solve your equations assuming constant density, and then, based on the changes in pressure you calculate, estimate the change in density. You will probably find that it will be a tiny fraction of an atmosphere, so that it can be neglected.

Chet

Chestermiller said:
Under the conditions you are working with, the change in density should be very little. Solve your equations assuming constant density, and then, based on the changes in pressure you calculate, estimate the change in density. You will probably find that it will be a tiny fraction of an atmosphere, so that it can be neglected.

Chet

Oh ok, thanks for that! Now I can proceed :D

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