1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does Bernoulli’s principle depend on the observer?

  1. Jan 21, 2009 #1
    Hello. I’ve found in the forum this thread that had the same question I’d thought.
    But there were too much things involved in it. I would like to know if Bernoulli’s principle effects change if the Observer velocity changes. For instance, everyone knows that a ping pong ball floats pushed by air stream. Besides, we know that inside this air “tube”, the air velocity is bigger than the air around it. So, the pressure in the tube is smaller. In this way, if we try to push the ball to one side, it comes back pushed to the middle of the tube by the larger external pressure.
    My question is: If something is moving inside the ascending tube air to the same velocity that stream (for instance, a fly) it will notice inside the tube a relative air velocity zero. However, outside the tube, it will notice a stream that moves fast in the opposite direction (-V). Will it detect a smaller pressure outside the tube? So, will it be pushed outside the tube?
    Regards from Spain!!
  2. jcsd
  3. Jan 21, 2009 #2


    User Avatar
    Homework Helper

    Bernoulli principle is not about absolute speeds between air and some object, but instead change in speed of the air and how that change occurred. The classic example for Bernoulli is flow in a pipe where no work is done (so the pipe is frictionless, and the fluid has no viscosity). The mass flow through any cross section of the pipe is constant or else mass would be accumulating. If a section of the pipe narrows, then the velocity of the fluid increases by the inverse of the decrease in cross sectional area (ignoring compression). Since the pipe does no work then the only remaining explanation for the increase in speed is a pressure differential from higher to lower to speed up the flow and vice versa.

    The key here is a change in speed and how that change occurred (was work involved). The exhaust from a powered turbine is high speed and high pressure because work is done. After exiting the the turbine, then Bernoulli principle applies (no work is being done), and the jet wash continues to accelerate as it's pressure decreases until the jet wash pressure returns back to the ambient pressure of the surrounding air.

    You can also think of Bernoulli as conservation of energy. The total energy of a mass of air is the sum of the pressure, kinetic energy, gravitational potential energy, and temperature. Bernoulii problems typically ignore the temperature aspect. If no work is done on that mass of air, then total energy remains constant, so an increase in kinetic energy coexists with a decrease in pressure, or vice versa. If work is done, then the total energy of that mass of air is changed, so Bernoulli principle doesn't apply during the time work is being performed.
    Last edited: Jan 21, 2009
  4. Jan 22, 2009 #3
    I am curious about the experiences of the fly in the airstream. Consider a fruit fly in a long, horizontally oriented clear glass tube. The glass tube has a reduction of its inside diameter for some portion of its length. An air compressor supplies pressure and the drosophilia is carried along with the air down the tube.

    When it gets to the portion of the tube with a reduced diameter I think it would experience itself as accelerating, and would concomitantly feel a drop in the air pressure of the air surrounding it in the tube.

    Now let's provide it with a little radar gun and some sort of pressure detector (ultrasonic?) to check the speed and pressure of the air outside the tube. While traveling in the large section of the tube it takes readings and gets values for the outside air speed and pressure. It repeats the readings after it gets to the constricted portion of the tube. How will they differ from the first set of readings? It seems clear the outside air speed will be greater in the second reading, but what about the outside air pressure?
  5. Jan 22, 2009 #4


    User Avatar
    Homework Helper

    A pump or fan causes the pressure of the air coming out of the tube to be higher than ambient. Complicating matters is that the air flows up and around the ball, so dynamic pressure and aerodynamic drag are factors here. The air from the tube continues to accelerate as it's pressure decreases back to ambient. Apparently, the air at the perimeter of the flow slows down faster than the air at the center of the flow, so the air at the perimeter would have higher pressure than the air at the center of the flow.

    The pressure isn't related to a relative velocity, but instead a change in the velocity. When no work is done on the air, then pressure differentials coexist with accelerations (or declerations) of air conforming to Bernoulli principle. The pressure outside the tube is ambient, regardless of the frame of reference, as long the observer doesn't interfere with the air by changing it's speed.
  6. Jan 22, 2009 #5
    It depends on where you are in relation to the air flow. Ambient air directly in the path of flow experiences it as higher pressure. Ambient air at right angles to the direction of air flow experience the flowing air as a lower pressure zone and will be entrained into it.
  7. Jan 22, 2009 #6


    User Avatar
    Homework Helper

    I'm assuming that the static pressure of the air coming out of the tube is sufficiently higher than ambient so that the dynamic pressure effects don't cause the air to the side of the flow is to be drawn inwards, similar to prop wash where the flow aft of the prop expands outwards not inwards.
  8. Jan 22, 2009 #7
    That would seem, intuitively, to be the case but it isn't what actually occurs. Take two sheets of paper and hold them up in front of your face parallel to each other (.75 - 1. inch apart) and blow between them. You will see that, instead of being blown apart, they are sucked together.

    You can try the same thing holding them at the angles you think might be represented by the expanding air flow, and the same sucking together occurs.

    Despite the fact the air stream coming out of your mouth is expanding it still represents a lower pressure area to the ambient air by virtue of its being in motion relative to that air.

    Here's another classic demonstration:

    http://www.faa.gov/education_research/education/educator_resources/educators_corner/grades_7_8/air_has_pressure/ [Broken]

    The expanding air coming out of the spool "should" blow the cardboard away. Instead the cardboard clings.
    Last edited by a moderator: May 3, 2017
  9. Jan 22, 2009 #8
    Then, do you think the fly moving in the air stream wouldn’t be pushed outside the tube, would it?
    You tell me as well the pressure doesn’t depend on the frame of reference. So, why don’t we choose a frame of reference that moves to a constant velocity which is the same to the stream’s one? I’m still thinking that in this frame the velocity inside the tube is zero and the air velocity outside –V. Why is the pressure smaller inside the tube if in this frame the air hasn’t got velocity?
    Thanks for your answers.
  10. Jan 22, 2009 #9


    User Avatar
    Homework Helper

    Vicosity (fiction of the air) is going to cause the air to mix in with the perimeter of the air stream regardless of the difference in static pressure. I was just pointing out that the stream from the blowdryer is an expanding outwards in a cone shape pattern, where the cross section gets larger with distance from the nozzle.

    However the end result is the same, the air at the perimeter of the stream is slower with higher pressure. However it's more complicated than just the air stream, since the shape of the object being suspened is also important. This web site offers a good explanation:


    The compression or expansion of air is very tiny at the relatively low pressures and speeds. At the spool hole paper interface, the air diverted outwards through a thin plane. At the center, there is some dynamic pressure effect, but the velocity of the outwards air is increased with little or no work done, so the pressure of the outwards air is reduced, causing the ambient pressure from outside to hold the cardboard in place.
    Last edited: Jan 22, 2009
  11. Jan 22, 2009 #10


    User Avatar
    Homework Helper

    The fly would get pushed up and out, because of it's shape. You need a near round shape to get an object to hover within the cone of air.

    Again a link to a good explanation:

    Except for dynamic pressure where the flow speed is changed via mechanical interaction, which is what happens with a solid in any relative air flow.

    Because it's how the air got to that state. The air has a total energy, kinetic energy, gravitational potential energy, pressure, and temperature. Bernoulli describes the situation where no work is done, and approximates the situation where a relatively small amount of work is done, where there is no or little change in total energy. If the velocity of air is changed without significant work being done, then there is an inverse relationship between pressure and speed^2.

    For interesting twist on the blow dryer + ping pong ball experiment, hold an empty toilet paper tube or paper towel tube just above the hovering ball and it will shoot through the tube going quite high.
  12. Jan 23, 2009 #11


    User Avatar
    Homework Helper

    I made a video of hovering ping pong ball with a blow dryer. I zoom in to show the rate of spin changing as I angle the blow dryer. I then held a cardboard tube over the hovering ball so it would shoot up through the tube.

    Last edited: Jan 23, 2009
  13. Jan 23, 2009 #12
    "hovering spinning shooting ping pong ball, the latest opus from director/producer Jeff Reid, takes us on a mind-altering journey into a dimension where natural forces seem to be defied; a land of hair dryers and toilet paper rolls, and mysterious, fleetingly glimpsed, button-pushing movers and shakers. It's a fantastic mindscape where gravity seems to lose all meaning, and ordinary objects manifest extraordinary powers. Dialog free, filmed in one continuous shot, and supported by a light, delightful New Age musical score, hovering spinning shooting ping pong ball is both art and entertinment for the whole family. Hailed by the N.Y. Times as "Cirque du Soleil, but with ping pong balls," this remarkable Indie film is a must see for all Reid fans! Two shoes up!"

    -Mr. Z. Shoe
    Cinema Correspondent
    The Zoobie Guide to Human Arts and Entertainment
  14. Jan 23, 2009 #13
    A great video, Jeff! Quite interesting the toilet paper tube. You say the fly would get pushed out because of its shape. Then, forget the fly. You think you throw a ping pong ball at the same velocity that the air stream. So, I repeat my question: will the ball detect inside the stream a small pressure? In that case, Would the ball be pushed out?

  15. Jan 23, 2009 #14


    User Avatar
    Homework Helper

    I recall reading about that a while back and didn't see any instance of this at youtube, so I made the video. I don't recall the physics involved. It's my guess that there is significant dynamic pressure below the ball, the tube captures a flow of air without allowing it to expand outwards (just upwards), and because the tube is just barely larger than the ball, it interferes with the air flow that would normally go around and above the ball, increasing the drag.

    Or a wad of paper that will just get shot up and out. Spherical like objects are required.

    Link to a good explanation:

    If you tilt the stream enough the ball falls out, but it can be tilted quite a bit before this happens. It's my opinion that that pressure in the stream is higher than ambient, so that flow accelerates to "exit velocity" as it's pressure returns to ambient, explained in the prop analysis at this link:


    Regardless of the pressure differences, gravity is always pulling down on the ball, so it could simply be a case of gravity providing more downwards force than the pressure and flow related effects if the flow is angled too much.
  16. Jan 24, 2009 #15
    Hello, I think I’ve not been understood yet. (Sorry for my English) So, I’ll try it again:
    We have a blow dryer vertically. When it’s working, we throw inside the air stream a ping pong ball vertically and up at the same velocity that air stream has. In ping pong ball’s frame of reference the ascending air velocity will be zero or quite small. However, the air surrounding the cone of air where the ball is, it will move with a high velocity in the opposite direction (-V). If is possible to apply Bernoulli’s principle under these conditions, it happens the ball will detect higher air velocity outside (the sides) and also a smaller pressure outside.
    And this is exactly the opposite thing that happens when the ball isn’t thrown but is static flowing in the air stream. (In this case, it has the tendency not to go out of the air cone).
    Would it be possible the example I’ve explained?
    Thanks a lot again.
  17. Jan 24, 2009 #16


    User Avatar
    Homework Helper

    It depends on the blow dryer (or the device) used to generate the updraft. The static pressure in the updraft could be lower or higher than ambient depending on the blow dryer. Each blow dryer is different. If the nozzle of the blow dryer is tapered so that the exit hole is sufficiently smaller the fan chamber diameter, and viscosity issues are relatively small, then venturi effect is involved, speeding up the air and reducing the pressure, and it would be possible for a blow dryer to output higher speed but lower than ambient static pressure. Most other devices, like a shop vac or room fan would have higher than static pressure.

    I did an experiment to test my blow dryer used in the video. I've seen what I consider misleading pictures of a straw in water with the blow dryer air flow perpendicular to the straw at the top of the straw, causing the water to rise, which would indicate that the pressure in the air flow is lower. It's my belief that the end of the straw in the air flow results in turbulence, which intereferes with the result. Just like real aircraft, I decided to fashion my own static port to reduce the turbulence issue.

    I took a spool of thread with a hole in it that just fit the straw. I then cut a slightly larger sized hole in a 6 inch piece of thin cardboard and rolled scotch tape to "double stick the cardboard to the spool with the holes aligned, and inserted the straw into the hole of the spool, and then held the straw in a glass of water. I then repeated the blow dryer test with my home made static port. I added some soap to the water to reduce surface tension effects, but still ended up with a hint of capillary like effect rasing the water a tiny amount inside the straw with no wind across my static port. I then turned on the blow dryer and directed the flow so it was parallel to the surface of my static port, perpendicular to the straw. The result was the water in the straw receded downwards a tiny amount. Even without the cardboard, but a sufficiently large spool of thread based static port, the results were about the same. The conclusion here is that the static pressure in the flow from my blow dryer is slightly higher than ambient.

    Getting back to your question, it depends. A similar thread was made at the mythbusters forum and some guys were using a shop vac hose and tube to do a similar thing on a much bigger scale. They were in a large building and used the tube from a paper towel which shot the ping pong ball quite high. If the ball were shot vertically enough, the ball would fall back down into the air flow again and return to hover. In this case, the ball had no tendency to be drawn outwards from the flow even though it's vertical speed through the tube was very close to the updraft speed.
    Last edited: Jan 24, 2009
  18. Jan 24, 2009 #17
    I'm not familiar with a "static port". What is the function?

    Also, what is the function of the spool in your experiment?
  19. Jan 24, 2009 #18


    User Avatar
    Homework Helper

    To allow an aircraft to accurately measure the ambient pressure near the aircraft without being affected by the relative wind speed of the aircraft.


    What I'm referring to is the static port, not the combo pitot-static tube

    I insert the straw into the spool, and tape cardboard onto the spool (with a hole in cardboard, that lines up with the hole in the spool and straw), to create a home made static port.

    The water movment is very small with a blow dryer, so a video wouldn't show much except that to show water movement is large (updwards) with a naked straw and very small (downwards in my case) with the home made static port. I'll try out our vacuum cleaner and/or electric leaf blower to see if those work better.
    Last edited: Jan 24, 2009
  20. Jan 24, 2009 #19
    My inquiry is exactly what you're saying. I want to know in this example if theorically the ball must be drawn outwards.

    Thanks again
  21. Jan 24, 2009 #20


    User Avatar
    Homework Helper

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Does Bernoulli’s principle depend on the observer?