Can a Closed Jar Measure the Weight of Flying Bees Inside?

[russ_watters]

Uh, it is - don't you see the rooster-tail? The plane goes fast enough that the pressure wave isn't directly under the plane.

Anyway, I'm pretty sure that's a painting.

[edit: There is a good actual photo of a B-1 doing this and you can see in the photo that the rooster-tail is actually diamond or wedge shaped because it is caused by the wingtip vortices, not the engines. It is difficult to see in this version of the pic, but there is a noticeable area of disturbed water just behind the plane and in front of the rooster-tail: http://www.freerepublic.com/focus/f-news/1496427/posts ]

edit2: apparently the B1 "photo" is a painting too. Anyway, they did a better job with it...

 

[russ_watters]

There is no need to look for complex examples of this - everyone has certainly seen pictures of helicopters hovering just over the water, blasting-out a circular disturbance below them. It is a very straghtforward calculation to figure out just how much water needs to be displaced to hold aloft a helicopter hovering just above the surface of a lake or ocean:

An SH-60 has a gross weight of 21,000 lb and a rotor diameter of 53'.

That's 2206 sq ft of area.
At 62.4 lb/ft^3 weight density for water, 21000 lb displaces 336 ft^3.

336 ft^3 / 2206 sq ft is 1.8 inches deep.

 

[peaceharris]

Uh, it is - don't you see the rooster-tail? The plane goes fast enough that the pressure wave isn't directly under the plane.

Anyway, I'm pretty sure that's a painting.

It's not a painting. The air pressure drops due to the increased velocity of air, water is sucked up. Just like drinking from a straw, you reduce the pressure, water is pulled up.

 

[russ_watters]

No, peaceharris, that isn't what happens at all. The air pressure on the bottom of the wing is higher and that air is pushed down. The air pressure above the wing is lower and that air is pulled down. Behind the plane, air is moving downward. Again, it has to - otherwise we'd have a violation of Newton's laws: http://www.mansfieldct.org/schools/MMS/staff/hand/flightbernoulliandNewton.htm

Googling for that painting comes up with discussions of it along with real pictures and video of what a plane in ground-effect looks like. For example: http://alexisparkinn.com/photogallery/Videos/Supersonic%20F14%20flyby.mpeg

Notice how the disturbance under/behind the plane looks a lot like what you see under a hovering helicopter. You can even see in that distrubance that he's accelerating past mach 1. It start's off in front of the plane and moves back and becomes wedge-shaped as he nears the ships.

Anyway, you can tell that painting is a painting because of the coloration, the weird "camera" angle, and the fact that they got the plane's effect on the water wrong!

 

[russ_watters]

Ok - here's a link to the artis'ts website, showing both paintings: http://www.drublair.com/comersus/store/comersus_listCategoriesAndProducts.asp?idCategory=55

 

[peaceharris]

OK... Thanks everyone for the explanation. Now I also think that the weight of flying bees will show on the scale outside.

 

[rcgldr]

This is easier to comprehend if a person understands how gases exert their weight inside closed containers.

The method for a gas to exert it's weight in a close container is via pressure differential that decreases with height. This net differential in pressure at the top and bottom of the container will generate a net downwards force equal to the weight of the gas inside the closed container. Note that pressure is exerted in all directions, not just downwards; it's the pressure differential versus height that results in a net downwards force.

A somewhat common example is if 6 pounds of air, about 80 cubic feet is put into a scuba tank (at around 3000 psi). The weight of the tank increases by the 6 pounds of air added into it.

If an object is flying or gliding inside a closed container, and there's no net vertical component of acceleration, then the pressure differential will increase so that the net downwards force will equal the sum of the weight of the air and the object.

 

[zumulko]

Some of the kinetic energy of the air particles moved by bee wing is transferred into the heat and radiated from the jar. Lost energy means lost momentum and hence less net force acting on the bottom of the jar.
Am I wrong?

 

[rcgldr]

Some of the kinetic energy of the air particles moved by bee wing is transferred into the heat and radiated from the jar. Lost energy means lost momentum and hence less net force acting on the bottom of the jar.

Not all of it gets converted to heat. Some of energy is used up in order to increase the pressure differential so that the net downforce of the air inside the jar exactly equals the sum of the weight of the air and the bee in side the jar.

All too often, how the air inside the jar exerts its own weight on the jar (via a pressure differential, higher at the bottom, lower at the top), isn't taken into account or understood.

One source of confusion, is all the "magical" Bernoulli stuff that seems to imply that lift can occur without acceleration of air downwards to create a reactionary upwards force. In the case of a bee, it's small, the flow is turbulent, and viscosity effects are significant, since the ratio of air affected by viscosity effects versus the size of a bees wings is relatively large. Insect with larger wings, such as a dragon fly, can glide, but bees and bumble bees can't.

Regarding the jet over water, here's a video of a F14 making supersonic passes, the first pass shows the shockwave as a moisture induced cone, the second pass has sound and you can see the shock wave, but no rooster tail on the water, and you'll hear a loud crack instead of a boom, because that's what a supersonic shock wave actually sounds like up close. As the shock wave continue to travel, eventually it transitions into a sound wave which will be a boom we're all much more familiar with.

f14flyby.wmv

 

[russ_watters]

Some of the kinetic energy of the air particles moved by bee wing is transferred into the heat and radiated from the jar. Lost energy means lost momentum and hence less net force acting on the bottom of the jar.
Am I wrong?

The lost heat just means the bee has to flap its wings faster than you would expect to stay aloft. It does not mean that the aerodynamic force isn't transferred through the air. Also, remember that in gases, changes in temperature cause changes in pressure.

 

[lushsector9]

this was tested

This is identical to a classic riddle involving a truck filled with birds crossing a bridge. (I wish I could find an example.) The owner bangs on the side of the truck to get the birds flying, hoping to reduce the weight of the truck. The answer is: no it doesn't work.

If one hasnt lived under a rock in a cave on the other side of town then one would have heard of the show mythbusters. now the show trys to be more scientific than it is but they did this exact expirement and showed that it does register so... Yes

 

[zumulko]

Let's assume that the bee is moving upward at some angle alpha to the vertical. Flaping is pushing the air particles in the opposite direction of bee movement, giving them momentum p. Then the air particles momentum p_y perpendicular to the bottom of the jar is p*cos(alpha), which is less then p. Does it mean the force acting on the bottom of the jar (and hence the weight) is less than if the bee is hovering?

 

[DaveC426913]

If one hasnt lived under a rock in a cave on the other side of town then one would have heard of the show mythbusters. now the show trys to be more scientific than it is but they did this exact expirement and showed that it does register so... Yes

I saw that episode; they busted that myth - the birds did not increase the weight of the container when they were in flight.

http://www.youtube.com/watch?v=DiQq_-_sfAc"

Have you checked the reception in that cave of yours?

 

[russ_watters]

We seem to be receiving an influx of people asking this question over and over. Time to lock the thread. It's run its course.