I just watched your show about making birds in a truck fly in order to reduce the gross weight of the truck. I still disagree. I believe both the birds and the helicopter were exerting a downward push of air that equaled their weight. While that would BUST the myth, I'd like you to carry the test a little further. Exactly what would happen if there was no roof on the truck and the birds and helicopter just flew away? Obviously, at some precise point, the weight in the truck would get less.

In a similar experiment, I've wondered if a human body gets lighter or heavier when a person "passes gas". If "gas" is lighter than air, a person should get heavier. I guess you can't really do that one on TV, but it would be interesting.

Warlock, it was a closed system for a reason. Weight equals mass times acceleration. The acceleration, in this case, is a constant---the earth's gravity. The experiment demonstrated that the mass of the closed system was the same---be the contents stationary or moving. Basic physics, which many people do not understand.

And if you took the sides off the truck and swung a broom at the birds...

You can't change the parameters of the myth.

It's a closed truck with birds in it.

The simplest logical solution to this I see is to imagine a tube closed at the base and with a top that can be put on it or removed. No imagine you make a disk with a one way valve on it. Put the disk in the tube inverted so it falls to the bottom, seal the tube and weigh it. Then open the tube, retrieve the valve, put it in right side up so the valve seals, let it sink a little (since it's unlikely to be a perfect seal) cap the tube and weigh it again.

If the weights come out different then you must have somehow either created or destroyed mass.

What you're proposing as an alternative is to weigh the tube, top and disk then remove the disk (let the birds/helicoptor fly out of the confines of the closed system) and just weigh the remaining 2 pieces.

Try this.
Have a balloon on the floor and a cylinder of Helium gas. Remotely open a valve and fill the balloon up.
Does the truck get lighter?

You guys missed the point in trying to bust this myth. To eliminate the weight of the birds or the helicopter, they must be out of 'ground-effect'. Until then, it is not FLYING, but just pushing on the ground. The helicopter would need to be at an altitude greater than the diameter of the rotor which results in the aerodymnamic lift carrying the weight - not ground effect. The weight of a 'flying object' does not increase the weight of the air.

I'm reminded of Mr. Hartman's Tidewater Community College physics class during which he posed the "birds in a truck" question. The entire class guessed "gets lighter", per his expectations. Except me. I answered "it depends on how open the bird/truck system is". From his reaction it seemed that no student had ever given the "correct" answer, much less the "it depends" answer. I'm an old farm boy who knows that chickens and other birds don't ship well in closed trucks, and I explained that in the *real* world the answer is "gets a bit lighter". We developed a bit of mutual respect after that.

The test on the episode simulated the closed system case. But one of the explanations was inaccurate. It doesn't matter if air is flowing downward against the truck bed or if the birds take flight in hot air or helium baloons. Everything in the closed system still has the same mass and still weighs the same. The only practical way to make the truck weigh less is to "bounce" the truck or its contents. You could see this on the data graph in the episode when the birds and helicopter "bounced" on the air or the bed. But you could also see the truck weighing more just as much as it was weighing less. So, over time, it weighed the same.

About the passing gas. Technically, if the "gas" is lighter than the air, the person would weigh slightly more on a scale, even though their mass would decrease. If you are floating in water, and breath out, you will probably sink. If you had a scale under water, you would increase your weight, even if your mass had decreased.

I don't see how you guys can call this myth busted. certainly within any "closed" system the results are going to be the same, but for hauling birds in a glass box is hardly realistic. try replacing the glass panels with chicken wire and retry the experiment. There will still be downward pressure on the truck bed however the outside air will likely absorb some of that downwards movement. so myth = not busted

By no means, Sir, you have a different myth. The purpose was NOT to haul birds but demonstrate a basic physics issue. One difficult to grasp, I grant you.

Good Grief!

First of all any hovering object (excluding any unknown alien technology) exerts a downward force equal to it own weight. Less force - the object descends; more force - the object gains altitude. Inside the truck the floor "feels" this force.

The only time this doesn't apply is when an airfoil moves forward (at speed) through the air. Then lift is created more by low pressure above the plane surface rather than a downward force, but the birds were hovering, not flying! The other question is if the latter was the case wouldn't the lower pressure above the birds wings pull down an equal weight on the TOP of the enclosed truck.

The real secret is as arthedain73 points out (all-be-it with a lot of unnecessary extra goble-de goope). The total mass of the enclosed space remains constant, regardless of what's going on inside.

Case-in-point: A beaker of water weighs 1 pound and a neutrally boyant object (hovering for all practical purposes ) inside the beaker weighs 1/10 lb. Regardless of what's going on inside the beaker the total weight will equal 1 and 1/10 lb.

again, that works well for a closed system. there is no doubt that the experiments they did were valid, really that the results were easily anticipated beforehand. The experiment however is not a realistic scenario. I imagine that when the bird flys it will have a low pressure area above it and a high pressure area below it. As long as both of these areas are contained in a glass box then the results will be as expected. But if you substitute chicken wire for glass then the zones may extend outside the box. as a result you will have leakage, possibly a little, possibly a lot. Now that test would have been interesting to see. As stated this was not what they tested. But they certainly cannot call the myth busted without considering these scenarios.

[quote]The weight of a 'flying object'[/quote]
They should have hung the copter from a string on the top of the truck. If the coptor gets above the width of the rotors in height, the truck should become lighter.

Closed systems and open systems are not the same. Hard to understand---witness some of the above postings.

if anything the truck would get lighter because to fly the birds need to create a downwerd push that is greater than the own weight.

downward*

The “Birds in a Truck” myth that was prematurely “Busted” is flawed due to a lack of precision in the scale used in the test. The registered weight of the truck should change, but OPPOSITE to common sense, and only in FLUX as the birds either rise or fall.

Birds gain loft when the normal force of the air flow they generate downward is GREATER than their weight, thus pushing them up. Since scales measure this normal force, birds taking flight in a truck should therefore INCREASE the weight of the truck. Now imagine that birds in flight suddenly stop flapping in midair and fall - they should therefore become “weightless” in freefall (they generate no normal force) and the truck should weigh LIGHTER. If the birds are at rest on a perch or maintaining a constant elevation, there should be no net weight change.

Or, think about it in terms of gravitational potential energy. Recalling that forces (like weight) are a derivative of energy with respect to distance, as the birds rise up in the truck, the truck must sink down onto the scale according to Newton’s third law, which registers as MORE WEIGHT. This explains why the weight should change only when the vertical position of the birds changes, and equal and opposite to their direction of change!

The above comments will only be observed in the most ideal conditions (for example, they ignore lateral forces), however they agree with theory, and anyone with a knowledge of Newtonian physics will agree - the truck will actually get HEAVIER as the birds FLY UP!

The problem with the Mythbusters method is that the scale used in the test was not precise enough to detect such a minute change in weight (they themselves noticed “noisy” data). The truck rig they used must have weighed at least 1000 lbs., and the total weight of birds was at most 1% of this. I imagine (but cannot confirm) that a difference of +/- 1% is difficult to detect on an industrial-sized scale used in this test. I recommend a much lighter rig and more birds before this myth can be confirmed or busted. Or, James’s balloon idea sounds good!

*notes on previous posts:

"normal force" is defined in my previous post a force counteracting gravity - it is an upward force.

arthedian73:
it's true that the MASS of the system doesn't change, but what you need to realize is that as the CENTER OF MASS of an object (truck + birds) changes within a gravitational field the WEIGHT MUST CHANGE!

The answer to this, is that it is a closed system, but often birds and such animals are carted around in trucks with holes in the sides for air. Certainly you have seen trucks hauling cows go down the road.

What this needs to try is to use sides of chicken wire. What should happen is the downward pressure of the wings flapping would then be spread not just within the closed container, but would escape to push down upon the outside of the truck. This would make for a lighter load.