Here is something to ponder : LUSENET : Junkyard Wars : One Thread

OK scappers.

A man is driving his 18 wheeler down the road. His trailer is the totally enclosed box kind, fairly well sealed. He has an extremely heavy load of live parrots. He is way overloaded! A state trooper spots his rig and decides to pull him over at the weigh station and check things out. The rig driver knows he is going to be fined for being over weight. He pulls on to the scale, gets out of the truck and walks to the meter on the scale. Just before he looks at the meter with the trooper he beats on the side of the trailer. This causes the parrots to fly about inside the enclosure. Was the trailer lighter now that the birds are flying around?

Careful! If you dare to answer, explain yourself.

Ron Lesseraux Captain - Mr. Fixit

-- Ron Lesseraux (, February 22, 2001


Yes. Do you feel the weight of a plane as it flies over? The planet is fairly well sealed. OK start the rocks flying!

-- Rob.Fitterling (, February 22, 2001.

I've heard it said that there are two critical rules in engineering, F=ma and "you can't push a rope." The second is more often forgotten than the first.

This is a case of another of newtons laws, usually summarized, "for every action there is an equal and opposite reaction. The birds have to exert a force in the downwards direction to stay in the air. That force will be equal to their weight. The air has to transmit the force somewhere, which will be the trailer bottom.

So, no, the trailer won't be any lighter.

-- Michael (Canadian P.Eng.) (, February 23, 2001.

When the birds first take off they push off the bottom and the truck weighs more. Before they beat their wings, the truck weighs less. Then it becomes chaos as the force of their wings displaces air that eventually applies force on the truck bottom. The long-time average weight will always equal the weight of all the birds resting. With enough birds the very large population of randomness will result in an average weight that varies very little while the whole population is in flight.

-- Devin T. Ross (, February 23, 2001.

Actually, there is one more issue here. For what its worth, submariners have been debating a version of this for about 30 years and they still don't have answer. The submarine version involves a 300 pound canary in the forward end of the boat, but the principle is the same.

Here's the submariner's answer that hasn't been given yet:

Birds fly by Bernoulli's Principle, the same thing that makes airplanes fly. Also, air is a fluid (much less dense than water, but still a fluid). Answer - the trailer is slightly lighter. The birds are exerting pressure against the air. The air pushes against the floor of the truck, but partially against the walls and ceiling too.

Another version of this same thing holds that the weight doesn't change, but the center of gravity does. Since many truck scales weight one axle at a time, this version would also have the weight being lighter since part of the weight shifts to the wheels which are not on the scale.

Which one is correct? I don't know. I've always wanted to get a 300 pound canary and find out.

Brian Torsk Bandits

-- Brian Flynn (, February 23, 2001.

The trailer is lighter. To max out the tractor-trailer payload, there has to be around 60,000 lbs. of parrots in the trailer. If a parrot weighs 5 lbs, that is 30,000 parrots. The heat generated and lack of air space (the trailer is fairly well sealed) kills the parrots. The parrots that are "flying" are therefore possessed and thus don't need to flap their wings to fly....

The trucker then needs Rita's help to get him out of trouble for hauling a biohazarded as deemed by the DOT.

(By the way, 30,000 parrots consume about 1/3 of the volume of the maximum trailer cube allowed on US highways. If they have been dead for over 72 hours, they fill 1/2 the volume...)

Sorry, couldn't resist!

-- Dan Denney (, February 23, 2001.


That was 3 lb. parrots to "seriously overload"...

Let that be a lesson...Don't read at work!

-- Dan Denney (, February 23, 2001.

Actually when the trooper pounded on the side of the truck he really scared the poo poo out of those poor birds, the thrust from their wings and the added weight of all that crap hitting the bed of the trailer would increase the trucks weight at lest three to four hundred pounds depending on how hard the trooper beat on the side of the rig.

-- Richard James Retey (, February 25, 2001.

most of the time when birds are scared they let out a loud squak, the squak of 30,000 birds would have to be VERY loud, thus scaring the "crap" out of the state trooper, while the state trooper goes to change his pants the truck driver gets away scott free.

plus if the box was fully sealed, 1) birds crap a whole lot! the methane would kill them all, 2) hopefully the cop or truck driver don't smoke, because methane is highly explosive, the truck might get a WHOLE lot lighter if they smoke around it...

-- Robert (, February 26, 2001.

Michael... it would appear that you misunderstand Newton's thirl law. Although it is true that for every action there is an equal and opposite reaction, you must remember the opposite part. the bird's wings exert a force on the air in the downward (-y) direction). If the force is to be opposite (as it must) the the air must exert an equal force BACK ONTO the bird's wings (+y direction). If the force were transferred to the floor of the trailer the bird would never get off the ground. if this was not true, how would the space shuttle accelerate/decelerate/change course in space? It certainly has nothing against which to push! The answer? The shuttle is propelled by the force exerted on it by the hot gases which rush outward from its engines. Of course, due to Newton's law of inertia, the engines could shut down once at the desired speed and heading. oh BTW... there are a few more things to know about physics, like K.E=1/2mv^2, (delta)L=(1/e)(F/A)(L0), or w/(w-w(prime))=p0gV, or even mg(sin theta)-mg(cos theta)(mu)=ma, or P=(l/g)^1/2... sorry... when the prysics starts it can be difficult to stop.

Love the show, Have a good day. Matt

-- matt perry (, February 27, 2001.

It's rare for parrots to fly in the dark, even when spooked, but that has nothing to do with the problem at hand. Carefully read original question which was; "Was the trailer lighter?", and the answer is a definite "nope," as the trailer itself is the same as it was, the birds were just thrown in for a diversion. The rest of the story was all a ruse to confound the physicists and engineers and leave them in total chaos, mired in numbers and symbols far beyond the comprehension of ordinary real life bubbaneers.

-- Waddy Thompson (, February 27, 2001.

Waddy, Can I watch as you try to explain to the state trooper that your load should not be counted against the scale's reading of the "trailer" weight? Maybe you are more convincing than me. I would never get away with it....

-- Devin T. Ross (, February 27, 2001.

Ahhh, c'mon guys!

Waddy wins this round. And is hopefully not a truck driver!

-- Dan Denney - Team Captain (, February 28, 2001.

The sailor is smater than the rest of you guys. I don't want to make it too easy for you guys so I'll give a hint.....think about NASAs KC135 that flies a parabola to acheive 0-G for the space program. Now image a load in the cargo hold.

-- apparently uncommon sense (, April 01, 2001.

Hey John, I am no expert on this subject. I just thought that I would give the guys something to argue about.

I am not sure about your theory being common sense though. If I am correct the KC135 is actually moving downward in the arc to create the illusion of 0-Gs. The truck never moves, only its cargo. If you think that the load is lifted from the bed of the truck, tell me where it is born. The birds just do not simply 'have no weight'.

Just thinking outloud.


-- Ron Lesseraux (, April 01, 2001.

I was always under the impression that wings generate lift by creating a vacuum on the trailing half of the upper wing surface- in essence, pulling the bird or plane up in to the vaccuum, rather than the bird or plane riding a mass of compressed air beneath the wing. Based on that, the trailer would register lighter on the scales if the birds inside it were flying. That also explains why I dislike the song, "Wind Beneath My Wings"- It ain't the wind beneath the wing that gets the job done.

-- Chip Haynes (, April 02, 2001.

I believe that all of those answers were quite logical and very well thought through just not complete, so here goes... according to newtons law their would be an amount of weight distributed down onto the scale whether it be from the suction of the vacuum created between the top of the trailor and the parrots, or the downward wind flow onto the bottom of the trailor and although this weight is not as much as the total weight of the resting birds because some of the pressure is placed on the sides and top of this trailor the load would be lighter than it would be had the birds not been in flight.

-- Aaron Cleveland (, February 19, 2003.

Moderation questions? read the FAQ