De-bunk a joke?

November 13, 2005 10:48 PM

When transporting parakeets, would keeping the birds airborne reduce the weight of the truck?

One of my father's favorite old jokes: While in line at a truck weigh-station, a trucker notices another trucker hitting the side of his trailer with a broomstick. When the trucker asked why he was doing this, the broom-sticker stated, "My rig's over-weight, but I'm hauling parakeets. I need to keep them in the air to make weight."

posted by jsteward to grab bag (25 comments total)

nope. this is an old physics chestnut. in order to stay aloft, the birds flapping their wings exert a downward force on the air inside the truck, which in turn exerts a downward force on the floor of the truck. that force must be equal to the weight of the birds, if they stay at the same height.
posted by sergeant sandwich at 10:52 PM on November 13, 2005

Exactly. Equal and opposite reaction.
posted by nanojath at 11:24 PM on November 13, 2005

No
posted by Mr T at 11:29 PM on November 13, 2005

Tackled by Unca Cec'

... though I coulda sworn he addressed the question more directly once.
posted by RavinDave at 11:29 PM on November 13, 2005

Would not the air pressure due to flight dissapate nearly instantly? The truck is not a closed atmosphere.
posted by Goofyy at 11:36 PM on November 13, 2005

What if the truck is not sealed? Any pressure in excess of atmospheric will be vented through the vents. There will be no downward force on the floor of the truck.
posted by five fresh fish at 11:40 PM on November 13, 2005

Which is, in fact, what Cecil says: if the system is not closed, the truck will in fact weigh less.
posted by five fresh fish at 11:42 PM on November 13, 2005

A somewhat related but interesting question is what happens to a helium filled balloon in an accelerating automobile. Passengers are pressed backward in their seats, but the balloon moves forward. Likewise when you drive around a curve, the passengers lean outward but the balloon moves inward.
posted by JackFlash at 12:01 AM on November 14, 2005

It's funny -- there was an "L" viaduct near where I lived in Evanston, and about once a year a delivery truck would get jammed up in it. The solution was invariably to release air from the tires.

The influence of this physics puzzle -- in which it is disproved that going over a bridge reduces the truck's weight -- has become transposed onto this trick. At least twice I've told someone in person the bit about the truck and the tires, and I've been told that's not true, that doesn't work.

Of course it works. I am glad people are skeptical, though.
posted by dhartung at 12:26 AM on November 14, 2005

Er, what bridge? What puzzle? Is there something missing from your post? Are you saying people believe that letting the tyres of a truck down doesn't lower the truck because of the thing with the birds? I am so confused right now.
posted by AmbroseChapel at 2:28 AM on November 14, 2005

What if the truck is not sealed? Any pressure in excess of atmospheric will be vented through the vents. There will be no downward force on the floor of the truck

The overall average pressure in the truck does not increase - when increasing the pressure under their wings, they at the same time decrease the pressure above it. Of course air could escape because of turbulence - but the same amount of air would float back in somewhere else.

A somewhat related but interesting question is what happens to a helium filled balloon in an accelerating automobile. Passengers are pressed backward in their seats, but the balloon moves forward. Likewise when you drive around a curve, the passengers lean outward but the balloon moves inward.

My first guess would be that the air gets pressed backwards/outwards just like the passengers - creating a higher air pressure in the back/outward side of the car. The helium balloon then floats in the direction of the lower pressure.

But I wonder if the effect is strong enough to be noticeable. Does this really work, or is this just a thought-experiment?
posted by uncle harold at 3:57 AM on November 14, 2005

Another point is that even if the air escaped out of the truck, it would do so *after* colliding with the floor on its way down, so there would still be pressure involved (think wind on a sailing ship).
posted by uncle harold at 4:03 AM on November 14, 2005

Of course if the birds where flying they would have to supply their own forward momentum, and would get slammed into the walls whenever you stopped or turned too sharp. So whilst in a closed system the weight would be the same, and in an open one the van would weigh less, you would probably end up with dead parakeets.

Of course the weight of the van would also oscillate with the flapping of the birds wings so at times it might actually weigh *more* than with the birds sitting still.

If you had a specially designed apparatus it would probably be possible to get the bird to hover whilst the van was driving forwards without it having to flap. It would still be tricky to keep the bird(s) moving forwards with the truck though.
posted by alexst at 4:05 AM on November 14, 2005

Another point is that even if the air escaped out of the truck, it would do so *after* colliding with the floor on its way down, so there would still be pressure involved (think wind on a sailing ship).

This depends on whether or not the bird was employing flapping or gliding flight to gain lift, as mentioned in the last paragraph of my previous post. If you had a strong enough air flow through the front of the truck then the bird could probably hover/glide quite well.

Obviously actually getting a bird to do any of this is still more or less impossible ;)
posted by alexst at 4:07 AM on November 14, 2005

So whilst in a closed system the weight would be the same, and in an open one the van would weigh less, you would probably end up with dead parakeets.

Is this the point where it is ok to start with the Monty Python references?
posted by uncle harold at 4:08 AM on November 14, 2005

In fact if it was using gliding flight it might even reduce the weight of the truck! You might need to tie it's feet to the floor though....
posted by alexst at 4:09 AM on November 14, 2005

Is this the point where it is ok to start with the Monty Python references?

Only if you where transporting Norwegian Blue's and not Parakeets.
posted by alexst at 4:10 AM on November 14, 2005

I've heard of similar where fishing ships fill their hulls beyond the displacement capacity of the ship but that as long as the fish keep momentum swimming in the hull the ship is not at risk of sinking. Highly dubious.
posted by furtive at 5:29 AM on November 14, 2005

AmbroseChapel: "Er, what bridge? What puzzle? Is there something missing from your post? Are you saying people believe that letting the tyres of a truck down doesn't lower the truck because of the thing with the birds? I am so confused right now."

Yes, that is essentially what dhartung is saying. People think "Wait, I've heard something with a truck before, and it made sense, but it ended up being wrong. This must be it."

dhartung: "I am glad people are skeptical, though."

Blind skepticism is just as foolish as blind acceptance. If you don't learn the correct lesson from the birds story, you might as well have never heard it. You're just a mouth saying words.
posted by Plutor at 7:19 AM on November 14, 2005

When it comes to physics problems, for every person there exists an amount of knowledge after which he or she becomes a "mouth saying words". That is, nobody is able to solve every physics problem.

Which is what I like about it.
posted by Eamon at 8:50 AM on November 14, 2005

AskMetafilter: You're just a mouth saying words.
posted by Four Flavors at 9:08 AM on November 14, 2005

This is one of those questions where my old, curmudgeonly quantum prof. would give you half of the answer, and then tell to get off your ass and actually do some empiricism. That is really the heart of physics, after all.

I tend to agree that the truck won't weigh less. But the question should ultimately be settled with experiment. So who's got a truck and a bunch of parakeets?
posted by teece at 10:25 AM on November 14, 2005

Uncle Harold regarding the balloon experiment: My first guess would be that the air gets pressed backwards/outwards just like the passengers - creating a higher air pressure in the back/outward side of the car. The helium balloon then floats in the direction of the lower pressure.

But I wonder if the effect is strong enough to be noticeable. Does this really work, or is this just a thought-experiment?

You have it exactly right -- there is a pressure differential in an accelerating car. So would this be noticeable? Well, if you accelerated at a rate of 1G, then the horizontal and vertical forces on the balloon would be equal so that the string on the balloon would be at 45 degrees. But 1G is a lot of acceleration -- about 22 mph per second. A more realistic acceleration would be perhaps 22 mph in 4 seconds or 1/4 G. So now the string on the balloon would have an angle of arcsin(.25) or about 15 degrees -- small but noticeable.
posted by JackFlash at 10:54 AM on November 14, 2005

That, of course, should be arctan, but for small angles is nearly the same.
posted by JackFlash at 11:03 AM on November 14, 2005

Surely you can test for this problem. You stand on a bathroom scale. You do a momentary mimicking of a parakeet's flight by jumping off the scale. (Arm flapping is optional.) The scale should register 0 when you're not on it. Unless you're eating porta/o/bellos/bella. In which case all bets are off. Or bettsaroffa. Or something.
posted by firstdrop at 12:15 PM on November 14, 2005