# Brain Teasers

### Brain Teasers

My answer in the chimney thread made me think about some of the brain teasers my friends & I used to ponder & debate.
Here's one we actually ended up calling the physics dept. of MIT to get resolved:

Situation:

An airliner is siting on the ground on a very sensitive scale.
The cabin is sealed & pressurized
Within the airliner there is a live, free bird.
When the bird is perching on the back of a seat, the aircraft weighs "X" (the weight of the aircraft itself)

PLUS

2 ounces (the weight of the bird)

Question:

When the bird leaves his perch & begins to fly around the cabin, what does the aircraft now weigh?

1. Same as before "X" + 2 ounces?
2. Just "X"?

or

3. "X" - 2 ounces?
Devil505

Stove/Furnace Make: Harman
Stove/Furnace Model: TLC-2000

### Re: Brain Teasers

The same I believe as the bird creates downward pressure when it flies...

There was mythbusters show on this and I beleive that was the results if memory serves me correctly. They did it in a container instead of a airplane.
Richard S.

Stoker Coal Boiler: Van Wert VA1200
Coal Size/Type: Buckwheat/Anthracite

### Re: Brain Teasers

Richard S. wrote:The same I believe as the bird creates downward pressure when it flies...

I'll wait for all any other guesses first Richard but the answer comes straight from a physics professor at MIT when I give it.
Devil505

Stove/Furnace Make: Harman
Stove/Furnace Model: TLC-2000

### Re: Brain Teasers

Richard S. wrote:The same I believe as the bird creates downward pressure when it flies...

Just to make you think a bit more:

The scale measures weight, not air pressure. Why would the air pressure (created by the flapping wings I assume you mean) affect the weight of the sealed aircraft in any way?....or maybe it does??

Now, suppose we de-pressurize the plane & open a window?..Any difference?
Devil505

Stove/Furnace Make: Harman
Stove/Furnace Model: TLC-2000

### Re: Brain Teasers

It would be the the pressure on the floor, which would be measured as weight by the scale assuming its outside the plane. If the window is open I'd imagine it would no longer matter. Again I seen it on mythbusters and I believe that was the conclusion but I could be wrong as it was a while ago..
Richard S.

Stoker Coal Boiler: Van Wert VA1200
Coal Size/Type: Buckwheat/Anthracite

### Re: Brain Teasers

Richard S. wrote:If the window is open I'd imagine it would no longer matter.

1. X
2. X +2 ounces
3. X-2 ounces
Devil505

Stove/Furnace Make: Harman
Stove/Furnace Model: TLC-2000

### Re: Brain Teasers

Devil5052 wrote:Why would the air pressure affect the weight of the sealed aircraft in any way?....or maybe it does??

Now, suppose we de-pressurize the plane & open a window?..Any difference?

The plane will weigh more with the cabin under pressure than it will with the window open.
coaledsweat

Stoker Coal Boiler: Axeman Anderson 260M
Coal Size/Type: Pea

### Re: Brain Teasers

coaledsweat wrote:The plane will weigh more with the cabin under pressure than it will with the window open.

Why, if the air used to pressurize the plane was already contained in a cylinder on board?

and

Why would that affect my original question in any case?
Devil505

Stove/Furnace Make: Harman
Stove/Furnace Model: TLC-2000

### Re: Brain Teasers

You didn't say from a cylinder on board and I did not say it would have an affect on the original question.
coaledsweat

Stoker Coal Boiler: Axeman Anderson 260M
Coal Size/Type: Pea

### Re: Brain Teasers

coaledsweat wrote:You didn't say from a cylinder on board and I did not say it would have an affect on the original question.

I am saying it now
Last edited by Devil505 on Fri Aug 08, 2008 5:34 pm, edited 1 time in total.
Devil505

Stove/Furnace Make: Harman
Stove/Furnace Model: TLC-2000

### Re: Brain Teasers

My view is the air in the cabin has weight. Therefor when the bird is in flight inside of the cabin it is producing lift to counter its weight but the lift is contained in the air buble carried by the aircraft so no weight change. X+2

If you remove the air I would expect a weight of X+2 minus the weight of the absent air
Steve.N

Stove/Furnace Make: Harman mkII
Stove/Furnace Model: Axeman Anderson 260 at store

### Re: Brain Teasers

OK...Here's the answer directrly from MIT physics:

The answer (whether pressurized or not...windows open or not) is..............

Drum roll

Just X

When the bird takes flight it's wings support it's weight in the air & therefore it's weight is no longer added to the aircraft.
Air pressure could be changed to a tiny degree but that would still have no affect on the weight of the aircraft.

(for those of you who reflexively doubt anything I say... ....Call your own damned University!! :
Devil505

Stove/Furnace Make: Harman
Stove/Furnace Model: TLC-2000

### Re: Brain Teasers

Here's another (easier) one.

Set up:

A 45 caliber bullet is held exactly 5' above a flat stretch of desert
A 45 Caliber pistol is clamped exactly 5' above the same desert & sighted perfectly horizontal to the ground.

The bullet is dropped (straight down) & the pistol is fired at exactly the same instant.

Which round hits the ground first?

1. The dropped one
2. The one fired from the pistol
3. Both hit the ground at the same time

(If you've seen/heard this teaser b4, please don't give it away)
Devil505

Stove/Furnace Make: Harman
Stove/Furnace Model: TLC-2000

### Re: Brain Teasers

Is that perfectly horizontal at the gun's mount or the bullets intended trajectory?
coaledsweat

Stoker Coal Boiler: Axeman Anderson 260M
Coal Size/Type: Pea

### Re: Brain Teasers

coaledsweat wrote:Is that perfectly horizontal at the gun's mount or the bullets intended trajectory?

Not sure if I understand your question but the barrel of the gun is perfectly horizontal (parallel) to the ground. (not aimed up or down) The bullet's trajectory will obviously be an arc, impacting the ground a some distance to the firing point, due to gravity.
Devil505

Stove/Furnace Make: Harman
Stove/Furnace Model: TLC-2000