# Bird in a box

#### cisamcgu

##### Legendary Member
This may have been posted before, if so, please ignore

A box weighs 1000g
A canary weighs 10g

Put canary in box, and then weigh them. Total weight = 1010g

Canary starts to fly in the box (OK, rather tricky, but bare with me) - what is the weight now ? (I think still 1010g)

Now, leaving the box on the scales, open the lid of the box, but let the canary land on the floor of the box. What is the weight now ? (I think still 1010g)

What happens if the canary flies, but stays within the open box ? What is the weight then ? 1000g ? 1010g ?

What happens if the canary starts to leave the box, but stays very close to the open lid ? (I have no idea about this bit )

Any thoughts ?

Andrew

#### Arch

##### Married to Night Train
You are Mr Hippo, and I claim my five pounds.

cisamcgu said:
A box weighs 1000g
A canary weighs 10g

Put canary in box, and then weigh them. Total weight = 1010g

Canary starts to fly in the box (OK, rather tricky, but bare with me) - what is the weight now ? (I think still 1010g)

Now, leaving the box on the scales, open the lid of the box, but let the canary land on the floor of the box. What is the weight now ? (I think still 1010g)

What happens if the canary flies, but stays within the open box ? What is the weight then ? 1000g ? 1010g ?

What happens if the canary starts to leave the box, but stays very close to the open lid ? (I have no idea about this bit )

Any thoughts ?

Hmm. Depends. Do you want the total weight of the items, or the weight shown on the scales.

I don't see how the weight of the canary registers if it's flying. If it's sitting on the floor or the edge of the box it counts, if it's flying it doesn't, because it's not exerting a pressure on the scales. It still weighs 10g or whatever, but it's not on the scales.

So, in terms of weight displayed on scales, it'll be a) 1010g, 1000g, c) 1010g and d)1000g.

#### alecstilleyedye

##### nothing in moderation
Moderator
what if the canary poops in the box?

#### Tetedelacourse

##### New Member
Arch is right i think.

OP
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#### cisamcgu

##### Legendary Member
lol - I'm not Mr Hippo

No, I ask it because I was on hoiliday last week (in The Netherlands - marvellous cycling country; rather flat, but so, so civilised), and mentioned the "plane on a treadmill problem" to a friend. He replied with the "bird in a box" - which I still cannot get my head around. When you open the lid, does the weight change ????

Andrew

#### Mr Celine

##### Discordian
The gram is a measure of mass and not weight. The mass of the box includes the canary, the box and the air inside it so does not change.

The weighing scales are showing the effect of gravity on that mass. I don't think that a canary can hover, but if it can then the air it pushes downwards to maintain its hover will have the same effect on the scales as if the canary was perched in the box.

OP
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#### cisamcgu

##### Legendary Member
Hmm. Depends. Do you want the total weight of the items, or the weight shown on the scales.

I don't see how the weight of the canary registers if it's flying. If it's sitting on the floor or the edge of the box it counts, if it's flying it doesn't, because it's not exerting a pressure on the scales. It still weighs 10g or whatever, but it's not on the scales.

So, in terms of weight displayed on scales, it'll be a) 1010g, 1000g, c) 1010g and d)1000g.

But the mass of the box+canary must be the same even if the canary is in the air of not. Surely conservation of mass must still be true, otherwise where has the mass of the bird gone ?

#### Arch

##### Married to Night Train
Mr Celine said:
The gram is a measure of mass and not weight. The mass of the box includes the canary, the box and the air inside it so does not change.

The weighing scales are showing the effect of gravity on that mass. I don't think that a canary can hover, but if it can then the air it pushes downwards to maintain its hover will have the same effect on the scales as if the canary was perched in the box.

Not unless the wings of the canary form an airtight seal with the box surely, and the pushed down air will escape up and around the wing? I don't think the downdraft of a canary wing would exert enough force to affect the scales, unless they were extremely sensitive.

#### bonj2

##### Guest
cisamcgu said:
But the mass of the box+canary must be the same even if the canary is in the air of not. Surely conservation of mass must still be true, otherwise where has the mass of the bird gone ?

The scales aren't measuring mass, they're measuring weight.

The only 'mysticism' associated with this apparent conundrum relies on the reader not understanding the difference between mass and weight. Which no offence cisamcgu but you obviously don't.

#### Cycling Naturalist

##### Guru
I hate to disagree with the fair Arch, but the box is a closed system and when the canary takes off, it can only do so by exerting enough pressure with its wings to overcome gravity. In other words it forces air downwards to the bottom of the box which will register as weight if the box is weighed. The weight will be less than 10g though because of the effect of turbulence as the down draught is dissipated.

If you don't believe me - think of a rocket taking off and the effect of the blast on the launch pad.

#### Cycling Naturalist

##### Guru
[QUOTE40181]
bonj;40173 said:
cisamcgu" said:
But the mass of the box+canary must be the same even if the canary is in the air of not. Surely conservation of mass must still be true, otherwise where has the mass of the bird gone ?

The scales aren't measuring mass, they're measuring weight.

The only 'mysticism' associated with this apparent conundrum relies on the reader not understanding the difference between mass and weight. Which no offence cisamcgu but you obviously don't.

No it doesn't. You misunderstand. As Patrick said, under certain conditions the flying bird would exert force on the bottom of the box.[/QUOTE]

Think of a hummingbird which also exerts a direct downforce.

#### Arch

##### Married to Night Train
Patrick Stevens said:
The weight will be less than 10g though because of the effect of turbulence as the down draught is dissipated.

Well, I think I'd almost come round to that conclusion...

I suspect the whole thing is designed as a heffalump trap for people like me to fall into. Which I did.

#### Arch

##### Married to Night Train
Or a stoat wearing a jet pack.

Or a vole on a pogo stick... Oh, hang on...

OP
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#### cisamcgu

##### Legendary Member
So - when the lid is closed, the scales will measure the both the bird and the box, irrespective of if it is flying or not(mass or weight, bonj - I know the difference, and it doesn't matter in this case (I think lol))

But what happens when the lid is opened, but the bird is still flying in the box ? (Hummingbird is better than canary)

Andrew