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Nick
2008-Dec-24, 12:13 PM
A question that always raises a debate (usually in pubs) when I ask it - and I still don't know the correct answer.

An enclosed van with a large cage in the back is transporting budgies to the local pet shop. Now, (ignoring acceleration/deceleration) the van comes to a long steep hill.

Does the van require more power to climb the hill at a constant rate if the budgies are all sitting on their perches, or if they are all in flight flitting about (i.e. will the van climb the hill faster at the same power if the budgies are flying around rather than being a dead weight)?

Ideas?

Nick

geonuc
2008-Dec-24, 12:50 PM
One thing to keep in mind which will help you arrive at the correct answer is that if the budgies are in flight when the van accelerates from a standstill to that constant rate, they are unrestrained. What happens to you when you are unrestrained in an accelerating vehicle?

Tog
2008-Dec-24, 01:07 PM
The official answer is that the air displaced by the wings to keep the birds in the air must be the same mass as the birds while at rest. Basically, the amount of matter in the van didn't change, and none of it converted to energy (though that's not technically true which will probably drive this thread to 38 pages by tomorrow).

So, if all of the matter is still present, then all the mass must be present as well.

Now, that's the answer I've always heard. It may not be the correct one, so we'll see how this works out.

Also, this might be more of a Q&A or Babbling thing than a Fun and Games.

cosmocrazy
2008-Dec-24, 04:14 PM
Because the birds are un-restrained they become independent of the vehicle, whilst in flight in the van, they have to maintain a forward flight motion to prevent all ending up against the back of the cage as the van accelerates to climb the hill. Therefore they use their own energy to maintain forward motion. So the van requires less energy to climb the hill. I guess?

hhEb09'1
2008-Dec-24, 04:26 PM
And what about a van (enclosed, as in the OP) full of helium balloons? When they are tied to the floor, or when they are drifting upwards, or when they come to rest against the ceiling? :)

cosmocrazy
2008-Dec-24, 04:45 PM
And what about a van (enclosed, as in the OP) full of helium balloons? When they are tied to the floor, or when they are drifting up wards, or when they come to rest against the ceiling? :)

Exactly.
The balloons are relying on the energy of the van to move, whether they are free floating or restrained. As the van accelerates the balloons will tend to be left behind and press against the back of the enclosure other than being pushed by the air molecules. In a sense they are restrained by the air trapped in the van. The air will not restrain the budgies, they have to use energy to maintain flight and unless they maintain a forward motion during that flight they will all end up at the back of the cage when the van accelerates. Each bird uses energy to maintain a forward motion of their own mass. This mass is deducted from the van because each bird becomes its own independent accelerating mass. As soon as a bird perches then that mass is added to the van's and the van requires more energy to accelerate. :)

hhEb09'1
2008-Dec-24, 05:14 PM
Exactly.
The balloons are relying on the energy of the van to move, whether they are free floating or restrained. As the van accelerates the balloons will tend to be left behind and press against the back of the enclosure other than being pushed by the air molecules.Actually (and weirdly I have done this) the balloons will go forward when the van accelerates forward.

cosmocrazy
2008-Dec-24, 05:21 PM
Actually (and weirdly I have done this) the balloons will go forward when the van accelerates forward.

That makes sense, the force of the trapped denser air being pressed against the back of the enclosure as the van accelerated would easily move the lighter balloons forward (a displacement effect). I suspect in a vacuum, not only would the balloons obviously not float but they would end up at the back of the enclosure.

hhEb09'1
2008-Dec-24, 05:36 PM
This mass is deducted from the van because each bird becomes its own independent accelerating mass. I thought the example of the balloons would show that they are not independent--in an enclosed van.

cosmocrazy
2008-Dec-24, 05:50 PM
Well they are not independent from the van while they are restrained in some form, whether thats by being perched or from the pressure of the air the are confined in (if the pressure or density is high enough to allow them to float without effort).

I think i can see where you are coming from and may see an error in my line of thinking. One could say the air in the van is accelerating forward while the birds float on that air, they are being carried forward by that air which in turn is moved forward by the van. So the overall mass of the system remains constant regardless, so the energy required to accelerate the van and its contents is the same regardless.(unless the birds assisted in some forward motion of their own) I think?

hhEb09'1
2008-Dec-24, 06:03 PM
I think that's the gist of the puzzle, but of course there are variants (what if the birds fly to the back?)
The official answer is that the air displaced by the wings to keep the birds in the air must be the same mass as the birds while at rest. Basically, the amount of matter in the van didn't change, and none of it converted to energy (though that's not technically true which will probably drive this thread to 38 pages by tomorrow).

So, if all of the matter is still present, then all the mass must be present as well.

Now, that's the answer I've always heard. It may not be the correct one, so we'll see how this works out.I'm not sure what distinction you are making there between matter and mass?

Also, this might be more of a Q&A or Babbling thing than a Fun and Games.I agree.

01101001
2008-Dec-24, 07:08 PM
Wikipedia: Mythbusters :: Birds in a Truck (http://en.wikipedia.org/wiki/MythBusters_(season_5)#Birds_in_a_Truck)


[Myth:] Birds flying around inside a truck will actually make the truck lighter. [Result:] Busted

cosmocrazy
2008-Dec-24, 07:15 PM
ok, lets change the scenario slightly and take gravity out of the equation. Lets say we had a large rocket cruising along in gravity free space. The birds are contained in a large air tight enclosure that has an atmosphere the same consistency and pressure as that on earth. The birds are free to fly about at will, ok they need not apply an "up wards" flight because they are in a gravity free environment. When the rocket accelerates the birds will surely end up at the back of the container, admittedly there would be some resistance form the air inside but not enough to prevent the birds hitting the back unless they maintained a forward motion independent of the rocket? So the rocket surely would only need enough energy to accelerate its own mass + a bit of air Resistance between the birds and the enclosure.Until the birds became "perched" on to the back of the container. Or would the air resistance applied by the birds forward motion equal enough pressure on the container to keep the overall energy acceleration requirements the same regardless?

Thinking about it more i guess if you have some sort of restraint between the objects then the mass of the system remains the same. An analogy would be to place an object at rest at the centre within a large container in zero gravity and a vacuum. If the container is accelerated then until the moment the object meets the wall of the container it would be totally independent. As soon as the object is restrained by matter or force then the mass of the whole system applies.

mugaliens
2008-Dec-24, 08:39 PM
Perched, flying around, or lying dead on the floor, the mass inside the van is the same, therefore the energy required to climb at a constant v up the hill remains the same.

Taking gravity out of the equation does nothing - the mass is the same. Therefore, F=ma, same mass, same force, results in the same acceleration.

cosmocrazy
2008-Dec-24, 10:34 PM
Perched, flying around, or lying dead on the floor, the mass inside the van is the same, therefore the energy required to climb at a constant v up the hill remains the same.

Taking gravity out of the equation does nothing - the mass is the same. Therefore, F=ma, same mass, same force, results in the same acceleration.

yes i can see how my initial line of thought was incorrect.


Thinking about it more i guess if you have some sort of restraint between the objects then the mass of the system remains the same. An analogy would be to place an object at relative rest at the centre of a large container in zero gravity and a vacuum. If the container is accelerated then until the moment the object meets the wall of the container it would be totally independent. As soon as the object is restrained by matter or force then the mass of the whole system applies

but is this correct?

Tog
2008-Dec-25, 08:56 AM
I'm not sure what distinction you are making there between matter and mass?

Honestly, neither am I. That was just the way I heard the question answered long ago. If nothing leaves the van, then the van won't know care where anything in it happens to be or what it's doing.

A variation that I thought about today (which is not new, I'm sure), is what of the back of the van wasn't sealed? What if there were a pair of openings at the front and rear of the cargo compartment?

There would be airflow through the box, and any air in the box that was displaced by the flapping birds could be pushed out, but more air would be coming in all the time. In this situation, I think the van would feel the weight of the load reduced, but the mass in the system should still be the same. How would this really work?

Also, I just thought of a tangent that I'd thought of years ago. I feel a new thread coming on...

mugaliens
2008-Dec-25, 11:54 AM
but is this correct?

Yep!

NEOWatcher
2008-Dec-30, 02:42 PM
An analogy would be to place an object at rest at the centre within a large container in zero gravity and a vacuum. If the container is accelerated then until the moment the object meets the wall of the container it would be totally independent. As soon as the object is restrained by matter or force then the mass of the whole system applies.
I'm not sure you are implying this, but I'd like to point out...
When the independent mass meets the container, then that mass needs energy to accelarate to the speed of the container.
That comes from either more power to keep the instantaneous acceleration, or from a loss of acceleration of the container. In other words, we need to note what happens on meeting the wall. Once it does, it's one system.


A variation that I thought about today (which is not new, I'm sure), is what of the back of the van wasn't sealed?
That would essentially be the same as if the van was an "open" cage if we don't want to complicate things like the size of the hole with pressure differentials, and flow frictions.

astromark
2008-Dec-30, 08:39 PM
"Better idea" Tie a string to each budgie and turn off the motor....

cosmocrazy
2008-Dec-30, 11:09 PM
I'm not sure you are implying this, but I'd like to point out...
When the independent mass meets the container, then that mass needs energy to accelerate to the speed of the container.
That comes from either more power to keep the instantaneous acceleration, or from a loss of acceleration of the container. In other words, we need to note what happens on meeting the wall. Once it does, it's one system.


Yes thats the point i'm making. Once the independent mass makes contact with the container the accelerating container requires more energy to accelerate the once previous independent object thus making it one system with now greater mass needing greater energy to maintain acceleration.

cosmocrazy
2008-Dec-30, 11:10 PM
"Better idea" Tie a string to each budgie and turn off the motor....

Yeah let them do the work! :lol: