# A Fly in a Moving Car

#### livingsacred

In a car moving forward at 60 mph a fly is buzzing freely. If the car suddenly comes to a complete stop, does the fly crash into the windshield?

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#### mbisCool

Why don't you test it out?

#### Stratosphere

In a car moving forward at 60 mph a fly is buzzing freely. If the car suddenly comes to a complete stop, does the fly crash into the windshield?
Of course.

#### Ivan Seeking

Staff Emeritus
Gold Member
Smoke, however, will move towards the back of the car. Why?

#### NeoDevin

Smoke, however, will move towards the back of the car. Why?
Same reason it floats upwards.

#### humanino

Smoke, however, will move towards the back of the car. Why?
Same direction as an inflatable balloon I would guess : because it's afraid of the windshield of course.

#### humanino

Same reason it floats upwards.
What a deep statement... indeed gravity is also an acceleration, and we're all afraid of falling.

#### Ivan Seeking

Staff Emeritus
Gold Member
Okay, how about the classic: If a bird is sitting inside and on the bottom of a sealed box, and the bird begins to fly - hovers in place inside of the box - does the box lose weight?

#### humanino

Okay, how about the classic: If a bird is sitting inside and on the bottom of a sealed box, and the bird begins to fly - hovers in place inside of the box - does the box lose weight?
The flying bird will release heat, increasing the temperature of the box which will radiate energy away : the box does loose weight until the bird dies of hunger.

#### junglebeast

Okay, how about the classic: If a bird is sitting inside and on the bottom of a sealed box, and the bird begins to fly - hovers in place inside of the box - does the box lose weight?
I'm thinking that the wing beats create a downward air pressure that increases the weight of the box.
edit -- hmm...that's probably wrong since there is no source of new air...

#### physics girl phd

Okay, how about the classic: If a bird is sitting inside and on the bottom of a sealed box, and the bird begins to fly - hovers in place inside of the box - does the box lose weight?
The mass "m" of the box itself doesn't change, and the height certainly doesn't change significantly enough to change the value of little "g" (even if it's bouncing around), so the weight of the box doesn't change. Now if the box was originally sitting on a scale, and you wanted the net force of the box/bird system on the scale, that's a different question.

#### Andre

Smoke, however, will move towards the back of the car. Why?
That would only be true if smoke is heavier than air. But is it?

Test it! have somebody who smokes (not me) blow a bubble filled with smoke and compare the speed at which it sinks with that of a normal bubble.

Then why does smoke rise? Not because it is lighter than air.

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#### Blenton

Yes the box on the scale would see a decrease ~ the bird is no longer part of the boxes structure.

#### NeoDevin

Okay, how about the classic: If a bird is sitting inside and on the bottom of a sealed box, and the bird begins to fly - hovers in place inside of the box - does the box lose weight?
The net downward force of the air on the bottom of the box has to equal the net upward force of the air on the bird. There'll be slight fluctuations, as the bird flaps and falls, but on average no weight will be lost.

#### tiny-tim

Homework Helper
Then why does smoke rise? Not because it is lighter than air.
I think it's because the smoke is hot, and heats the air, and the smoke is then carried on the updraft of hot air.

That's why smoke will be attracted to a chimney even if the chimney is not directly above … it follow the air current.
In a car moving forward at 60 mph a fly is buzzing freely. If the car suddenly comes to a complete stop, does the fly crash into the windshield?
Of course.
I dunno … won't the fly reach terminal velocity very quickly … alternatively, won't the air suddenly become very "viscous"?

#### Pythagorean

Gold Member
Yes the box on the scale would see a decrease ~ the bird is no longer part of the boxes structure.
actually, it is part of the boxes structure through the air in the box. If the bird lifts up off the box, he will have to be exerting force downwards (on the air) some of which will push down on the box, registering on the scale.

Of course, the full weight of the bird probably won't show up on the scale as some of the air molecules push against the sides of the box, which doesn't register on the scale. I don't know how they would relate mathematically, but I'm guessing you'd have to use navier-stokes or some what crap.

#### Ivan Seeking

Staff Emeritus
Gold Member
That would only be true if smoke is heavier than air.
Really?

Btw, I have tried it.

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#### Ivan Seeking

Staff Emeritus
Gold Member
Now if the box was originally sitting on a scale, and you wanted the net force of the box/bird system on the scale, that's a different question.
Alright, the box is sitting on a scale and the bird begins to fly inside of the box... :tongue:

It was intended as a tricky question but not a trick question.

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#### humanino

Alright, the box is sitting on a scale and the bird begins to fly inside of the box... :tongue:
Can you provide the accuracy of the scale, an order of magnitude for the proper mass of the box (not counting air inside), the inner volume of the box, the mass of the bird, and also the temperature and pressure if they are not standard. Then we could make an actual statement : this is not measurable...

#### Pythagorean

Gold Member
Can you provide the accuracy of the scale, an order of magnitude for the proper mass of the box (not counting air inside), the inner volume of the box, the mass of the bird, and also the temperature and pressure if they are not standard. Then we could make an actual statement : this is not measurable...
just keep it theoretical

V = volume
m = mass of bird
M = mass of box

etc...

#### junglebeast

actually, it is part of the boxes structure through the air in the box. If the bird lifts up off the box, he will have to be exerting force downwards (on the air) some of which will push down on the box, registering on the scale.

Of course, the full weight of the bird probably won't show up on the scale as some of the air molecules push against the sides of the box, which doesn't register on the scale. I don't know how they would relate mathematically, but I'm guessing you'd have to use navier-stokes or some what crap.
Although the air pressure directly the bird will be greater, if the box is large enough to hold a flying / flapping bird, then the air molecules (which can travel much faster than the bird's wing) will likely dissipate that downward pressure so that it is a uniform pressure in all directions, such that the added weight to the box is an imperceptible fraction of the bird's weight.

#### humanino

just keep it theoretical
Fine, but I already answered : the mass decreases due to loss of energy by electromagnetic radiation ! This is obviously negligible compared to even the thermal fluctuations of the scale, but theoretically, it is the only energy out of the box.

#### Pythagorean

Gold Member
Although the air pressure directly the bird will be greater, if the box is large enough to hold a flying / flapping bird, then the air molecules (which can travel much faster than the bird's wing) will likely dissipate that downward pressure so that it is a uniform pressure in all directions, such that the added weight to the box is an imperceptible fraction of the bird's weight.
yeah, I think that would be a specific case. While we're building the equation to describe this physical event, we should include the term and if epsilon goes to zero, so be it, but then we can find the critical point (box volume to bird's wing surface area or something) at which this is true.

Gold Member

#### junglebeast

just keep it theoretical

V = volume
m = mass of bird
M = mass of box

etc...
This question is governed by the chaotic fluid dynamics of air currents. How can you write that as a simple algebraic equation without neglecting the factors that actually cause the real world result? If you only look at mass, volume, etc, you would conclude that an airplane cannot possibly fly because you ignored the effect of air resistance.

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