Does a Fly Crash Into the Windshield When a Car Suddenly Stops?

[OAQfirst]

Don't you just love how A fly in a moving car goes to the bird in the box question.
:D

Oh boy. Now we'll have to deal with the fly in a box tied to a bird in a car scenario!

 

[Ivan Seeking]

Don't you just love how A fly in a moving car goes to the bird in the box question.
:D

Someone had to make it interesting.

 

[DaveC426913]

hummingbirds can hover, and this particular hummingbird is very cooperative.

Yes they can, but the experiment was about whether the bird's flight would make the box lighter by lifting it. So I don't see how you could have been imagining the scenario.

 

[Ivan Seeking]

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.

Of course you are ignoring energy going into the box.

 

[Ivan Seeking]

Oh boy. Now we'll have to deal with the fly in a box tied to a bird in a car scenario!

A mechanism is designed such that after a predetermined period of time, there is a 50% chance that a bird is alive and hovering in a sealed box that is sitting on a scale. If we don't look at the scale...

 

[Pythagorean]

Yes they can, but the experiment was about whether the bird's flight would make the box lighter by lifting it. So I don't see how you could have been imagining the scenario.

I thought this was about whether the bird's mass contributed to the scale's measurement when the bird wasn't in contact with the ground. As the hummingbird hovers above the floor of the box, he's still exerting a force downward to keep himself up. This should register on the scale.

I'm not well versed in fluid dynamics, but I would think, even in a sealed box, that as long as it's not a perfectly rigid box, that some of that energy will be absorbed by the box itself. I'm also assuming there's also some amount of turbulence that would absorb the energy in collisions between air molecules.

 

[Pythagorean]

Of course you are ignoring energy going into the box.

yeah, that, and air molecules colliding with each other, however (in)significant that would be.

 

[humanino]

Of course you are ignoring energy going into the box.

No, I have assumed thermodynamical equilibrium to begin with and failed to find an incoming source of energy from an undefined external thermal bath. Although strictly speaking, by thermal equilibrium we should have the bird dead and decomposed long ago. Unless the cat comes in... Is it what you suggest ?

 

[NeoDevin]

I'm not well versed in fluid dynamics, but I would think, even in a sealed box, that as long as it's not a perfectly rigid box, that some of that energy will be absorbed by the box itself. I'm also assuming there's also some amount of turbulence that would absorb the energy in collisions between air molecules.

Conservation of momentum: To support itself in the air, the bird must exert a force on the air molecules around it, giving them some (downward) momentum. That momentum cannot be 'dissipated' by the side of the box, or by diffusion.

 

[Office_Shredder]

Assume a spherical frictionless bird...

 

[Andre]

Really?

Btw, I have tried it.

well here is an interesting demo too.

izLjvAm-bG8[/youtube] Initially ...emperature, it's definitely heavier than air.

 

[BobG]

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?

If the interior of the car is a vacuum, the fly will crash into the windshield at 60 mph (plus or minus the fly's current velocity). Or, alternatively, if the car smashes into a rock wall at 60 mph, the fly will crash into the windshield, but at less than 60 mph.

In practice, the fly is a lot less likely to crash into the windshield than the driver, since the fly has a lot lower terminal velocity. (In fact, using the fly as an example instead of the driver is just obfuscation.)

The box with the bird won't change it's net weight since either the bird or the air displaced by the bird will push down on the bottom of the box with a constant amount of force (the air pressure caused by the bird's weight or the flapping of its wings is related to the fly scenario, since it's the fly's inability to compress the air between it and the windshield that would keep it from hitting the windshield in most real world situations).

The fact that the air molecules have momentum carrying them into the windshield is also why the helium balloon will move to the back of the car during a collision. The momentum of the air molecules displace the balloon, which has less momentum than the air molecules (p=mv; the balloon has the same velocity as the air, but less mass for the same volume).

The more interesting question is whether the fly buzzing around in a car that drives into a rock wall at 60 mph would hit the windshield at a high enough speed to leave a bug splat. If you hit a bug driving down the highway at 60 mph, the bug leaves a splat, but the air can escape from between the bug and the windshield. Inside the car, the volume is restricted and the overall air pressure in the car can't change just because the car came to a stop.

Actually, you can simulate this very easily by throwing a fly at a wall as hard as you can. It's not a perfect simulation since the average room allows a lot more airflow than your average car, but the fact that I can't throw a fly hard enough to leave a bug splat suggests there's no way the fly will leave a splat on the inside of the window in a 60 mph collision (maybe, since I may not actually be able to throw anything at even a paltry 60 mph).

Throwing a fly at a wall as hard as I can will give the fly a concussion and it will lie around on the floor completely stunned. This makes it a lot easier to put into a vial, which can be stashed into the freezer. The low temperature leaves the fly sluggish or unconscious so I can add more flies later on.

 

[Ivan Seeking]

Initially the smoke bubbles rise, suggesting that smoke is lighter than air, however if we are patient enough to watch the whole trailer, we see them settling back again.

We can't see how the bubbles are made, but hot smoke rises because of the lesser density or the air, like it does above the fire, that's convection. However, when smoke cools to ambient temperature, it's definitely heavier than air.

You are missing the point. The cold air is heavier than warm air, so when one hits the brake, the heavier air goes towards the front of car, and the warmer air [and smoke or a helium balloon] go towards the rear of the car.

 

[Andre]

No as cold smoke is heavier than air, it will move towards the front. A helium balloon would move to the back

Another demonstration of heavy smoke:

 

[Ivan Seeking]

No as cold smoke is heavier than air, it will move towards the front. A helium balloon would move to the back

I didn't say cold smoke. And as I said, I've done it. Besides that, your initial objection was that my suggestion would imply that smoke is heavier than air. So now you have completely reversed your position.

 

[Ivan Seeking]

That would only be true if smoke is heavier than air. But is it?.

So as I said, you have completely reversed your position.

 

[BobG]

We can't see how the bubbles are made, but hot smoke rises because of the lesser density or the air, like it does above the fire, that's convection. However, when smoke cools to ambient temperature, it's definitely heavier than air.

You are missing the point. The cold air is heavier than warm air, so when one hits the brake, the heavier air goes towards the front of car, and the warmer air [and smoke or a helium balloon] go towards the rear of the car.

I don't think Andre is missing the point. Cold air is denser than warm air, not heavier. If the cold air and the warm air have the same chemical composition, one mole of cold air will have the same mass as one mole of warm air. If the mole of cold air and warm air were each enclosed in a balloon in a vacuum, they'd each weigh the same (even though the balloon of warm air would be bigger).

You'd need to know the chemical composition of the "smoke" in question, but I think it's safe to say that most "smoke" would have a higher atomic mass than normal air (but not definite - if the "smoke" were actually steam, that would be one obvious exception in more ways than one).

Smoke is not the same as a helium balloon, though. I think it's safe to say the only way the helium balloon is going to be "heavier" than the rest of the atmosphere is if the rest of the atmosphere happens to be hydrogen.

 

[Pythagorean]

when they've been saying "heavier than", "lighter than", I've been taken it to mean "heavier per unit volume" and "lighter per unit volume" (i.e. density).

It would be odd to call one gas heavier than the other while secretly assuming two different volumes but not letting anyone know about it.

 

[Ivan Seeking]

when they've been saying "heavier than", "lighter than", I've been taken it to mean "heavier per unit volume" and "lighter per unit volume" (i.e. density).

It would be odd to call one gas heavier than the other while secretly assuming two different volumes but not letting anyone know about it.

Yes, I thought that was pretty obvious. It is also a common reference.

 

[Ivan Seeking]

You'd need to know the chemical composition of the "smoke" in question, but I think it's safe to say that most "smoke" would have a higher atomic mass than normal air (but not definite - if the "smoke" were actually steam, that would be one obvious exception in more ways than one)

In any event, Andre's position completely reversed and the initial objection was exactly incorrect.

I said the smoke in a braking car will move to the back of the car. Why? The answer is that the smoke is lighter [less dense] than air, not heavier [more dense]. It doesn't matter what the smoke is made of given the result observed.