Thought Experiment to argue about.....

From a reference point in the car, the air would be accelerated toward the front of the car. The fly would also be accelerated in the same direction. The air in front of the fly would become denser, increasing the drag as the fly tries to continue towards the front. Then the air would start to settle back causing the fly to have to fly against a slight "headwind." For a short time the air density might go back and forth like water sloshing in a bucket, causing the fly to experience cycles of headwinds and tailwinds.

If I calculated it right, the acceleration is about 6.75 m/sec2. So, as was said, it less than one G, so I don't think it will bother the fly too much.

In conclusion, don't slam on your brakes to try to get rid of a fly.
 
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I think that most people have come to similar conclusions to what I came up with. I think that while it seems a simple question in some ways, the physics itself is quite complex with quite a few factors.

So yeah, the conclusion that I came to was that the deceleration of the car would be passed on to the fly via the air being compressed slightly and increasing the resistance to the fly meaning that while it might experience a short increase in velocity relative to the car, it would quickly return to its previous velocity.
 
This is why it's sensible to keep things like feathers on the rear parcel tray of a hatchback, but anything much heavier or denser than a grape is a bad idea. Bowling balls are right out.
 
PhantomWolf said:
I came up with this while driving home yesterday and being annoyed by a fly.

You are driving down on a straight country highway at 100km/h (60mph) relative to the ground. A fly on the back window takes off and starts to fly towards the windscreen at about 10km/h (6mph) relative to the car.

Suddenly a deer runs out ahead of you and you slam on the brakes, stopping in 4 seconds.

What happens to the fly?

I'm not going to make it a poll, so feel free to give and answer and show your working. :D
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Assuming, of course, that he fly is inside the car: Once the fly alights, it is essentially it's own inertial frame, flying 110 Km/h relative to the ground. So since it is not affected by the brakes of the car, it should continue at 110 km/h, even as the car stops. This is what happens to the suitcase you left in the rear-window (sans 10 km/h).

HOWEVER, since the fly is very much subject to air resistance, and the air in the car follows the speed of the car, it will in fact slow down with the car, and will only reach the front screen marginally faster than if the car had not slowed down.

Hans
 
theprestige said:
You're sitting on the back seat of the car. The car suddenly stops moving. What happens to you?
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I'm stopped by the seat-belt.

There's an apple balanced on your knee. What happens to the apple?
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It continues forward till it hits something.

You're bouncing the apple in your hand. The car stops moving. What happens to the apple?
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The same as the other apple.

There's a fly buzzing around the apple. What happens to the fly?
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It is braked by air resistance and will not move forward much relative to the car.

(I am here assuming the car stops by using the brakes, or some other reasonable cause, not some silliness about instantly stopping.)

Hans
 
theprestige said:
The physics are pretty simple and obvious. The dynamics of the specific scenario are a little more complicated.
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I'm very seriously considering using it as my question of choice to determine if someone (i.e. anti-science dumbasses such as flat earthers) has enough physics knowledge to understand why when a plane takes off on an east/west runway on the equator, it's not trying to slide sideways at 1,000 mph after turning to land on a north/south runway.
 
The Atheist said:
Would the fly act like a helium balloon?
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No, most definitely not. The helium balloon acts as it does because it's less dense than the surrounding medium. A fly is more dense than the surrounding medium - flies, though light, are denser than air - so it'll experience an apparent acceleration in the same direction as the air rather than the opposite direction.

Dave
 
theprestige said:
Reasonable enough, but beyond the scope of the question, I think. How the fly handles the entrained air in the car follows from the fact that the fly - like everything else in the car - will keep moving forward when the car stops. Regardless of whether the fly is in the air or perched somewhere when the stop happens.
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His comment had nothing to do with whether or not the fly was perched. It was related only to the fact that momentum is proportional to mass but air resistance is proportional to surface area. That's why the behavior of the fly (alive or dead) will be very different from the behavior of the apple.
 
Dave Rogers said:
No, most definitely not. The helium balloon acts as it does because it's less dense than the surrounding medium. A fly is more dense than the surrounding medium - flies, though light, are denser than air - so it'll experience an apparent acceleration in the same direction as the air rather than the opposite direction.

Dave
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Not so sure that's right at all. As long as the fly continues to fly, then its effects should be somewhat similar to the balloons, simply because it acts effectively like it is buoyant.

Remember the old saying about how a bumble bee shouldn't be able to fly according to the physics of aerodynamics? Clearly this saying was wrong..but in a troll way...the truth was it followed a different set of dynamics that took us a while to figure out. We did though. And part of that whole dynamics is actually taking into account the "viscosity" of the air and so called micro "hurricanes".

In lay terms, flies fly by rotating their wings, which creates pockets of low air pressure, which in turn create small eddies above the fly’s wing which lift it into the air and, thus, grant it the ability to fly. This low pressure is very similar to the balloons buoyancy!
 
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Red Baron Farms said:
Not so sure that's right at all. As long as the fly continues to fly, then its effects should be somewhat similar to the balloons, simply because it acts effectively like it is buoyant.
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Except that the direction of boyancy from flight doesn't have to change when the car accelerates or decelerates, unlike the balloon. Plus it won't scale proportionally to that acceleration.
 
Ziggurat said:
Except that the direction of boyancy from flight doesn't have to change when the car accelerates or decelerates, unlike the balloon. Plus it won't scale proportionally to that acceleration.
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Actually, from the viewpoint of the fly, the perceived direction of 'down' will change as the car decelerates. It will angle in the direction of the car's movement and the composite gravity and braking G force will lead the fly to perceive an increased gravitational pull.

Thus, if the fly is determined to fly straight and level (admittedly not a typical flying mode for flies), we should see its path curve upwards.

Hans
 
MRC_Hans said:
Thus, if the fly is determined to fly straight and level (admittedly not a typical flying mode for flies), we should see its path curve upwards.
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Yes, but that part of the problem isn't well defined. Is the fly trying to go straight and level as defined by the reference frame of the air around it, or is it trying to go to the front windshield? Could be either. Could be some third option. But either way, the fly has a choice in the matter, unlike the helium balloon.
 
MRC_Hans said:
Thus, if the fly is determined to fly straight and level (admittedly not a typical flying mode for flies), we should see its path curve upwards.
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The vertical component of gravity won’t change so the fly probably wouldn’t curve upwards

Ziggurat said:
Is the fly trying to go straight and level as defined by the reference frame of the air around it, or is it trying to go to the front windshield? Could be either. Could be some third option. But either way, the fly has a choice in the matter, unlike the helium balloon.
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My guess is that the fly’s priority would be opposing air resistance because normally that would mean a gust of wind is blowing it somewhere that may not be safe. If this is the case it would fly towards the front window while looking for a place to land, but fly behavior and survival strategies are a little out of scope.
 
lomiller said:
The vertical component of gravity won’t change so the fly probably wouldn’t curve upwards



My guess is that the fly’s priority would be opposing air resistance because normally that would mean a gust of wind is blowing it somewhere that may not be safe. If this is the case it would fly towards the front window while looking for a place to land, but fly behavior and survival strategies are a little out of scope.
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The fly can't differentiate the "vertical component of gravity" from the air accelerating it. The acceleration due to gravity is added to the acceleration due to the air. The result is a vector. (Edit: that isn't exactly right, but the point is correct. The affect of gravity is hidden in the flies perception of acceleration.)

So, yes, if the fly is not using its eyes to guide it toward the front window, it will curve up relative to the bottom of the car.
 
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lomiller said:
My guess is that the fly’s priority would be opposing air resistance because normally that would mean a gust of wind is blowing it somewhere that may not be safe.
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I don't think flies have multiple accelerometers and internal gyroscopes like a spaceship, so a fly can't use just acceleration and/or the opposing wind resistance to navigate. Once it takes off, the "wind" it senses is the apparent wind due to its movement through the air. The acceleration it feels is due to its changes in speed and/or direction.

So, the only way to tell you from down or do any other kind of navigation is to rely on other senses such as sight.
 
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Ever since I started watching Worldomoter and the cases/deaths keep ticking up and up every day, I’ve wondered what the numbers for the virus would be.
How many SARS-CoV-2 viruses are there worldwide today?
How many newborns and deaths amongst the SARS-CoV-2 community per day (per second)?
I have absolutely no idea.
I think each infected cell can produce like a million viruses.
 
jadebox said:
I don't think flies have multiple accelerometers and internal gyroscopes like a spaceship, so a fly can't use just acceleration and/or the opposing wind resistance to navigate. Once it takes off, the "wind" it senses is the apparent wind due to its movement through the air. The acceleration it feels is due to its changes in speed and/or direction.

So, the only way to tell you from down or do any other kind of navigation is to rely on other senses such as sight.
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I don't really know about flies or other insects, but mammals, including people do have the multiple accelerometers and the functional equivalent of gyroscopes in their inner ears. It would not surprise me in the least if flying insects have sensory organs that serve a similar function.
 
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