Flies Free To Fly

[Lynn_M]

Every now and then, I'll get a fly flying around the interior of my car. Why doesn't the motion of the car moving forward cause the fly to be pinned against the rear windscreen?

[Gromit]

I'm guessing, but I imagine it is because the air in the car is moving at the same speed as the car.

[Khandro]

Good question Lynn. Looking to Albert Einstein and his theory of relativity, if you replace the fly in your question with say a cricket ball on the floor of the boot of your car, if you accelerate from still, the ball will appear to do just that, and roll to the back of boot. In fact it isn't rolling 'back' at all, it is 'trying' to stand still in relation to the earth's gravity, and it is the car that is moving forward. Similarly, you too would try to stand still in relation to the earth's gravity if you didn't have a seat -back to keep you in place.

The fly is also being affected by the earth's gravitational pull but as weighs in at about 300 micrograms that pull is very slight and if it is in flight it is more conditioned by the air pocket within your car. However if it were standing on your cars window-cill it would undergo the same force as the cricket ball but this would have almost no effect on it because of its tiny weight.

[Old_Geezer]

Unless you are permanently accelerating, why would it ?

[Khandro]

OG; Even with increasing acceleration if the fly was airborne it would have no real effect on it in a sealed car, the air isn't moving in relation to the car. You could witness this by cigar smoke (now not allowed in nanny states :0)

[Old_Geezer]

Strange. The astronauts in the rockets seem pulled into their seat during acceleration. Indeed if I accelerate in my car I experience the same effect.

[Khandro]

Of course! they like you are attracted by the earth's gravity which they are attempting to escape, the fly on the cill would be subject to the same forward g-force as the driver (as well as gravity) but because of its 300 micron weight it would be almost negligible.

[ToraToraTora]

for these thought experiments disregard the air. Ok it's the same as standing up on a train and jumping up in the air, does the train pass under you? No because it's all relative. on the train you are moving at the speed of the train, when you jump you are jumping in a curve and landing where you took off. A viewer from the side would see your shoes arcing through the air but a viewer on the train would see you go up and down. Same with the fly it gets up to speed on the car once it takes off it already has the speed of the car,any movements it makes are then relative to the speed of the car. Thank you Mr Einstein.

[Old_Geezer]

They may be equivalent but I think it is the acceleration rather than the gravity that dominates. I don't not know how large its effect on a fly would be but accelerate fast enough and it'll get effected at some rate.

[Khandro]

TTT; Yes, but don't jump up if you are standing on the roof of the train, particularly if you're on the last carriage!

[ToraToraTora]

works the same on the roof khandro.

[jim360]

People who are thanking Mr Einstein are giving credit to the wrong person. For motion at these speeds, it's actually enough to thank Galileo.

[bhg481]

I think you might get blown backwards off the roof TTT. The air there isn't moving at the same speed as the train and your wind resistance is not negligible.

[ToraToraTora]

yes bhg that is why i said above that these thought experiments illustrate a principle that is effected by the presence of the air, so generally a vacuum is implied so that we can illustrate this area of relativity without getting side lined.

[THECORBYLOON]

If there were trains on the moon, you could jump up and down on the roof of a moving train and land on the same spot.

[jim360]

Also it is important to consider the presence of the air, I think. The instant the fly takes off then yes, air or not it would carry on moving at the speed of the car and be safe from squishing. But as soon as it started to change direction under its own impetus then the fly's and the car's motions would start to decouple and the fly would be at great risk of getting splattered. What saves it from this fate is the air in the car, that also moves at the speed of the car and so keeps the fly in motion with the car, on top of whatever else it tries to do.

So keeping the air involved is important to explain why the fly stays safe from a grisly fate -- even though (Galileo's, not Einstein's) relativity is enough to explain why it would be safe to start with.

[Khandro]

ToraToraTora
//works the same on the roof khandro,// no it doesn't, its not even the same in an open top car for the fly or anything, try standing on the seat of an open top car at 100mph and jumping in the air. You might have to be scraped off the road surface!

[ToraToraTora]

did you read what I said about the air?

[johnny37]

If the earth spins at 1000mph and an aeroplane flies west at 500mph, why does the aeroplane not go backwards, or even take off?

[ToraToraTora]

because it is doing 1000mph on the ground before it starts.

[ToraToraTora]

speed only has meaning when it is relative to something else. Eg you are travelling at 60,000mph right now, can you feel it?