Fly Flying.

August 10, 2006 3:33 PM

Imagine, if you will, that you are driving down the road at at least 60km/h or more. There is a fly sitting on your dashboard. Suddenly the fly lifts off... what happens next?

I'm a student of politics, not of physics, so I've always wondered about what would happen to the fly in this situation.

I imagine that the fly, now that it is no longer perched on a dashboard which is part of the vehicle travelling at 60km/h or more, would slam headlong into the inside-back window of the car and die. However, I think I've read somewhere about an 'air bubble' (or something to that effect) which suggests that the fly, trapped inside the car whose 'air pocket' is also moving at 60km/h or more, should have relatively unhindered motion within that pocket of space. If this is the case, would it slam into the back of the car and die if it lifted off and one of the car windows was open?

posted by Effigy2000 to science & nature (24 comments total)

What happens to you when you're in a car travelling at 60km/h? Do you feel a 60km/h wind in your face? Why would it be different for the fly?
posted by blue mustard at 3:35 PM on August 10, 2006

Because you're still sitting down on the seat driving. The fly, on the other hand, is no longer physically attached to the vehicle in any way shape or form and is hovering in 360 degree space.
posted by Effigy2000 at 3:37 PM on August 10, 2006

That fly is also traveling at 60km/h. What force would make him stop suddenly in your scenario?
posted by found missing at 3:38 PM on August 10, 2006

Try dropping a quater when you're in a car. See how it doesn't move to the back of the car? Same thing.
posted by aubilenon at 3:40 PM on August 10, 2006

If the fly gets smashed against the windshield, as you're thinking might happen, you'd better *never* jump up in an airplane, or you're going to be 300mph pate on the front bulkhead.
posted by jammer at 3:43 PM on August 10, 2006 [1 favorite has favorites]

Er, rear window and read bulkhead. Gimme a break. I just got home from work.
posted by jammer at 3:44 PM on August 10, 2006

Because you're still sitting down on the seat driving.

Suppose you had no windshield--you'd feel the wind, despite being in the seat, no?
posted by equalpants at 3:45 PM on August 10, 2006

You may be forgetting that the fly has had motion imparted to it while sitting on the dashboard. The air inside the car is also moving at 60 km/h.

Now, if the fly sticks his snout out the window, whole different proposition: then we get fly paste, only on someone else's windshield behind you, not necessarily yours.

Put another way, everything inside the car's passenger compartment is moving at 60 km/h in the same direction until otherwise acted upon. Objects in motion tend to stay in motion, etcetera.

This is why car accidents tend to be so terrifically interesting in bad ways for unrestrained passengers: the car is acted upon by an unbalanced force, but the unrestrained passenger keeps on going at the same speed and in the same direction as before.
posted by scrump at 3:45 PM on August 10, 2006

Its not about the "air bubble" Its that the car is going at 60 km/h, you are going at 60 km/h, the fly is going at 60 km/h, etc.

The fly's body doesnt somehow "know" that there's a ground outside going by quickly or air rushing past the windows and so on. This is all basic Newton's Laws, in fact the First Law:

Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
posted by vacapinta at 3:48 PM on August 10, 2006

This is exactly why they bolt the seats to the floor nowadays. Back in the day, they just set up lawn chairs in cars. AND WE LIKED IT!
posted by found missing at 3:49 PM on August 10, 2006

Inertia is the key. The others have it right. Inertia says that objects in motion will tend to stay in motion. The fly is moving along with the car, you, the air inside the car, etc at 60km/h. Until a force acts on it, it will continue to travel at that same speed.

If, however, it flew outside the window, it would be up against air that is not moving, which would significantly impact its forward speed. Very quickly the fly is slowed down to a stop.
posted by knave at 3:50 PM on August 10, 2006

This has nothing to do with bubbles and everything to do with Newton's First Law of Motion:

"An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force."

The fly is moving at 60 km/h on the Dash and when it lifts off its still moving at 60kmh along with you because nothing has changed that velocity. The air in the car is also moving at 60km/h too so everything is stationary relative to each other.

However relative to the earth you are still moving 60km/h, relative to the solar system you are moving even faster, and relative to the galaxy you are moving quite a bit faster. Food for thought.
posted by dendrite at 3:50 PM on August 10, 2006

Just to clarify that there's nothing exotic going on with "air pockets" or anything: suppose you had a sealed air tank in the seat next to you. The air inside that tank is traveling at 60mph along with you and the rest of the car, right? Well, same thing for the air in the car--your car is a large, slightly leaky, air tank.
posted by equalpants at 3:54 PM on August 10, 2006

I'd like to add that I commend you for asking this question - questioning things and expanding one's knowledge of how the world works is always a good thing.

That said, I also feel this is very basic stuff which people should learn as children. I doubt even among scientifically-minded people, most could come up with the correct explanation for why astronauts float when they're in orbit.
posted by vacapinta at 3:55 PM on August 10, 2006

As the Earth is spinning on its axis, does this cause a fly to land in a radically different longitude every time it takes flight?
posted by peeedro at 4:05 PM on August 10, 2006

The earth is moving through space at tremendous velocity. Do we fling off into nearby objects when we jump? Draw a box around the car. Everything in the box operates as if at rest, at least with respect to each other. That truck crossing against the light in front of the car though, it is outside of the box. Ouch.
posted by caddis at 4:59 PM on August 10, 2006

Second what others have said, especially dendrite, but note that you feel the effects of speed when you acccelerate and decelerate. When you're moving at 60kpm, or 800kph on a plane, you don't really feel that speed. Everything's normal. But when you slam on the brakes, or when your plane fires up the jets to take off, then you feel the forces, because your speed is changing. You will probably move around a bit, feel yourself getting pushed back in your seat. That is the effect of the plane pushing you, so that you are also going 800kph.

In the case of the fly, if the fly lifted off the dashboard and you slammed on the brakes, then it would probably slam into the windscreen.
posted by Jimbob at 6:02 PM on August 10, 2006

If you want a real easy demo of the physics involved, the next time you are braking at a stop sign (and be sure no other cars are nearby!), reach down with one hand and hit the seat release lever to adjust the driver's seat. You should thwap rather nicely into the steering wheel and possibly the windshield. I did this to myself (and my front seat passenger/little sister) the very day I got my driver's license and was proudly driving home from the DMV in my mother's car. It was pretty funny, I thought. Oddly, Mom wasn't so amused.

It was a good long while before I got the keys to the old Vista Cruiser again. (Not a 70's Show reference - a real 1976 Oldsmobile Vista Cruiser. Much cooler than Eric Foreman's.)
posted by John Smallberries at 8:47 PM on August 10, 2006

Vacapinta: I doubt even among scientifically-minded people, most could come up with the correct explanation for why astronauts float when they're in orbit.

I'm wondering what Vacapinta could possibly mean. "Floating" is an product of bouyancy. Are his astronauts in some kind of magic swimming pool that is orbiting around the planet? Or are his astronauts orbiting the drain in the tub?

I'm guessing that most scientifically-minded people, given a correctly phrased question, could probably give a decent answer.
posted by Dunwitty at 10:43 PM on August 10, 2006

That seat-release thing is more complicated than simple deceleration - there's a positive-feedback loop involved as well.

If your foot's on the brake pedal when you release the seat, the car decelerates while you and your seat don't, which pushes your foot harder onto the brake pedal, which makes the car decelerate harder still, which slides you even harder into the brake pedal, which makes the car decelerate harder, which pretty quickly ends in tears.

It's like the story my ex-boss told me about accidentally touching the antenna connection of a high-powered SSB transmitter with an open microphone. His initial yelp made the shock worse which make him yell which made it worse still... yyaAAAOHFUCK!!!!!
posted by flabdablet at 11:13 PM on August 10, 2006

I'm wondering what Vacapinta could possibly mean.

He means 'float' in the sense of 'not falling', smarty pants.
posted by chrismear at 12:50 AM on August 11, 2006

Or does he?! Bwah-hah-hah...
posted by chrismear at 12:51 AM on August 11, 2006

If you live about the same distance from the equator as Washington DC or San Francisco, you are spinning around the Earth's axis toward the sunrise at around 800 miles per hour. Earth revolves around the sun at around 66,650 miles per hour. Our solar system revolves around our galaxy at 559,000 miles per hour. Our local group of galaxies is headed away from the Center of the Universe at 1,332,000 miles per hour. So find something solid and hold on!
posted by gregoreo at 5:39 AM on August 11, 2006

What happens next? The damn fly gets in your field of vision, and you wreck the car, of course.
posted by drstein at 1:13 PM on August 14, 2006