How can a fly fly within a moving vehicle?

July 9, 2008 7:22 PM

Physics of a fly maneuvering within a moving vehicle.

Quick physics question.

How is it possible for a fly to navigate its way around the inside of a car, while that car is travelling forward in space at any significant rate of speed?

Wouldn’t the fly, as soon as it detaches itself from the car to fly around inside, be instantly catapulted to the back window as its speed is no longer being maintained? Is it being affected in this way, but just in such a minute amount, its not noticeable?

Lets say you had a large, hollow, 100’ cylinder. This cylinder is on the back of a flat bed truck, travelling down the road at 60 mph. At the top of the cylinder is a small piece of string, holding a tennis ball approximately 12” from the ceiling. That piece of string is cut. Would the ball, as it falls towards the floor of the cylinder, travel towards the back of the cylinder as its speed decreases?

posted by aleahey to science & nature (15 comments total)

The same reason that if you're reading a newspaper inside a moving car, it isn't plastered against your face--when your car moves, the space inside it, along with the air, moves too--so it's as though things are still in the car.
posted by DMan at 7:26 PM on July 9, 2008

I asked this question on AskMetafilter once. Here's the link. I marked a best answer too. I think it should solve your problem for you. You can give me a best answer now, if you want. :P
posted by Effigy2000 at 7:29 PM on July 9, 2008

Lets say you had a large, hollow, 100’ cylinder. This cylinder is on the back of a flat bed truck, travelling down the road at 60 mph. At the top of the cylinder is a small piece of string, holding a tennis ball approximately 12” from the ceiling. That piece of string is cut. Would the ball, as it falls towards the floor of the cylinder, travel towards the back of the cylinder as its speed decreases?

Nope; assuming that there aren't significant air currents inside the cylinder, the ball will hit the bottom of the tube.

This video doesn't quite address the situation you asked, but it's too good not to link to. The point of the video is that they manage to make the ball fall straight down by firing the ball at the opposite direction at the same velocity the car is travelling at.
posted by Johnny Assay at 7:39 PM on July 9, 2008

The cylinder question, depends on whether the cylinder is sealed or not. If the air on the highway is traveling through the cylinder (because there's nothing capping the ends), then the ball will encounter air resistance and slow down -- that is, move backward relative to the truck. If it's sealed, then the air inside is going the same speed as the cylinder, the ball will not slow down.

You need to forget the idea that there is any concept of absolute speed. All motion is relative to something else. If you were floating through space, say, away from the earth at 1000mph, and you released a tennis ball, it would stay right next to you, moving in the same direction at the same speed. You can say that you and the ball are sitting still, and earth is moving away at 1000mph. Or you can say vice versa. They mean the same thing, because there's no such thing as absolute motion.

Inside the context of your car, the same thing is true. The fly isn't doing 60mph. It's just doing its usual business, which happens to be in the context of your car. The fact that the pavement below is whizzing by at 60mph means nothing.
posted by knave at 7:43 PM on July 9, 2008

You may enjoy the wikipedia article on Inertial Frame of Reference. Basically, the idea is that velocity only makes sense in terms of being relative to something else. Acceleration (changes in velocity) are detectable, but velocity isn't. If you step on the gas, the fly will appear to move backwards; nothing will happen if you keep your velocity constant.

There are special cases (when the frame of reference is rotating, or subject to strong tidal forces near a black hole), but you can probably ignore them for this thought experiment.
posted by jenkinsEar at 7:57 PM on July 9, 2008

jenkinsEar has it. Everything is moving, the Earth, our solar system, and of course cars, buses and planes. Once everything in your frame of reference has settled into a constant speed things within that frame will act as if at rest, until your frame runs into another frame having a different velocity.
posted by caddis at 8:06 PM on July 9, 2008

At the top of the cylinder is a small piece of string, holding a tennis ball approximately 12” from the ceiling. That piece of string is cut. Would the ball, as it falls towards the floor of the cylinder, travel towards the back of the cylinder as its speed decreases?

No.
posted by delmoi at 8:10 PM on July 9, 2008

To the OP: Everything is relative.

Also think of you standing in a car going 30 miles per hour forward and throwing a ball out the back at a speed of 30 miles per hour.
posted by Brian Puccio at 8:37 PM on July 9, 2008

The car is moving, the air is not.
posted by rhizome at 10:00 PM on July 9, 2008

If that were true, rhizome, the fly would be plastered to the back window as the car moves away from the air. Both are moving. You must distinguish between velocity and acceleration. If the car were to accelerate suddenly, both your head and the fly would appear to snap towards the rear of the car. In fact it would be the car snapping forwards against the constant velocity of your head and the fly. We are moving at a constant velocity of 1000 miles per hour as the earth rotates, plus we're orbiting the sun, plus our entire solar system is hurtling through space, and you can stand up, walk into the kitchen and pour a coffee.
posted by weapons-grade pandemonium at 10:34 PM on July 9, 2008

Einstein's Theory of Relativity in words of four letters or less.
posted by weapons-grade pandemonium at 10:37 PM on July 9, 2008

Related question:

If you are travelling in a moving car and stomp on the brakes, all of the loose items in the car (that book on the passenger seat, and the unbelted passenger holding it) continue to move forward at the original velocity until they are stopped by hitting the dashboard. This is equivalent to what would happen if you tilted the car forward, which is how those motion simulator rides work.

If you were in a motion simulator holding a helium balloon, and the seat tilted forward, you would tilt forward and the balloon would tilt backward.

If you are in a car with a helium balloon and slam on the brakes, does the balloon go forward like everything else? Or does it go backward?

I haven't tried this, but it should go backward. Gravity isn't distinguishable from acceleration.
posted by fantabulous timewaster at 8:56 AM on July 10, 2008 [1 favorite]

WGP: I should clarify. The car is moving, the air inside of it is not. This may be oversimplified, but I think it gets the relativity point across. :)
posted by rhizome at 9:04 AM on July 10, 2008

Very, very good insight fantabulous timewaster. Your intuition is correct, that the balloon would go the "wrong" way in the car. If you made a hard right turn with a helium balloon in your car, all the objects inside would feel pushed to the outside of the turn (to the left, relatively speaking). This includes all the air in the car, which is denser than the helium balloon. As the air moves to the left, the balloon will be pushed to the inside of the corner, in the same direction the car is turning. If you hit the brakes, the balloon would go backwards.

There was a great video of this effect up on Robert Krampf's website, but apparently it's subscription only now. I can't find any other demo videos of it, but it was really neat to watch.
posted by knave at 9:09 AM on July 10, 2008

knave, that was a question from my advisor (on a car trip, after a sudden stop). Not original to me.

Krampf seems like an entertaining guy. It's probably cheaper to drive to the store and get my own balloon, though. Plus I would have a balloon afterwards.
posted by fantabulous timewaster at 9:32 AM on July 10, 2008