MrDudeMan
Lifer
- Jan 15, 2001
- 15,069
- 94
- 91
Every physicist in the history of the world is rolling in their grave right now.
The fly and car are initially at rest in terms of their horizontal velocity with respect to the ground. The only thing inside the car applying any amount of force to the fly is the random air movement, which statistically is the same on all sides of the fly, and obviously gravity. The fly is opposing the force of gravity on its own, and nothing about the movement of the car is impacting this at low speeds like what a car is capable of.
Saying this has anything to do with frame of reference is a misapplication of that concept. Throwing a ball in a car and catching it without moving your hand is an actual application of "frame of reference" but still doesn't mean much. The ball had an initial velocity matching that of your body/hand, so throwing it in the air, while an observer would see its path as parabolic, seems straight up and down to you.
The fly has an initial velocity of 0 and will continue to do so until a force acts on it. The air inside the vehicle is most likely not the right density to be the cause of the fly's forward movement. A very small nudge will be felt by the fly as the air toward the back seat becomes more dense, but at the speeds a car is capable of it would be surprising if the fly moved very much at all. To make this simpler, if everything else remained the same but there was no air, only vacuum, the fly would not move unless the car physically contacted it. There would be literally no transfer of force onto the fly due to the car's movement. Frame of reference means nothing here.
This also has literally nothing to do with what effects you feel as you ride in the car. You are attached to the car and thus must move when it does. A fly is subject to inertia just as you are, but again your inertia is opposed by the movement of the seat because you are attached to it, and the fly has no connection to the car other than the very slight compression of the air inside.
The fly and car are initially at rest in terms of their horizontal velocity with respect to the ground. The only thing inside the car applying any amount of force to the fly is the random air movement, which statistically is the same on all sides of the fly, and obviously gravity. The fly is opposing the force of gravity on its own, and nothing about the movement of the car is impacting this at low speeds like what a car is capable of.
Saying this has anything to do with frame of reference is a misapplication of that concept. Throwing a ball in a car and catching it without moving your hand is an actual application of "frame of reference" but still doesn't mean much. The ball had an initial velocity matching that of your body/hand, so throwing it in the air, while an observer would see its path as parabolic, seems straight up and down to you.
The fly has an initial velocity of 0 and will continue to do so until a force acts on it. The air inside the vehicle is most likely not the right density to be the cause of the fly's forward movement. A very small nudge will be felt by the fly as the air toward the back seat becomes more dense, but at the speeds a car is capable of it would be surprising if the fly moved very much at all. To make this simpler, if everything else remained the same but there was no air, only vacuum, the fly would not move unless the car physically contacted it. There would be literally no transfer of force onto the fly due to the car's movement. Frame of reference means nothing here.
This also has literally nothing to do with what effects you feel as you ride in the car. You are attached to the car and thus must move when it does. A fly is subject to inertia just as you are, but again your inertia is opposed by the movement of the seat because you are attached to it, and the fly has no connection to the car other than the very slight compression of the air inside.
MrDudeMan
Lifer
- Jan 15, 2001
- 15,069
- 94
- 91
Originally posted by: Oyeve
Heres a test. Kill a fat fly, hang by string in center of car, have someone video it, slam on gas, if string moves, you have one answer. Then, catch fly, keep alive but hang on string in center of car, video it and slam on gas. Compare results.
Connecting it to the car in any way drastically changes the problem. I know (hope) you were being ridiclous but just to be sure I thought I would reply.
DrPizza
Administrator Elite Member Goat Whisperer
No human being has ever been in an environment where there wasn't a net gravitational force. This includes astronauts. "Zero-gravity" is a misnomer for free-fall.
In western NY. That's an odd question for this thread.
I think the probable source of confusion for this problem is the non-intuitive behavior of a helium balloon in a car as it accelerates (covered thoroughly in other threads & all over the internet if someone wants to read more about it.) A fly isn't buoyant in air, thus will go backwards (with respect to the car.) But forward with respect to the ground outside the car. (Damn, frame of reference does matter.)
Speaking of buoyancy, here's something to twist your brain around:
If you had a sealed jar, completely filled with water, except for a ping pong ball inside, the ping pong ball would be at the top of the jar (the side furthest from the surface of the earth. Now, take that jar with you on a plane, strap on your parachute & get ready to jump. After jumping out of the plane (and carrying the jar with you), throw the jar toward the earth (in such a way that the jar isn't spinning end over end.) Describe what happens to the ball from the point when you jumped out of the plane until the jar smashes on the ground.
In plane - ball floats to top (gravity pulling on water, "pushing" ball to top)
In freefall - jar, water, ball are accelerated equally by gravity, ball wanders around jar
Throw jar - accelerated more than gravity, ball is front of jar (farthest from thrower, closer to earth)
After initial acceleration of throw you return to freefall state as the only acceleration is by gravity
-edit- at the point where you weren't accelerating perfectly gravitationally the ball would float to the top as that's about the same as standing. As long as you aren't doing 9.8m^2 acceleration the ball floats to top, if not it then wander around, if greater acceleration than 9.8 then ball goes to bottom of jar (closest to earth).
In freefall - jar, water, ball are accelerated equally by gravity, ball wanders around jar
Throw jar - accelerated more than gravity, ball is front of jar (farthest from thrower, closer to earth)
After initial acceleration of throw you return to freefall state as the only acceleration is by gravity
-edit- at the point where you weren't accelerating perfectly gravitationally the ball would float to the top as that's about the same as standing. As long as you aren't doing 9.8m^2 acceleration the ball floats to top, if not it then wander around, if greater acceleration than 9.8 then ball goes to bottom of jar (closest to earth).
speg
Diamond Member
DrPizza
Administrator Elite Member Goat Whisperer
Sure, but where does the ping pong ball "float" to when you're throwing the jar & it's accelerating at greater than 9.81m/s² toward the earth.
ConstipatedVigilante
Diamond Member
- Feb 22, 2006
- 7,670
- 1
- 0
Doesn't the whole jar and its contents have the same velocity, thus the ping pong ball keeps floating at the top, due to density?
Jeff7
Lifer
- Jan 4, 2001
- 41,596
- 20
- 81
It is floating in a cup of beer, as a college student has already decided to play beer pong with it.
Buoyancy is dependent on gravity, no? I'd guess that the ping pong ball would be able to move and tumble about freely in the water, just as air bubbles do within a sphere of water on the space station or shuttle, where they're also in freefall.
Edit: I missed the part about throwing it downward. The acceleration would force the ball to the side of the jar facing the ground.
No, the resistance of the air molecules would pretty much keep the fly in place. The fly has too little mass to get much movement. Now a bowling ball on the hood of the car would smash right through your windshield.
Is your son is messing with you maybe?
A regular old house fly or fruit fly can't hover can it?
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