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?

[Khandro]

TTT: You said earlier // 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.//

But I don't think that is strictly true either, if the Train is travelling at 100 mph, then you are too- natch, but when you jump you loose contact with the train so you begin to slow down, don't forget that even though you are travelling at speed you are still subject to the gravitational pull of the earth. You are airborne for such a short amount of time that where you land on the floor of the train must be fractionally different from where you took off from.

[ToraToraTora]

why would I be slowing down? in a vacuum, remember, Newtons 1st.

[Khandro]

Why do you talk about vacuum, we are dealing with real flies, cars and trains? a say, 12 stone man, when he jumps has lost contact with the energy of the train and only has, while airborne, the momentum, which must weaken during the time disconnected, so he is travelling slower than the train and must come down in a different position, microscopic I suppose, but different.

[mibn2cweus]

Why would an object's forward velocity change simply because it is no longer held to the floor by the downward force of gravity? What force would slow its forward momentum?

[mibn2cweus]

If you want to get from the back of a bus to the front in a hurry, try jumping in the aisle when the bus driver slams on the brakes.

[Khandro]

//What force would slow its forward momentum?// The Earths gravitational pull - it hasn't gone away just because you are travelling at 100mph.

[Hypognosis]

@thread

Lynn specified //a fly flying around//

Sorry, TTT but the role of the air in the car needs explanation.
Also, "flying around" implies forward, back, left, right, up and down.
No hovering going on, perhaps we could deal with that later.

I don't have a proper answer yet. bbl

[jim360]

At the scales involved, I'd say that gravity may as well be neglected -- if you were interested in how time varies from the top to the bottom of a train then the difference you are talking about is, I believe, just about at the limits of our ability to measure, but otherwise can be ignored as acting on the train equally with the man jumping up and down inside it.

The air is important because it acts as sort of a coupling between the car and anything moving about inside the car. If you threw a sweet back and forth the air would have no real importance, but for a fly it would matter. The key difference is that a sweet once thrown does nothing on its own to change the way it moves, but a fly moves under its own impetus -- and so would soon decouple from the car in terms of relative motion once it tried to change direction. That changes if it's moving against the air that is also inside, and moving along with, the car. I think.

[ToraToraTora]

the same force works on the train too kandro

[Clanad]

In this case it's all in the air... the air is captive within the car, the fly is moving within the captive air and doesn't recognize the car is moving, since the air is moving right along with the car, regardless of the direction of the car.

An airplane is in the same fix... the air it is flying in will always produce exactly the same airspeed on the airspeed indicator at say, cruise speed (doesn't matter the speed chosen). If the airplane makes a turn one direction or another, the airspeed indicator remains exactly the same (except at steeply banked turns... more about that in a moment). If the air is moving relative to the earth's surface (called wind) the airplane doesn't "care" (as if it could)... it will still record the same "normal" airspeed on the indicator. An observer on the earth's surface watching closely might see the airplane slow down or speed up relative to the wind, and close to the earth, the pilot may recognize that change... but only relative to the earth's surface... not the air speed indicator. Never changes for a given power setting and given attitude of the aircraft (no, not mad or happy).

In steeply banked turns, there will be a loss of airspeed until the maneuver is recovered, but this is due to "G" forces causing the aircraft to "weigh" more... 1g, 2G's, etc., depending on the steepness of the bank.

Your fly only moves relative to the body of air that it's flying in, again, regardless of that body of air's motion relative to the earth outside or even the air outside...

[scowie]

Khandro: No linear momentum is lost as a result of gravity since the momentum is directed forwards and gravity acts downwards. Jumping *will* actually cause you to land marginally further back though, not because of loss of forward momentum, but because being at a higher altitude requires you to have a marginally greater forward momentum in order to stay over the same spot on the floor (you need to have the same *rotational* momentum as the floor). Whilst standing stationary, your head is moving marginally faster, as the earth rotates, than your feet are. If it didn't you'd end up being upside-down after 12 hours :)

[Clanad]

Absolutely, dive guy...

[Khandro]

scowie; // No linear momentum is lost as a result of gravity since the momentum is directed forwards and gravity acts downwards.//

Once the human jumps inside the train, contact with the momentum-forming-force is lost. --- Let's 'reduction ad absurdum'; the carriage is no longer 10 feet high but 1,000 ft., when it's stationary drop from a marked point on the ceiling a 1" dia. steel ball bearing and mark the spot where it hits the floor. Take the train up to 100mph and drop the steel ball from the same spot on the ceiling, after falling 1,000 ft. are you saying it will hit the same spot on the floor?

[jomifl]

The Earth's gravity has nothing to do with it and neither does the weight of the fly. The fly will be 'left behind' when the car is accelerating but as soon as the car maintains a constant speed there will be no effect upon the fly. It will buzz around as if it was in a stationary car.

[jomifl]

Khandro, there is no 'momentum forming' force.

[mibn2cweus]

//Let's 'reduction ad absurdum'; the carriage is no longer 10 feet high but 1,000 ft., when it's stationary drop from a marked point on the ceiling a 1" dia. steel ball bearing and mark the spot where it hits the floor. Take the train up to 100mph and drop the steel ball from the same spot on the ceiling, after falling 1,000 ft. are you saying it will hit the same spot on the floor?//

The top of a moving train following the curvature of the Earth is actually moving faster than the bottom so the ball would land at a point forward of a point in the train directly below the point from which the ball was dropped.

[Khandro]

jomifl; I thought I dealt with the fly in the second post after the OP - no one has contradicted it anyway. But I have moved on to the secondary subject of a human on a train, in response to TT.
The momentum forming force is from the energy of the train's movement transmitted to the passenger through physical contact with it and this contact is produced by the earth's gravity, which as I've said never goes away but is slightly reduced by forward motion. Is that correct?