Utterly pointless, tenuous and hypothetical arguement
 

OK, here it is....

Sitting at East Midlands last week waiting to board a jet for Sweden, we were watching the bloke at the gift shop was playing with one of those little indoor remote control helicopter toys. Totally bored and fueled by Red Bull and coffee, here's the question we posed:

737 at the end of the runway, radio controlled model in the hover in the cabin. Jet opens the taps and rolls down the runway.

Does the helicopter remain in position or does it end up plastered on the rear bulkhead or hitting the bloke leaving the toilet in the face? Remember, the helicopter model is in the hover in the cabin at the point the 737 rolls.

Max.

Easy...

It stays in the hover, assuming the R/C pilot can maintain the hover whilst the 737 is accelerating.

But while we are on the subject of hypotheticals, what happens if you are travelling at the speed of light and you turn your headlights on???

It stays dark?


Reminds me of the question of a load of geese in an aircraft. Does the a/c weigh less if they all take flight at once or something like that?.

OK, so lets say it's on the deck when the jet rolls, then lifts off when it's in the cruise? Will it still go backwards?

By the way, we wanted to buy two and race them around the cabin, but thought that the crew might not see the funny side.....

It gets very dark.... ?

Depends on whether theres enough space for them to get speed up to get lift.

What I want to know is, if you fart, does cabin pressure increase?, and if so is that reason my ears pop in economy?

It should hover ok.

since its skids are in contact initialy with the a/c going at 440kts... ( -i know ur example is different but this should get a firm understanding on the forces at work.)

...it is given that boost of horizontal velocity, therefore must hover ok as it is also traveling at 440kts realative to the ground, but 0kts realative to the a/c.
thats the way we're taught in the world of physics anyway.

and if you think about it, the earth is spinning at thousands of miles per hour.. now correct me if im wrong, but that is similar to the situation we are discussing? It proves that since we are on the earth, we too are spinning at those thousands of miles per hour, thus when the wheels leave the ground we are given that invisible boost of horizontal velocity, realative to the ground yes we are moving at 400kts, but relative to the universe as a whole we are going at say <>1000400kts?< (a random number i have no idea of the actual speed we are turning)

what im saying is basicaly a huge version of-

when u drop a feather in your car, it does not shoot to the back, simply falls normally, since your hand has transferred horizontal velocity thus it keeps up with the car.

in a hover the helicopter doesnt fall downwards so this kinetic energy in the horizontal velocity would soon bleed of so it would eventually end up at the back of the a/c



however everything i have just said requires a bit of though and i hope iv put it across clearly :)

Talking about speeds...

Imagine running around a light pole.

If you run fast enough, will you eventually reach your a**e. :}

As Svenestron quite rightly points out, if you are hovering the little helicopter it will start to move towards the back of the plane as the plane takes off. However, due partly to air resistance, but mainly the fact that the stability of these little helis is due largely to the flap-back effect it will end up travelling at a slow speed towards the tail of the aircraft rather than accelerating and going splat against the rear bulkhead.

With regards to turning on your headlights when you are travelling at the speed of light:

• You can't travel at the speed of light (unless you go on a serious Jenny Craig program).

• Even if you do travel close to the speed of light (say 99.9999% of the speed of light), The light from your headlights will still travel away from you at the speed of light (from your point of view). Surely the lorentz transformation is a basic part of the POF training? ;)

Well, every time I've been in an airliner, I certainly get pushed against my seatback as the plane accelerates, so I would guess that your toy helicopter would too. But the bigger question is: Why is there a man coming out of the toilet while the airliner is on the takeoff roll?

Would an observer behind you see your indicators winking if you were travelling the speed of light ?.

Depends on how fast and in what direction relative to you he's going. Doppler shift and all that. Slower than you and you get red shift. Faster and you get blue shift. Blue indicators. That would be confusing. The flash rate would also be different as time itself runs at a different rate depending on relative speed.
More importantly, if you were about to run into the back end of someone and you were going much faster than him, his brake lights would go blue so you wouldnt know you had to stop.:p

To you its normal. To an observer travelling at a different speed its not.
All these things are covered by the theory of general relativity.

Edit: Lots of interesting posts recently. I've covered everything from a horses arse to Einstein on Rotorheads.:ok:

Was going away for Christmas. Just got chucked off my flight because I refused to put away my model helicopter.

However - for years as a kid I would try to knock myself out by standing in the bit of the railway carriage nearest the door. I would stand facing the direction of travel, about an inch away from the bulkhead. Once the train got up to speed, I would jump off the floor to see if the train would knock me out.

It worked.

But only because I jumped so hard I collided with the ceiling........

and I always thought that "relativity" was a Christmas play starring my cousins.

thank your lucky stars you didn't get curious about the ol' elevator cable snap scenario.

If I were travelling at the speed of light, I would be obnoxious enough to not use my indicators

If a helicopter is hovering in still air (ie in the cabin) why should it move on take off when the air inside remains still???

It would be hard to prove as to hover requires constant input from the pilot to maintain it and any movement on that take off would/should/could be controlled by the pilot.

This said I now feel the urge to prove this theory one way or another... Anyone got a large enough plane to have a go???:E

It would move on take off as it would be in an accelerating reference frame (not an inertial reference frame - one moving at a constant velocity).

Put another way, while the air may remain stationary relative to the plane*, the helicopter has mass and will want to remain stationary unless a force acts on it to accelerate it. In this case, the force will come from the air starting to move relative to the helicopter. This will cause the rotor to "flap-back", tilting the lift vector and providing the acceleration need to keep the model helicopter accelerating with the plane. In the course of achieving this, the model will have moved slightly towards the rear of the plane.

As Svenestron has quite correctly pointed out, if the plane has a constant acceleration, then that should be indistinguishable from a tilted, slightly stronger gravity force. As such, we'd expect that after the initial shift towards the rear of the plane, the helicopter would adopt a stable hover with the disk tilted slightly forward to provide the required acceleration.

Remember here as well that we are talking about those teeny, weeny little model helicopters that use flap-back to provide their stability.

Daniel

* The air actually isn't completely stationary relative to the plane. As the plane accelerates there will be a slight shift of the airmass in the plane towards the rear. This will cause a slight (very slight) pressure gradient from a slightly lower pressure at the front of the plane to a slightly higher pressure at the rear of the plane. It is this pressure differential that allows the air in the plane to accelerate at the same rate as the plane itself (much in the same way the pressure gradient between the earth's surface and space stops the air from accelerating downwards due to gravity). Of course, even this picture is simplified, ignoring transient "sloshing" effects as the air moves about.

Can't wait for Monday. And Nick Lappos.

O27PMR, my understanding is (ready to be corrected:ouch:) ... It'd move toward the back because it has mass and as Newton's 1st law of motion states - a mass will remain at rest or continue at its existing velocity unless an external force is applied. The acceleration of the plane is being applied to the helo only via the air around it, which'll be much less force than the inertia of the mass of the helo. The force you feel in your back as the a/c accelerates when you're in it is the accelerating force being applied to your mass (as it were :)).

What I really don't get though, is the speed of light piece ...:eek:. Isn't an object's mass meant to change as it approaches the speed of light? if so, then what's the story with 2 objects travelling away from each other at half the speed of light each? Their mass won't change (?) yet relative to each other they're doing the speed of light. Confuses the heck out of me :sad:

Gravity, as sensed by the helicopter, would increase and be tilted towards the rear of the plane. (Try, as above, dangling a shoe lace, but add a heavy weight to the end. Not only will it tilt backwards but the "weight" will increase.)

Due to the inherend aerodynamics of the rotor system it will tilt towards the nose of the aircraft and, unless more power is applied to counteract the increase in gravity, it will sink.

OA

I wouldn't wait for any other responses, Deemar got it bang on and I agree with overtauk's addition about the sink due to increased "apparent" gravity.

As far as mass increase goes, rest mass is the lowest you can measure of an object, which occurs when it is not moving relative to you. The faster the object moves, the greater the mass you will measure (negligible until very close to the speed of light). The rest is beyond the scope of this forum. There are many good books on special relativity.

Matthew.

If you want to know the answer to the speed of light question, come and observe the A4 autostrada at rush hour. (although it is true, you have to factor in talking on your mobile phone at the same time, and you have to observe via your rear-view mirror!):}