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I wouldn't mind betting that someone, somewhere, has just called his wife over and said "just look at what this daft lot are getting all serious and technical about now".
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I'd say it was because as you accelerate the body of the car tilts up at the front and the balloons rise in that direction. Whe you brake the nose dips and the balloons float upwards towards the back. Remember these balloons are not floating free, but pressing against the roof, which yaws under acceleration and braking.
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I'm tempted to agree with you ChrisR, but I prefer the simple answer.
That as you accelerate, the air is pressed to the back of the car as you would expect. The only visible effect is that it displaces the balloon forwards. The balloon is also being urged back by the acceleration, but less forcefully as it has less mass=less inertia. So the air wins and the balloon goes forward.
For deceleration, right turns, left turns etc, mutatis mutandis.
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Indeed. Mutatis and, as you say, mutandis [sage nod].
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That sounds plausible in theory, but I'm not convinced the air inside a car moves enough or for long enough to increase the pressure behind the balloon to that extent. If the balloon was floating free it would discount my theory--all it takes is a little weight added.
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It wouldn't move if there was nothing floating on top of it. Imagine the car was half full of water. Where would the water go when you put the brakes on? And anything that was floating on top would still be "on top" i.e. behind it due to revised direction of "g"-force.
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Wow, I was expecting this to be a short thread, and now look what I've started.
It all, rather unbelievably, comes back to Einstein's General Theory of relativity.
If you are placed in a lift in the middle of space, you will experience weightlessness. If the lift is then accelerated upwards, the floor will move towards you and then press itself to you. From your perspective inside the lift, you will be pulled towards the floor, then experience a force pressing you to the floor. In all possible ways, the effect of this will be the same as gravity. Einstein said that in that case, it isn't just the effect you see; instead what you are experiencing actually *is* gravity. (if you go on with the experiment and imagine the effect on the beam of a torch shining across an accelerating lift, you can prove that gravity will bend a light beam, later proved by experiment)
So, when you accelerate a car by putting your foot to the floor, you are creating a gravitational force that acts towards the back of the car. Helium being lighter than the air in the car, it moves away from the direction in which the gravity is pulling, hence, towards the front of the car.
It's pleasing to know that in some small way, Einstein's great theories come into play when you put your foot down or brake your car. It's not all about light beams and time travel.
V
PS. Several previous posters seem to have answered this accurately and much more succinctly.
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"And why does a spoked wheel look like it's rotating in the opposite direction to which the vehicle is moving? Nature has some explaining to do about all these inconsistancies."
Because if you're watching on TV, what appears to be steady movement is in fact 25 frames or so a second. If a spoked wheel in the time between frames has moved to a point just behind where the previous spoke was in the last frame, then what you will see is an apparent slow rotation backwards.
You can also get the same effect (but only as a passenger!) if you look at alloy wheels on other cars at night. The "steady" light from streetlights actually flickers 60 times a second. Thusm if, during the time the streetlight is off, the spokes of the wheel move to say, exactly the same point as the previous spoke was during the last flash of light, you'll see a stationary wheel.
Dunno if that makes sense, but that's my best shot.
V
PS a quick search gave:
www.thecarconnection.com/index.asp?n=158,209&sid=2...7
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I must stop reading threads like this. I was in my local filling station tonight and I found myself browsing round the shop to see if they sold balloons. Thankfully they didn't, and with a bit of luck a good nights sleep should do the trick.
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So, when you accelerate a car by putting your foot to the floor, you are creating a gravitational force that acts towards the back of the car. Helium being lighter than the air in the car, it moves away from the direction in which the gravity is pulling, hence, towards the front of the car.
Well put, Vin.
But I think that is conventional physics, not relativity. What Einstein actually said was that you do NOT feel gravity as a force.
He famously came to this conclusion after talking to a builder who had fallen off a roof. The man said he didn't experience any force at all: he just "found himself moving towards the ground". What Einstein said was happening was that the space around the Earth was distorted because of its mass. That is the point where I stop following the argument, but one thing is clear- it is not a force.
Imagine free-falling inside a lift. You would experience weightlessness, with no apparent force, yet would somehow arrive at the bottom.
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The reason I chose a relativistic explanation is that it is the simpler picture, otherwise, in classical physics, you're left looking at inertial explanations. These will give the right explanation, but are much more complex. If you simply accept that there is additional gravitational force acting towards the rear of the car, the picture becomes simpler.
Forgive me for use of the word "force"; you're right. It is more accurate to suggest that you are greating a curvature of space towards the rear of your car when you accelerate. Whether that makes it clearer to picture is open to doubt.
I'd love to go on; I love this stuff, but I think the mods have been remarkably lenient up to now, and I don't feel they need and help from me to get the scissors out.
V
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Wouldn't the fact too that most cars have dedicated air vents to allow stale air to be expelled have a slight/small effect on the movement of air within the vehicle; the air itself could never be still if the vehicle is moving.
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It is sadly a much observed phenomenon that to end a discussion about physics, all you need to do is attempt a relatavistic explanation. At that point, everyone's brain starts to hurt and they give up.
Except Einstien's, but I suspect he is not a BR member.
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You're all quite, quite mad.
No Dosh - Backroom Moderator
mailto:moderators@honestjohn.co.uk
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Relatively speaking, of course ;-)
--
Espada III - well if you have a family and need a Lamborghini, what else do you drive?
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