Another Physics Conundrum [POF]
 

I've been looking at the standard debates which arise when talking about hovering a model helicopter in an enclosed space (like an aeroplane with a model heli in it) and the Principles behind the semi-aerobatic manouvers carried out by some display pilots.

This looks like its got nothing to do with helicopters, but It may be relevant to the arguments which involve frames of reference.

Have a think about this one.

Ballon filled with helium tied to an inflexible anchor point in the middle of a car.

When the car accelerates (forwards) the balloon moves forward relative to the bit of the car its tied to. When the car decelerates the balloon moves backwards relative to the anchor point, when the cars speed is constant the balloon effectively hovers above its anchor point.

Windows and vents closed, blowers off.

Tried with several different lengths of chord and same results each time.
I can't say whether accel / decel was constant rate as I don't have a G meter in my car (funny old thing).

And yes I am certain about the directions, which is way have questioned such an inane detail, seems counterintuitive.

Gary
Guess whose just been to Pizza Hut with a four yr old?

Splot this probably wants adding to the POF Conundrums thread after a day or two, just though it was a newish observation and warranted seperate attention on a Sunday afternoon.

bouyancy effect. When you accelerate the air has inertia so is compressed at the rear of the vehicle. Density increases so bouyant balloon moves in opposite direction. Or something like that.

Is that the right way round? Similarly, today I had trays of Kitty Chunks placed on the front seat of my car. When I accelerated quickly from the lights, the Kitty Chunks tipped over towards the back of the seat; when I braked heavily to avoid Norfolk's finest drivers, the Kitty Chunks end up in the passenger footwell, neatly following Newton's First Law.

Cheers

Whirls

This is very similar to the candle in a jar on a rotating turntable, the flame goes towards the centre of rotation.

The whole point, as alluded to by Gaseous, is that the heavier object, either the air in the car or the tin of kitty chunks, moves to the back as one accelerates and to the front as one brakes. The balloon is therefore forced where the air isn't going (for want of a better phrase) so moves forward as one accelerates.

I think......................

G force meter
 

If you have a macbook or macbook pro, try this program for metering: http://www.suitable.com/tools/seismac.html

It's the sum of vectors. Lift pulls the balloon up, the string pulls the balloon forward.

Ummm...Fake, the string can't pull forward of its own pivot point, and i have never seen string PUSH anything yet.:bored:

The way AOPA PILOT Magazine explains this is that the balloon always opposes the forces of gravity. As the car accelerates, "gravity" pushes us rearward but causes the balloon to tend to go forward.

Works for me, I guess.

Things float because they're less dense than what's around them.

When the car's stopped, or moving at a constant rate, the balloon floats straight up.

If the car accelerates forwards, everything attached to it goes too, but the air isn't nailed to the car, so it sits where it was. However, it can't completely squash itself onto the back window - one of the properties of gases is that they tend to spread themselves out evenly if left on their own.

So what happens is, the air gets slightly denser towards the back of the car while the 'push' of acceleration is on (really, the car's just racing off and the air's not coming along at the same rate just yet).

This makes the balloon 'float' towards the front of the car. If it was heavier than air, it would go backwards, but in this case, it's lighter, so acts as an accelerometer (points in the direction of acceleration).

You can make an accelerometer by putting a bit of styrofoam in a jar of water held on its side. Whichever way you accelerate the bottle, the styrofoam will go that way too, because the more dense stuff lags behind and therefore pushes the less dense stuff out of the way.

I am nearly following this thread, so, if I stand in the back of of a large empty trailer travelling at say 40 Mph in comparison with the trailer I am static and equall to the floor of the trailer, if I then jump into the air I am no longer part of that trailer floor, er ... so would I splat into the back of that trailer?:confused:


Peter R-B

Peter - it depends on how dense you are;)

Hello Peter.

You're nearly there. If the trailers doing a constant 40MPH you will simply jump up and down on the spot feeling a bit of a dick. If its accelerating rapidly, you are denser than the air(or were last time I saw you) so you will splat on the back of the trailer.

If you were inflated with helium to the point where you float to the ceiling, You would splat on the front of the trailer if you jumped down off the ceiling while the trailer accelerates. (what a bizarre image!) Simple.

Phil

PofF
 

Can I join in?

Now we're playing tennis in a 40 knot wind.

But using golf balls.

My first serve is into the 40 knot wind and leaves the raquet at 60 knots and sadly strikes you on the forehead. Due to its inertia, for the first ten feet or so the golf ball speed is the initial 60kts regardless of the wind. So if you are standing just ten feet away, you're going to have a nasty bruise if not something worse. If however you are standing on the far baseline as inertia fades, you get struck by the golf ball at 20 knots ... a minor bruise perhaps.

We change ends and I serve a 40 knot golf ball aimed at your forehead. Once again you are standing too close at ten feet, and due to its inertia, you are again struck at 60 knots and another nasty but similar bruise. However, if you are on the far baseline, the groundspeed increases and you get struck by the golf ball at 100 knots. A possible fatality.

I think regardless of the 40 knot wind .. the ten feet bruising situation remains the same due to inertia and/or earth/space/ground speed.

Its been a bad Monday ... anyone for tennis?

Take care all,

DennisK

The car is pulling the string forward. Another important point is the air mass inside the car is moving with the car. Otherwise it'd would be just like running holding a balloon, drag would force the balloon back.

Jeez, you got nothing better to do Gary??? :hmm:

I can also bet that whilst discovering this car borne phenomena, you said to your wife 'look, isn't this amazing?' and she said 'eh? you having a laugh, nutter!'

Don't laugh - my wife asked what I'd just been up to, I said 'writing on Pprune about why a balloon in a car does ... etc', she says, yep, you're a nerd!

Soon after that, you'll probably be arrested for indecency.

So .. lets get this right, if I was stood on floor of said trailer doing 40Mph or Knts, and I was juggling table tennis balls and golf balls, the denser balls whilst in the air with trailer accelerating would move rearward away from the direction of acceleration, whilst the t/tennis balls being very light would move forwards:confused:

Peter R-B

Wot if you were juggling golf balls and helium balloons in a trailer which was being towed by a jumbo jet on a treadmill??? Would you reach escape velocity?

If it takes a yard and a half of calico to weave an elephant's underpants, how long will it take a double-breasted cockroach to tap-dance through a barrel of treacle?:eek:

Man I want to watch that tennis game Dennis...er from a distance that is.

You seriously have some interesting techniques for teaching physics.

Cant wait for your lessons on Newtons 3 laws.

Is it often your students survive to make it through to actual hands on flight training.

I bet they have reactions of a boxer and nerves of a high strung cat.

You lesson reminds me of the teachings of Dodgeball by Patches O'Houlihan.

" First you must learn the 5 D's..Dodge, dive, dip, duck, Dodge"!!!

"If you can dodge a wrench, you can dodge a ball"!!
Anyhoo

HF

Advanced Physics
 

Teaching techniques?

Don't get too many complaints - as they say ... from the guess whos!

Come on Gary ... launch another thread. I'm due another laugh this week.

Dennis K