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Pop Quiz - How smart are cycling geeks?(23 posts)

Pop Quiz - How smart are cycling geeks?jose_Tex_mex
Jun 13, 2002 9:14 PM
I see a lot of Physics related questions around so I thought I would post one myself. Here goes. Remember to explain your answers. I will give no credit to those who just guess - since it's a 50/50 shot.

You are in your car (sorry, cannot change this to a bike) at a red light with the windows closed. There's a balloon attached to your gear shifter which is floating in the air - straight upwards. When the light goes green you hammer the gas and accelerate forward. The acceleration pushes you back in to your seat. WHICH WAY DOES THE BALLON GO AND WHY? For the adventurous: while at speed you make a left turn, which way does the balloon lean.

Extra Credit: When replying put either "Forwards" or "Backwards" as the first word in your Topic area.

Depending on how much interest there is (and discussion) I will post the answer. Perhaps, before the Friday 5 c'clock whistle.
Jun 14, 2002 5:31 AM
I think I remember this one from physics class. The air in the car is more dense than the helium (I presume) in the balloon. When the car speeds up, the denser air will tend to move to the back of the car (relatively speaking), pushing the balloon forward. In a left turn, the balloon would go left. Same reason.

Now, if you were in a convertible with the top down, I have no idea. :-)

Why would you put your windows up in a convertable? (nm)Kristin
Jun 14, 2002 6:26 AM
backwards the baloon 'appears' to move, but actually the car128
Jun 14, 2002 6:10 AM
moves forward, and due to inertia, the baloon tends to remain at rest.
Then there is the point at which the baloon, since attached to the moving body, has force acted upon it and it accelerates to the velocity of the car. Once equalibrium is reached...da da da.
When the car turns into the corner, again, inertia: an abjects resistence to change, dictates that the 'loon wil continue at the velocity (speed and direction) of the car, i.e forward, but will 'appear' to move right when car goes left, until the attached string causes a 'new' force to act upon the 'loon and it will proceed in some centripital tangent, i suppose??, to the new course of the car.

I think this is the principle of point of view and inertia, like parallax motion?. And if I'm correct, then you are wrong: the balloon does not move either forward or backward when the light turns green; it is the car which changes position- to forward.
If I remember correctly, This is the correct answer....(nm)Dave Hickey
Jun 14, 2002 6:17 AM
relative motionDougSloan
Jun 14, 2002 6:45 AM
I think we are talking about relative motion here. Sure, if the car moves forward, everything in the car will move forward, too, else it would go out the back of the car.

We are talking about relative motion, within the frame of reference of the interior of the car. What does the balloon do with repect to the car?

Forget the car for a while. Why do helium balloons rise? It's because air is denser. The denser air is pulled down by gravity more strongly than the balloon is pulled down. When that happens (in a fluid), the air pushes the balloon up.

In a car, the air has more "resistance" to change in speed and direction than the balloon (inertia). The denser air tends to want to move to the back of the car (relatively) than the balloon. When it does that, it pushes the balloon forward with repect to the car interior.

Jun 14, 2002 7:17 AM
I actually would have guessed backwards until I read this.

Smarty pants!
my head hurts. nmJS Haiku Shop
Jun 14, 2002 7:18 AM
yeah, but which direction does it go when you accelerate? nmSpinchick
Jun 14, 2002 7:58 AM
toward the porcelain phone, where i talk to god. nmJS Haiku Shop
Jun 14, 2002 9:24 AM
What color is the balloon?bikedodger
Jun 14, 2002 8:56 AM
Good Question - The balloon is done in 2002 Mapei Team Colorsjose_Tex_mex
Jun 14, 2002 9:04 AM
That certainly simplifies the analysis!bikedodger
Jun 14, 2002 9:22 AM
Answer Time - I guess we are all "kind of correct."jose_Tex_mex
Jun 14, 2002 11:37 AM
I am sure you are just eagerly awaiting the answer so here goes. As for the extra credit - kudos to Doug for reading and answering the question. His answer is basically mine. However, as stated below I should have set the question up better...

When the car accelerates forward the balloon leans forward.
While at speed if you hit the brakes the balloon moves back.
In a left turn the balloon moves to the left. In a right turn the lean is to the right.

Why is this? Because the balloon is floating the gas inside must be lighter than air - say helium. Balloons go up because of this. Balloons do not go too high because the air gets thinner - air pressure is greatest at sea level. That is, the higher up you go the less dense the air is. Back to the car.
When you accelerate forward there's a rush of air to the back, you create a higher pressure of air in the back of the car. Instead of floating upward away from the denser air, the balloon now wants to "float" forward.
The same in the other cases. The air pressure pushes the balloon in the direction of the acceleration.

Now I just thought of something else - feel free to add in your 2 cents. During a very short time interval when the car is just accelerating the motion of the air is to the back - air pressure has not kicked in. I suppose the balloon initially will go with the air (towards the back) and then forward. Definitely, the overall viewable motion is to the front. Sorry, I guess I should have structured the question better
'move' relative to the car, or to the air 'in the car'??128
Jun 14, 2002 12:04 PM
at the point of acceleration it seems even if the ba-freakin'-loon does get pushed forward in the gasseous arena, it still moves backward with reference to the car? I guess the question would be: does the car move enough to make this true before the helium v. denser air reaction occurs....

Science is Fun!

But I hear ya mun, good question. Doug gets the gold star of the day....!
Jun 19, 2002 9:08 AM
Since we are mashing on the gas and then braking, are we not considering the natural movement of the car.

Acceleration, especially in a rear wheel drive car would force the front end upwards, thus creating the impression the balloon went forward. Same thing when braking, the car will tend to lean forwards, thus giving the impression of it going backwards. Thus the turns as well, turning right will typically cause a car to lean to the left, and vice versa to the right when turning left.

I would think that this will have significantly more impact on the perceived "motion" of the balloon relative to the car than the weight of the air...
Wrong - From Any Frame of Referencejose_Tex_mex
Jun 19, 2002 2:47 PM
An obeserver outside sees the baloon move in the way we have described.
The driver, in the same reference frame as the car sees the balloon move in the way we described. Thus, your idea that it's the motion of the car which makes the balloon appear to move disproved.
Get a helium balloon and try it. You'll see it has very little to do with the body of the car.
the REAL answer is...ColnagoFE
Jun 14, 2002 12:47 PM
who gives a sh*t. what do i win?
You win a Langstrom two-handled cheese straightener....128
Jun 14, 2002 12:52 PM
...and a slow crawl over broken glass to the bonus round!
I was hoping for the electric dog polisher nmDougSloan
Jun 14, 2002 4:22 PM
maybe you can swap for a gasoline powered turtle neck sweater?nm128
Jun 17, 2002 11:49 AM
Jun 14, 2002 8:07 PM
Inside or Outsidejose_Tex_mex
Jun 19, 2002 2:48 PM
Observers inside or outside will relatively see the same thing.
There's slight differences. However, I doubt if we could perceive them.