Physics majors, Part Deax
Thread Starter
Safety Car
Joined: May 2000
Posts: 4,820
Likes: 347
From: Albuquerque NM
Physics majors, Part Deax
So you are driving your Vette, with a helium filled balloon on a string, tied to your shifter. Windows up, no air movement in the car, steady speed. If you step on the gas and accelerate the car, what will the balloon do?
Will it:
A. Move back
B. Stay where it is
C. Move forward
Will it:
A. Move back
B. Stay where it is
C. Move forward
Burning Brakes
Joined: Nov 2004
Posts: 993
Likes: 0
From: PA
I say: it will move back. The only method of acceleration imparted to the baloon is via the string attached to the shifter. As the vehicle accelerates, the energy is imparted to the baloon via that string.
Burning Brakes
Joined: Sep 2001
Posts: 1,173
Likes: 1
From: Aguila AZ
The balloon will move forward. This is because air is heavier than helium and the accelleration of the car moves the air mass back forcing the lighter balloon forward. This is like a centrifuge only in a straight line...
Shirl Dickey
Shirl Dickey
Le Mans Master
Joined: Jun 2004
Posts: 6,269
Likes: 50
From: Dallas Texas
St. Jude Donor '06-'07-'09-'10
Simple as Newton's First Law of Motion. Every object in a state of uniform motion tends to stay in that motion unless an outside force is applied. In this case, it's a state of non-motion and the force is the string. The question is about the air that supports the balloon? You would think it cause the balloon to move backward because it, too is at rest? 
Last edited by sxeC7; Jul 26, 2006 at 11:07 AM.
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Burning Brakes
Joined: Jan 2006
Posts: 1,226
Likes: 45
From: Castleton VA
Cruise-In VIII Veteran
Originally Posted by itchynackers
Actually, thats not exactly true.
The balloon will move forward RELATIVE to the pavement.
The balloon will move backward RELATIVE to the car.
The balloon will move forward RELATIVE to the pavement.
The balloon will move backward RELATIVE to the car.
The balloon moves forward relative to the car (as well as the pavement).
This is not because the car acceleration somehow causes the air to move "back" to the rear of the car (to then push the balloon forward), but because the air's inertia prevents it from moving with the car until the car's rear window starts to push the air molecules forward. As the rear window advances, it essentially compresses the air from behind, creating a higher pressure in the air in the rear portion of the passenger compartment. It is this transient higher pressure air which pushes the balloon forward.
Le Mans Master
Joined: Oct 2005
Posts: 7,088
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From: Metro Detroit Michigan
Originally Posted by MrEracer
The balloon will move forward. This is because air is heavier than helium and the accelleration of the car moves the air mass back forcing the lighter balloon forward. This is like a centrifuge only in a straight line...
Shirl Dickey
Shirl Dickey
Except that the balloon might initially move backwards very slightly, following the air movement toward the rear of the car.
Net movement would be forward.
Team Owner
Joined: Jul 2004
Posts: 124,339
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From: .... NJ
Cruise-In VII Veteran
St. Jude Donor '05 thru '19
Ok, so you have an enclosed C5 with a bunch of bees in it. At first all the bees are resting on the floor of the C5. Next all the bees are flying in the C5. What happens to the weight of the C5 when the bees go from resting to flying?
Safety Car
Joined: Nov 2004
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From: Si Vis Pacem Habeatis Filios Vestros, Ad Bellum Parare Modo
Originally Posted by StephenT
Ok, so you have an enclosed C5 with a bunch of bees in it. At first all the bees are resting on the floor of the C5. Next all the bees are flying in the C5. What happens to the weight of the C5 when the bees go from resting to flying?
Melting Slicks
Joined: Jul 2005
Posts: 2,572
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From: Rehoboth MA . . . NRA Life Member
St. Jude Donor '06-'07
Originally Posted by StephenT
Ok, so you have an enclosed C5 with a bunch of bees in it. At first all the bees are resting on the floor of the C5. Next all the bees are flying in the C5. What happens to the weight of the C5 when the bees go from resting to flying?
