The puzzling balloon

[friartuck]

Just a puzzling question here--I wonder if anyone has ever noticed this strange physical characteristic of a helium balloon.

One day I was in my car driving home from the local fair when I noticed that the helium balloon thatone of my children had was doing something that didn't look right.

As it floated on the end of a piece of string, we went round a sharp bend.

The balloon leaned over but not away from the bend, it actually leaned into it.

If you don't understand the question, just think of it in reverse. Suppose I had a piece of string and dangled a weight on the end of it. As I would go round a right hand bend, you would expect centrifugal force to throw it outwards (To the left).... but the balloon does exactly the opposite.

Try it and you will see what I mean.


Any suggestions????



Friar Tuck of Sherwood

 

[GregLocock]

Did that one at the age of 15

Big clue - which is heavier, a balloon sized lump of air, or the balloon?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[25362]

As Greg Locock implies, the mass of heavier air moves outward; the net result being that the lighter helium balloon leans inward.

 

[Skogsgurra]

You can do it in a large glass of water. Tie a cork to a string and fix to the bottom with some water resistant glue. Fill glass with water so cork is submersed. Move glass forward - cork moves forward.

It is not much of a puzzle - but having an object moving the wrong direction in a car is probably a bit puzzling. Now, where do I find a helium ballon?

Gunnar Englund

http://www.gke.org

 

[MikeyP]

2 premises

1) When stationary on the surface of the earth, I experience a downward force due to gravity and the balloon experiences an upward force due to bouyancy (which is simply an arse-about-face way of looking at gravity). So the force I experience is the opposite of the force experienced by the balloon.

2) Einstein said that the force experienced by a body due to gravity is the same as the force experienced by a body in an accelerated reference frame. This is the "principle of equivalence" and it implies that the laws of physics are the same even for observers in an accelerating reference frame.

In the car going round in circles I am in a constantly accelerating reference frame. So when I feel an outward force due to the acceleration of the car, the balloon must still feel the opposite, ie. an inward force because the laws of physics in the car must be the same as when I am stationary.

M

--
Dr Michael F Platten

 

[25362]

I think Dr Platten is right. A car rounding a turn is actually an accelerated frame of reference. From your perspective as a passenger, you're at rest in an accelerating reference frame. Yet you seem to experience a force that pushes you towards the outside of the turn, and you may actually slide across the seat.

Passenger and car are simply obeying Newton's laws. The important point to understand what is going on is to apply Newton's laws not in the car's frame of reference but in the inertial frame in which the car is explicitly accelerating.

A frictional force acts to keep the car accelerating in its circular path. The force acting on the passenger is the friction of the seat against her/his bottom, and it's directed toward the center of the curved path that the passenger and the car are following.

If the frictional force isn't big enough to provide the force mv²/r needed to keep the passenger in circular motion with radius r, then the passenger will describe a larger radius, and will therefore move with respect to the car.

The balloon, along with everything else in the car, is accelerating because it is in circular motion. The only force capable of providing this horizontal acceleration is the horizontal component of the tension of the string. Thus the balloon is bound to describe a smaller radius.

By measuring [θ] the angle of deflection of the string one can find the car's velocity

v = [√] (r.g.tan[θ])​

 

[strokersix]

Agree with the acceleration and buoyancy arguments above.

Remember also that (assuming closed windows and the HVAC fan isn't on high speed) the mass of air in the car will tend to remain in a fixed position while the car enclosure rotates around the air when entering a turn. If the balloon is in the back seat this will cause the balloon to appear to lean into the turn.

 

[MikeyP]

I don't think it matters if the air in the car is moving or not (apart from the fact that the balloon would be buffeted about). The upward bouyancy force is unrelated to the motion of the medium.

Ha. I think I've just invented a new item for the applied mathematicians tool box. Together with the massless infinitly flexible string and the frictionless pully, we now have the "dragless" helium balloon!

M

--
Dr Michael F Platten