MadSci Network: Physics
Query:

Re: HELIUM AND WATER BALLOONS IN CAR!

Date: Fri Nov 19 22:41:17 1999
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Physics
ID: 942721351.Ph
Message:

Greetings, Byungsoo Kim:

There are a few items of background information which must be combined, before your question can properly be answered. Please bear with me as I present some things which undoubtedly you already know.

When you are driving down a straight road at a constant velocity, every loose item in the car handily obeys Newton's First Law of Motion: "An object at rest or in motion will tend to remain in that state, unless acted upon by an outside force." The auto also obeys this Law, but because it constantly pushes air out of its way, and has frictional losses associated with the wheels on the pavement and the internal drive train, the engine must keep running to maintain a steady speed. Still, it takes less than 20 horsepower to cruise at 100 kilometers per hour (or about 62 miles per hour), thanks to the First Law of Motion. (The extra horsepower that an auto possesses is there only to allow rapid acceleration. No one wants to take 5 minutes to go from zero to 100 kph!)

Loose objects in an auto are "acted upon" by the body, seats, or some other part of the vehicle (whatever is touching a loose object), whenever the whole thing accelerates. The two most important points related to this are:

  1. "Velocity" is a concept that includes both the speed and the direction of motion, and..
  2. "Acceleration" exists whenever there is ANY change in a velocity.
Most people are quite aware that acceleration is always associated with a change in speed; it often takes some effort to convince them that when an object changes its direction of motion, acceleration is present there, also.

However, a perfect demonstration of this effect occurs whenever the auto follows a curve in the road. Let's assume the road curves towards the left, but let's wait a bit before following the auto down that path.

Let us first return to the straight road, and magically do two things:

  1. Stop the car instantly.
  2. Turn the car so that it faces left, also instantly.
What happens to all the loose objects inside the car? They are still going to obey the First Law of Motion, and try to continue going down the straight road. However, since the car is now both stopped and facing left, THE RIGHT WALL OF THE CAR IS IN EVERYTHING'S WAY. What happens next in this magical scenario is pretty messy, and we need not discuss it.

But I hope the point has been made: When an auto merely follows the curve of the road towards the left, a less drastic version of the same thing happens: The right wall of the vehicle gets in the way of every loose object's natural tendency to keep going straight. When each object comes to rest against that wall, then it begins experiencing acceleration towards its own left. At the end of the curve, when the auto goes straight again, everything in in will have finished accelerating towards the left, so the various objects will lay loosely once more.

To people inside a leftwards-turning auto, it may appear that loose items are being forced to the right, and it is certainly true that they are observed to move to the right. However, what is actually happening is that the right wall of the car is merely moving leftwards, and thereby intercepting/interrupting the original motion of the loose items. This is a simple 'relativity' type of phenomenon/illusion. (Another common example of such an illusion involves stopping your auto in a parking lot, just as a neighboring vehical begins moving; many people feel a momentary panic when they interpret the neighboring motion to mean that THEY are still moving!)


Now we have to cover a different topic for a bit, an airy one. Air is pretty slippery stuff, but like any other substance on Earth, it possesses both mass and weight. Mass is a fundamental property of any type of matter; weight is a measure of the Earth's gravitational force upon a mass. That last fact is important enough to repeat and simplify: WEIGHT IS A MEASUREMENT OF A FORCE.

There is a nice simple experiment which proves that air has weight (and therefore also has mass, since forces and masses are inextricably linked). One starts with two balloons of the same weight, and the kind of see-saw scale (a "balance") that can compare their weights. Then ONE of the balloons is inflated. The weight of the balloon does not change, but now it holds a quantity of air which is just a bit denser than the surrounding air. (The air HAS to be denser, because it is getting squeezed by the balloon, which is trying to shrink back to its deflated size.)

It is a common experience that of two objects the same size, the denser one will weigh more. Let us consider the balloon which has NOT been inflated: We can imagine a volume of air surrounding it which is exactly the same size and shape as the inflated balloon. This air is at normal density, of course, being uncontained. When the two balloons, and their accompanying volumes of air, are placed on the balance (and all OTHER air can be ignored, since there is the same amount on both sides), the inflated balloon will be found to be heavier, because the air on that side of the scale is denser than the air on the other side. But since we are detecting the extra weight of the denser air, we are detecting some weight of air! And therefore air has mass, as mentioned.

It might be noted that if the Earth's atmosphere was made of pure helium, and we repeated this experiment, the results would be the same: The inflated balloon will weigh more than the uninflated balloon, because the inflated balloon will contain squeezed/denser gas. The normal behavior of helium is due to the simple fact that its natural density is a great deal less than that of ordinary air. A volume of helium tends to float upwards in air for the same reason that a volume of ordinary wood tends to float upwards in water. The fluid nature of the denser and heavier material allows it to seep underneath the less-dense substance, and stay there. Gravity provides the force to make this happen....


Finally we can approach the answer to your question. When your auto turns towards the left, most loose objects, such as a water balloon, will appear to move to the right, as already described. However, one very important loose object has to be mentioned: the air in the car! All the ordinary objects in the vehicle are denser than air, but the helium balloon is an exception. As the auto turns leftwards, the air it contains will invisibly seem to move rightwards, just like all the other loose objects. The force provided by the leftwards-moving right wall of the auto causes a tiny pressure imbalance in the air, across the width of the vehicle. This imbalance is enough to cause a helium balloon to move to the left! The balloon is simply getting pushed aside by all the denser air piling up on the right, in exactly the same way that gravity pulls heavier air underneath a helium balloon, thereby causing it to rise.


One of the neat things about Physics is that when the explanation of some scenario is correct, predictions can be made concerning slight variations in the scenario. (If the predictions are fulfilled, then that is evidence in favor of the correctness of the explanation.) I therefore venture to say that if the windows of the automobile were open, and then a left turn occurred, air pressure could not build up on the right side, and so a helium balloon should appear to move to the right, along with everything else. If I am mistaken, then there is some other factor which my explanation has failed to take into account.


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