Balloon Ride: Newsletter Challenge (September 2014)

It moves the opposite way you would think. In this case to the left.

Negative buoyancy is the reason.

The string is heavier than the balloon so it is acted upon by forces opposite to those which would act if you had a weight suspended by a string.

Kids love this.

Inertia is base on the mass not the weight. As the balloon has positive mass the movement would be to the right after the turn. See my last post of a more complete explanation.

According to the laws of inertia or Newton's first law; bodies in motion stay in motion and in the same direction unless acted upon by another force. Or something like that.

It is the same as when your head and body moves and leans opposite to the direction of the turn in a car. If you are turning right, your body will want to keep going straight but the car movement will force you to the left until the car staightens out and then you resume a straight motion relative to the car after it turned.

The balloon, assuming it floats in the car, will move to the left. The air molecules will exert a force all moving left. The balloon will try to move in a straight line relative to the car. Now if you are driving a convertible, expect the balloon to move in the direction you were travelling prior to the curve.

GA- but only if the windows are up!

You are right about that. There are so many variables that could affect this test. We all know what the right answer is IF you consider the right answer to be the most obvious and leave out all other factors such as the windows and or the fact that when the car turns the air in the car does NOT turn at the same rate thus causing wind inside the car even it the windows are up. And if turning fast enough in a long skinny car and the turn being very sharp at the 30 MPH givin in the question the air currents caused by the turn would overpower the higher density answer the person/s that made up this question were looking for. With the windows open at 30 MPH the ballon is all over the place and the test is useless. A good example of this kind of lack of data for the question is the barometer question we got in school and the many answers that are in fact all correct but never the less NOT what the teacher wanted. So we had to go to the principle to get him to override the bad grade some of us were given for in Fact THINKING. And our answers were correct. See the Movie Phenomonin with John Travolta when he takes the IQ test. Now that is a good example of these test questions. But I still like them. Cesare Cesare

Correct, right. It does move counterintuitively, to the right, not the left. My mistake.

The air molecules move to the left, pushing the balloon to the right.

Kids still like it.

I saw this on one of the science programs in TV. When the vehicle makes that right turn the balloon moves towards the left side of the car simply due to inertia. Then something interesting happens - after a short time the balloon moves to the right. Why - because when the car makes that right turn the air inside the car moves towards the left side of the car due to inertia just as the balloon initially did.

But then the air "bounces" off the left side of the car and moves towards the right side. Since a balloon has a very low mass/volume ratio it is easily pushed by the bouncing air towards the right side.

The same thing happens when the brake is applied. The balloon initially surges forward and shortly after surges backwards while the car is still decelerating.

The forward speed is slowing so one would expect the balloon to be pushed backwards as well.

AH -

What happened to the person holding the other end of the string when the car turned and negotiated the curve?

His/her movement, reaction's to the car turning will have a direct effect which should also influence the balloon's net motion if any...

The balloon turns to the right.

If the car is travelling horizontally at a uniform velocity the balloon will rise straight up. As the car turns, 'straight up' is now at an angle veering toward the right and the balloon will want to rise in that direction instead, with the string trailing to the left.

Looking at it another way: rather than the car turning to the right, assume instead that the car is stationary but is rotated about its roll axis through the same angle as the net acceleration vector in the turn. The balloon still wants to rise straight up, except that 'straight up' has now been redefined with respect to the car.

It's the New Up.

The balloon moves to the right, toward the inside of the curve. The air in the car is heavier and moves to the left, toward the outside of the curve. And if you put on the brakes, it flies to the back of the car.

You've done this before.

Oh, yeah. Balloon bouquet on Valentine's Day!

What about the static charge on the balloon?

Illusionists just love to make things that look wrong.

You get the same effect in an air conditioned urban train - with or without the balloon. When the train slows & stops, the warm (upper) air runs to the back, cooler (lower) air to the front.

Vice versa when it takes off again.

A film temp sensor (fast acting) should show this - have one at the front, one at the back of the train.

Smarter every day:

https://www.youtube.com/watch?v=y8mzDvpKzfY

Short Answer When you make a right turn the ballon will tilt to right. The reason is because of acceleration ( Breaking and Acceleration are the same thing in Physics in most and in this case ) of air that even though the car turns to the right the air wants to keep going the direction and speed it was going. All things do if not acted on. That causes the air to move to the left of the car and become more dense on the left side of the car As the air becomes more dense the ballon being filled with Helium will move to the right as it is lighter than the heavier air. Now Longer answer. This is both a great question from the standpoint of making and helping a person learn to think. There are other factors. So the answer above is only in part correct. In Fact the ballon will move to the right due to the higher air density but it will also move to left to some degree depending on the speed of the turn. That data was not given in the question only the speed the car was traveling. But if the turn is sharp the air not only builds up and gets heavy on the left side forcing the lighter ballon to the right but also the air is being forced to turn with the car in part but not as fast. Since the car is turning to the right and the air wants to keep going the direction it was going unless the car was round and had no parasitic drag from seats and from the fact the most cars are rectangle the car is going to force the air to turn to varying degrees based on how tall the seats are and the length of the car etc..As the car turns and the air does not as turn as much as the car and if the ballon is near the front of the car the ballon will be affected by the air that is forced to turn with the car thus forcing the halon to the left. Also the we do not know the weight of the string thus we do not know how much of a reverse effect it will have on pulling the ballon to the left as the car turns to the right. But back to the air. How many of you have driven on a hot day with the air conditioning on. Most of the air from the air cooling going and staying mostly in the front of the car. If you take a fast turn to the right or left or even better a 180 degree turn in rather direction you not that for a short time the air is hotter as some of the air that was in back in now in front. If the car had no driver in the cab of the car where the ballon was and if the inside was round without any protrusions of any kind the air would not turn with the car much if at all. Thus the only action of the centrifical forces and acceleration in the turn thus the ballon goes to the right as stated in the sort version. So many many factors will have an effect. This question is a great and at the same time a crazy question from a physics standpoint. And I love it for that reason. Another example of a question asked in a high school science class was "How can you tell the hight of a tall building using a barometer. This question is similar as there is one answer that the teacher is looking for. But there are in fact a lot of answers. One student who was clearly board with the question gave 3 answers. None of the what the teacher was after. But all correct answers. First the student said you could take the barometer to the top of the building and drop it off and time how long it takes to smash into the ground using the 1 Gravity value of acceleration of 32 feet per second per second. Another answer the kid gave was to find the owner of the building and say " Hey, I will give you a good barometer if you tell me how tall your building is" Anyway you get the point . I would love to go on and on about both the barometer and the ballon. Cesare

Your answer not not take into account the Kintic Theory of Gasses which states that all gas at the same level within an enclosed space are at an equal pressure. Air does not form into denser regions within a vessel due to the motion of that vessel.

Equal pressure, yes. Equal density, no. The density of any particular gas also has to take into account its molecular (NOT atomic, bitten myself on that one before) weight as well as pressure and temperature factors. So the effect of turning the vehicle is similar to a gas centrifuge, causing denser gases such as CO2 as well as cooler (and therefore more dense) air to move towards the outside of the turn, displacing gases of lower density (including the helium inside the balloon, and therefore the balloon itself) towards the inside of the turn.

Having said that, I should add that the question does not specify the following points that are needed to give an accurate answer:

1. Is the helium balloon properly inflated, and actually pulling upwards on the string?? My kids will keep playing with them long after they've lost any semblance of buoyancy (then POP, of course!).
2. Is the car sealed- windows up, convertibles and leaky old Model T's need not apply.
3. Is driver wearing socks with sandals.....

Not even equal pressure. Pressure is directly proportional to density of a gas. PV=nRT

Not true.

When a container of gas is accelerated, the force of acceleration acts on each gas molecule independently. Each molecule acts according to Newton's laws of physics.

The same holds true with the Earth's atmosphere and it is densest at the point closest to the Earth's surface and eventually thins out as you ascend upward, where the force of gravity is much lower.

The key here is acceleration. If there is no acceleration (or even gravity), then the gas will be of uniform density inside the container).

The balloon will appear to move to the left because it is a free-floating mass and the car is turning.

If the balloon is visible through the car windows, observed from outside the car, the balloon turns left with the car. The additional oscillations created by the turn could also be observed from the outside, but as any observer within the car has reoriented through 90° and the external observer has remained stationary they each see the movements with a 90° rotational displacement.

Three separate oscillations are imparted to the balloon within the car.

Because it is only tethered vertically, the balloon will attempt to continue along the original track of the car. From inside the car this is observed as a move to the right after the turn is completed. The amplitude of the movement is determined by inertia which is a function of the car's forward speed, the balloon mass, and the speed of deceleration along the original track (sharpness or radius of the turn). During the turn the movement will at first appear to be forward and will then swing to the right. The movement will be countered by air resistance, determined by the size and surface texture of the balloon, the length of the tether, and the fact that the tether arcs away from the vertical so the balloon moves below it's apogee. Buoyancy will pull the balloon back to the vertical, but it will overshoot due to inertia, so a damped oscillation is created.

As the car rotates a torque is created within the tether. The tether will attempt to return to it's natural state exerting a force that rotates the balloon around the axis of the tether. Again there will be an overshoot and a second damped oscillation will be created. The amplitude of this oscillation is determined by the speed of the turn, the torsional rigidity of the tether, and the mass distribution and the diameter (moment of inertia) of the balloon.

Any disturbance of the tether will cause it to elongate and contract with an amplitude and frequency based on the elasticity and length of the tether. Again this will create a damped oscillating movement at right angles to the axis off the tether.

While these movements will occur, their amplitude may be so small and the rate of damping so high as to be unobservable without sophisticated instrumentation. Given additional information it would be possible to mathematically predict all the movements with a high degree of accuracy.

Sorry I did not correctly read the question. Zero marks for part one of my answer as I read it that the car turned left. Subsituting left where I put right and right wher I put left corrects my mistake.

Since the other end of the string is handheld, the balloon will be pulled and follow the direction of the person holding it.

The centrifugal force affecting that person will prevail so the net balloon motion will be to the Left..

Your all wrong! It moves down......after it pops on one of the zip screws sticking through the roof of the car holding my redneck sunroof on.

the driver sees a motion at the balloon to the left, this motion is stopped by the string and slowed down by the air resistance inside the car. when finished the turn, the balloon turns to the right and stays then again. when driving a circle the balloon stays with an angel of the string other than 90 degrees to the horizontal.

depends on the density of the gas inside the blloon and outside; is the balloongas heavier than the air inside the car?

What happens to the balloon when the car jumps to ludicrous speed?

In a slow turn, the balloon will remain in the original position as the inertia of movement is not sufficient to overcome the inertia of the stationery object. However, in a sharp turn (as in a movie chase) the balloon will move to the apparent left as the balloon is still moving forward or in original direction of travel (inertia) as the car turns right. The same effect may be observed without the danger by using the same balloon and starting quickly from a stop. The balloon will move to the rear of the car.

OK, with all of the very valid "what if" scenarios proposed here I'd like to make a proposal. I propose this scenario/question instead of the proffered question of this blog:

The string of a helium balloon is floating in the back seat of a car because a child has just released it. The rear, right window of this 825 Sterling sedan has only one open window when the driver makes a quick [no braking] 90° turn and the balloon floats out this right rear window to the bewilderment of the infant in the child seat. Was it more likely that the driver made a right or left turn to lose this balloon? How fast would the car have to be moving for just the Bernoulli pressure differential at the window to make the change in direction meaningless concern? The partial vacuum differential would cause enough of a draft that this untethered balloon would eventually leave.

Right, but it might depend on continuing the turn for a bit.

Been there, done that but there is a caveat. Provided the "air" inside the car is denser than the Helium and the string holding the ballon has negligible mass, the ballon will move to the inside of the corner i.e. to the right. If the atmosphere inside the car was less dense than Helium, the ballon would move to the left (relative to the car). A similar effect can be felt in a car on a cold day soon after you turn the heater on (provided you are a sensitive new age person!!). Turning a right hand corner (if you drive on the correct side of the road as we do in Australia), will wash the drivers head in warmer air. If you happen to drive on the wrong side of the road, for example in the US, then the driver would need to turn left to feel the warm air around his head.

If the balloon is untethered and floating free within the car, the mass of the helium and it's casing is exactly equal to the air displaced. Would it move? If the balloon is untethered and lighter than the displaced air, it interacts with the roof of the car and the roof shape/texture and frictions characteristics will be the determining factors. If the balloon is tethered then any displacement sets up complex oscillations within the tether. The question then becomes the direction of the initial displacement.

There are two forces at work. Inertia, and the supposed movement of air around the car. Many contributors would appear to have malfunctioning heating/cooling/air conditioning systems within their vehicles. My air conditioner maintains an evenly distributed air temperature due to the fan. Even allowing that I switched off the air conditioning to conduct the experiment, temperature gradients with a 2-3m³ space with a 1.2m height and filled with seats acting as baffles is not going to be excessive. The air pressure difference due to gravity over 1m is approximately 3.8 x 10^-4Psi. The general opinion is that air will slosh around the car like water in a partially filled tank. This only happens because there is a large pressure difference between the air and water at the surface so the water has somewhere to move to. With a totally filled container, be it water or air, movement is restricted by the lack of a place to go. Try dispersing a dye in water by a stirring paddle. With a partially full tank the dye disperses quickly. With a totally full tank the water moves as a block with the paddle and the dye does not disperse. All mixing tanks are designed with an ullage space for this very reason. It is my belief that the air does not move.

Initially the balloon moves to the left of the car due simply to inertia. Then after a second or so the balloon moves to the right! Why? Because when the car makes that right turn the air inside the car just like the balloon moves towards the left. But after a short moment the air "bounces" off the left side and toward the right side. Since the balloon has a very low weight/volume ratio, it is easily pushed by the air to the right side.

It's the same thing if you suddenly brake. The balloon moves toward the front and shortly after moves towards the rear!

Here's the answer to your question. I had no idea the balloon would do what it did. Great Question!

Driving a Car with a Helium Balloon: Physics

http://www.youtube.com/watch?v=XXpURFYgR2E

Touche for finding a video that shows what actually happens. A demonstration reduces the discussion to just why this happens instead of what will happen.

It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong.

Richard P. Feynman

Read more at http://www.brainyquote.com/quotes/quotes/r/richardpf160383.html#Zx8Sh08xTBSLddpp.99