# Movement of a balloon in dead air

I had a helium balloon in my car tied so it hangs down by string toughing nothing. Windows are rolled up. Heat and AC are off. Air inside the car is not moving and car is parked with motor running. When I step on the gas pedal and car takes off forward balloon goes forward towards the windshield.

Repeat the same experiment with air inside the balloon hanging from ceiling by string. When car takes off forward balloon goes forward too.

Next experiment hand a rock from ceiling with string just like the balloon when car takes off forward rocks goes to the rear and balloon goes forward.

Inertia makes the rock stay in place as the car takes off.

I assume inertia makes air in the car stay in place too this produces a high pressure area in the rear and low pressure area in front so air pressure pushed balloon forward?

russ_watters
Mentor
It's not about inertia, it is about "up". The balloon goes up. What happens to "up" when the car is accelerating forward?

Bystander
A.T.
I assume inertia makes air in the car stay in place too this produces a high pressure area in the rear and low pressure area in front so air pressure pushed balloon forward?
Yes, the force of buoyancy is opposite to the pressure gradient, which has a backward component due to the inertia of the air.

A.T.
What happens to "up" when the car is accelerating forward?
Nothing happens to "up", if I define "up" as vertical to the ground. Without defining "up", you explain nothing by talking about "up".

russ_watters
Mentor
Nothing happens to "up", if I define "up" as vertical to the ground. Without defining "up", you explain nothing by talking about "up".
The context of what I said implies the definition I'm using. You aren't actually confused about it, are you? You don't actually think that definition of yours is better, much less even useful here, do you?

My approach here is to teach the broader principle first and push the OP to apply it, because it will be useful in other contexts. Sometimes teaching only to the question doesn't get there (er...though the OP does reference "down").

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berkeman
Chestermiller
Mentor
I think what Russ is alluding to is that, when the car is accelerating, "artificial gravity" is being created toward the rear of the compartment. This is a pseudo body force resulting from the use of an accelerating frame of reference. This artificial gravitational force creates a pressure gradient in the air within the compartment to accelerate it in the direction of motion (as reckoned from an inertial frame of reference). This, in turn, produces a horizontal artificial "buoyant force" on the objects within the compartment. The helium balloon moves toward the front of the car in response to this artificial "buoyant force." The rock, on the other hand, although experiencing the same buoyant force "sinks" toward the rear of the compartment under its own artificial gravitational (weight) force.

russ_watters
jbriggs444
Homework Helper
I assume inertia makes air in the car stay in place too this produces a high pressure area in the rear and low pressure area in front so air pressure pushed balloon forward?
I would say that this explanation is exactly right.

russ_watters
russ_watters
Mentor
I would say that this explanation is exactly right.
Fair enough. I guess my main concern was that citing inertia, while not wrong, seems superfluous. The acceleration vectors are the givens in the problem, so adding them together tells you the direction the balloon moves ("up") without using inertia. Using inertia will enable finding the magnitude of the pressure gradient and new buoyant force, but that wasn't asked.

jbriggs444
russ_watters
Mentor
I think what Russ is alluding to is that, when the car is accelerating, "artificial gravity" is being created toward the rear of the compartment. This is a pseudo body force resulting from the use of an accelerating frame of reference.
Yes, though I do mean it more directly in that for most purposes I can think of, "up" is the direction of the acceleration vector. I don't think the qualifiers (pseudo, artificial) are necessary. If you don't have a clear view of the horizon or some other context clue, you can't even tell what is "pseudo" and what is "real(?)".

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Chestermiller
With an air filled balloon hanging from ceiling by a string and a helium filled balloon attached to the seat and the rock hanging from the ceiling too both balloons go forward when the car takes off and the rock goes to the rear.

The thing that convinced me air pressure inside the car is pushing both balloon around is, if car accelerates slow balloons go forward slow but if car accelerates fast both balloons shoot forward very fast. Rock has several time more mass than balloons plus very little wind resistance compared to balloons so rock always goes to the rear no matter what speed car accelerates. If car takes off very fast enough rock goes back and hits the ceiling and both balloons go forward and hit the front window.

Another interesting observation is while driving down the road at 50 or 60 mph rock and balloons hang there is place as long as car speed stays the same the mass of air inside the car is moving at the same speed of the car. If I drive 50 mph around a sharp curve to the left balloons move to the left while rock goes to the right.

I put 1 helium balloon inside the car with no string it stays up on the ceiling near center it is extremely hard to accelerate slow enough to keep the balloons in place on the ceiling. While driving along going places like Walmart etc balloons goes forward when car stops at traffic lights then goes to the rear as car takes off. Balloon goes left when car turns left then it goes right when car turns right.

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berkeman
A.T.
jbriggs444
Homework Helper
The thing that convinced me air pressure inside the car is pushing both balloon around is
You realize that buoyancy and air pressure gradients are the same thing?

russ_watters
Mentor
With an air filled balloon hanging from ceiling by a string and a helium filled balloon attached to the seat and the rock hanging from the ceiling too both balloons go forward when the car takes off and the rock goes to the rear.
Have you actually seen this happen? The hanging balloon should go backwards at the same angle as the rock. With a positive (up) net force (buoyancy - weight), the object should move/angle forward and with a negative net force it should move/angle back.
Rock has several time more mass than balloons plus very little wind resistance compared to balloons so rock always goes to the rear no matter what speed car accelerates.
Buoyancy, not wind resistance but otherwise yes. The buoyancy of the rock is typically ignored because in all cases we generally see, the rock is much, much denser than air. But the same principles/equations are at work for all the objects here.

russ_watters
Mentor
I put 1 helium balloon inside the car with no string it stays up on the ceiling near center it is extremely hard to accelerate slow enough to keep the balloons in place on the ceiling.
The effect is magnified by the fact that the ceiling is near flat. The balloon is actually "seeking" a surface perpendicular to the force vector; a "ceiling" that is "up". This is why bubble levels are made nearly flat (it increases their sensitivity).

jbriggs444
Homework Helper
Have you actually seen this happen? The hanging balloon should go backwards at the same angle as the rock.
If the air in the car were well stratified with hot on top and cold on bottom, buoyancy could drive the top air forward during acceleration, carrying both a helium balloon and an air balloon forward together. This would be wind pressure rather than direct buoyant force in action.

russ_watters
russ_watters
Mentor
If the air in the car were well stratified with hot on top and cold on bottom, buoyancy could drive the top air forward during acceleration, carrying both a helium balloon and an air balloon forward together. This would be wind pressure rather than direct buoyant force in action.
Good point. Particularly when it is cold outside and you have the heat on (large gradients), you can feel the air in the car "slosh around" due to accelerations.

sophiecentaur