When your car turns, does the air mass inside it turn with the car or is there an inertia to it, like water that has a delayed reaction when you rotate a glass?
I was thinking about this while in the car today. The A/C was on. It seems like I can feel the cool breeze change a bit when turning from a traffic intersection. Maybe it’s just the warmth of the sun moving around, maybe it’s my imagination?
A little of both. The air has its own inertia, including rotational inertia, and so will want to maintain its orientation independent of the car.
But the air is also strongly connected to the interior of the car by friction, and so the doors, windows, windshield, roof, floor, and seats will all help drag the air around with them as they turn.
What you’ll end up with is some mixing, some turbulence, where the central mass of the air does not rotate, but where the periphery of the air rotates as it is dragged by the car interior.
If some of the air in you car is warm and some is cool, the cool air will move to the outside of the curve as you make the turn. So, if you’re making a left turn, it’ll move to the left side of the car. This is more noticeable, when you’re going around a curve or making a turn without stopping rather then sitting at a light/stop sign and then turning. When that happens the cool air first moves to the back.
This is basically Newton’s First Law. The cool air is dense and pushes it’s way through the hot air. It’s no different then hanging something from your rear view mirror and watching it move around as it pushes itself through the (considerably, to the point of being negligibly) less dense air that surrounds it. The difference is that you can only feel it and not see it so it’s really strange until you know what the hell is going on.
If you have a helium balloon floating free inside your car, it will move towards the front of the car when you accelerate and it will move towards the rear of the car when you brake. If you picture the cooler and heavier air sloshing around almost like a liquid, the movements of the balloon (pushing up towards the lighter air) make a lot of sense.
(moving air from your A/C may disrupt this)
The other difference between air and water is that air has a lot less mass, hence you need smaller forces to get it moving.
Technically, both actions are “acceleration”. But I think you have it backwards. Hit the gas, and the balloon will appear to move to the rear. Hit the break, and it will “move” towards the front. Just as you do, in your seat. Or is there some phenomenon I’m missing that affects helium balloons differently than other objects?
Yep, you missed. The air is actually heavier than the balloon so it pushes to the back of the car when you accelerate, forcing the balloon forward. And the air rushes to the front of the car when you brake, pushing the balloon backward.
Bubbadog
(who has had a bunch of balloons attack him in the front seat as soon as he drove away from the store)
engineer_comp_geek has it right, but it’s an easy one to get wrong.
Very cool about the helium. (No pun intended.) Inertia, acceleration. Friction, convection.
Add some thermoclines to the helium and you’ve got yourself a nice little analytical bon bon.
An easy way to think of the helium balloon is that acceleration is indistinguishable from gravity. If you let go of a helium balloon in an atmosphere in a gravitational field, the balloon “falls” in the direction opposite of gravity, and so too in the accelerating car.
What a cute video! Perfect demonstration.
Side note: A family member of mine once passed out from one inhale off a helium balloon. Lack of oxygen. Dropped right to the pavement in mere seconds. So be careful doing that.
Thanks for the great answers. My hubby sometimes makes fun of the obscure things I wonder about so I can’t wait to tell him this is really real!
I don’t think you can necessarily attribute the change in the direction of the air to it’s being denser. As it comes out of the vent, it is moving, and will tend to keep moving in that same direction as the car is rotating. That will also make it deflect. Think of a rotating sprinkler. The water will spiral outwards, but not because the air is blowing it around.
To really know if it’s because the cold air is denser, you’d have to also try blowing hot air, and see if the hot air deflects in the opposite direction of the cold air, or in the same direction as the cold air.
This is a different effect from my long hair blowing forward when I drive in a open convertible, right?
Yes. Your hair blowing forward is due to the air flow over the convertible. The air basically gets pushed up over the front window then swirls around over your head and comes back at you from behind. This isn’t the greatest picture but I think it gets the point across:
http://www.v12performance.com/images/side-view-arrow-before.jpg
Airflow in an open convertible is going to be quite different depending on the aerodynamics of the car. You need a glass shield behind you.
Doesn’t have to be glass. Some convertibles have a little pop-up frame behind the seats containing mesh like window screening. That’s disrupts the airflow enough to make the ride much less windy.
Not your imagination. I notice this all the time, though it seems to me more noticeably in summer with cool air from the A/C than in winter with warm air from the heater. I’ll go with the stated theory of inertia and change in air mixing when you turn the corner.