Q. Once, on a very long, boring trip, my passenger and I were reduced to asking each other dumb questions. We came upon one question that we couldn't settle. Imagine that I'm driving along at highway speed with the air conditioner on and the windows closed. A hummingbird that was caged in the back seat gets loose, and with nowhere really to go, it ends up just hovering there in midcar. Our question is this: If I hit the brakes hard, does the hummingbird crash into the windshield? My friend said of course it would, that its momentum would cause it to keep moving forward as the car slowed. But I, being the more educated of us (not necessarily a good thing, as my friend claimed), said that the hummingbird's position would depend on its air speed, not its ground speed, and as the car slowed down, the air inside the car would slow down at an equal rate, as would the hummingbird, thereby avoiding becoming windshield splatter. So guys, please settle this question so I can finally get some sleep.
You can read the answer in the website. But of course, it is "obvious" that the poor bird will go SPLAT onto the windshield.
But then, there was a followup question on what would happen to cigarette smoke. Now, presumably, the question is being asked because the cigarette smoke is "less dense" than the surrounding air, which is why it rises. The answer given on that website is still the same as that for the hummingbird, which should not be correct.
I gave a similar problem in one of my "Revamping the Undergraduate Physics Lab" series, but instead, I used a helium-filled balloon. The whole point is to find something that has a smaller density than the air surrounding it. So if we have the balloon in the car instead of the cigarette smoke, hitting the brakes will actually cause the balloon to move BACKWARDS. This is because the more dense air surrounding it will be the one that gets pushed forward more than the balloon, thus displacing the balloon backwards.
One can always try this oneself. I've seen this counter-intuitive observation during a few train rides, so I know for a fact that this is the correct observation. I haven't done it with smoke, though, since obviously smoking isn't allowed on trains and in many vehicles that I have been in. But if a volume of smoke rises because it is less dense than air, then it should behave just like the helium-filled balloon, presuming that it doesn't get mixed too much with the surrounding air.