Does a Fly Crash Into the Windshield When a Car Suddenly Stops?

[Ivan Seeking]

That would only be true if smoke is heavier than air. But is it?.

So as I said, you have completely reversed your position.

 

[BobG]

We can't see how the bubbles are made, but hot smoke rises because of the lesser density or the air, like it does above the fire, that's convection. However, when smoke cools to ambient temperature, it's definitely heavier than air.

You are missing the point. The cold air is heavier than warm air, so when one hits the brake, the heavier air goes towards the front of car, and the warmer air [and smoke or a helium balloon] go towards the rear of the car.

I don't think Andre is missing the point. Cold air is denser than warm air, not heavier. If the cold air and the warm air have the same chemical composition, one mole of cold air will have the same mass as one mole of warm air. If the mole of cold air and warm air were each enclosed in a balloon in a vacuum, they'd each weigh the same (even though the balloon of warm air would be bigger).

You'd need to know the chemical composition of the "smoke" in question, but I think it's safe to say that most "smoke" would have a higher atomic mass than normal air (but not definite - if the "smoke" were actually steam, that would be one obvious exception in more ways than one).

Smoke is not the same as a helium balloon, though. I think it's safe to say the only way the helium balloon is going to be "heavier" than the rest of the atmosphere is if the rest of the atmosphere happens to be hydrogen.

 

[Pythagorean]

when they've been saying "heavier than", "lighter than", I've been taken it to mean "heavier per unit volume" and "lighter per unit volume" (i.e. density).

It would be odd to call one gas heavier than the other while secretly assuming two different volumes but not letting anyone know about it.

 

[Ivan Seeking]

when they've been saying "heavier than", "lighter than", I've been taken it to mean "heavier per unit volume" and "lighter per unit volume" (i.e. density).

It would be odd to call one gas heavier than the other while secretly assuming two different volumes but not letting anyone know about it.

Yes, I thought that was pretty obvious. It is also a common reference.

 

[Ivan Seeking]

You'd need to know the chemical composition of the "smoke" in question, but I think it's safe to say that most "smoke" would have a higher atomic mass than normal air (but not definite - if the "smoke" were actually steam, that would be one obvious exception in more ways than one)

In any event, Andre's position completely reversed and the initial objection was exactly incorrect.

I said the smoke in a braking car will move to the back of the car. Why? The answer is that the smoke is lighter [less dense] than air, not heavier [more dense]. It doesn't matter what the smoke is made of given the result observed.