[paul.fr]

So everyone understands what i am asking (well, hopfully they do!) this may be a bit long winded.

a common house fly is in your living room flying on the spot, hovering, it has no forward speed.

now lets put that fly in your car, travelling at say 30 miles per hour. It is still hovering. if you then crash your car, you will be propelled forward and hopfully get saved by your seatbelt.

what happens to the fly? does it continue to hover, or is it propelled forward in to the windscreen?

[elegantlywasted]

I would guess liquefaction.

[lyner]

The fly is travelling, along with the air, at 30mph.
The car stops, sharpish.
The fly continues to move forwards through the air and towards the windscreen.
However, the air will have stopped moving fairly soon because of the force on it from the (now stationary) windscreen.
Air, to a fly, is a bit like treacle.   The fly is light and the viscosity of the air is significant.  By the time the fly has moved just a few cm through the air, its speed will have dropped dramatically due to friction in the air and because its mass is probably only 0.1gram or less. Lucky fly.
Not so lucky when it's the car which is moving, because the car doesn't slow up for the fly - it's too massive. Splat.
btw, if you had a helium balloon, tethered  on a string in the middle of the cab, it would  move backwards, with a jerk, when the car crashes.
Also, if you ever have a helium balloon in a car with you, watch how it behaves as you go round a corner - it leans INTO the bend, not out of it.

[chris]

And why does the helium balloon do that?

[neilep]

Is the fly wearing a seatbelt ?

[paul.fr]

Is the fly wearing a seatbelt ?

No, and he has a pizza!

[neilep]

I agree with sophiecentaur...I expect the fly will move towards the screen and either come to halt just before or gently enjoy windscreen familiarity and luff !

.....and that's what I have to say about that.

[Karen W.]

And why does the helium balloon do that?

I remember this question is from the forum before, but I can't remember the answer! Was it because the helium was lighter then the air and the air has enrtia. In the other case we were talking about a car accelerating and the ballon actually goes forward. So does this change the scenario if the car suddenly stops, does that change the direction the balloon travels making it move rearward instead on impact? I am confused Chris?

Here is the link to the other question and its answer.

http://www.thenakedscientists.com/forum/index.php?topic=2819.msg22190;topicseen#msg22190

[lightarrow]

And why does the helium balloon do that?

Helium is lighter than air. When the car brakes, air, more massive, goes ahead and so Helium goes back; when the car accelerates, the air, more massive, goes back, so Helium ahead. When the car bends to the right, centrifugal force pushes the more massive air to the left (so Helium goes right) ecc.

For example, take a bottle filled with water but not completely, so that an air bubble remains inside. then close the bottle and lean it horizontally. The air bubble will be in the top center, inside the bottle. Now gently accelerate ahead the bottle with your hand.

Where do you expect the air bubble will move?

[Karen W.]

Thanks alberto that makes sense and correlates with the other thread!