fly in a moving car

[A Childs Mind]

ok i was staring at a fly in my car oneday and then i was like...... why dosint the fly fly to the back window.

 

so the car is drinving 75 mph. and the fly is going 12 mph why dosint the fly start flying bacwards to hith the back windshield.

 

and if the car is traviling 75mph and the fly flys forward in the car wouldint that mean the fly is moving faster than the car

[Sisyphus]

All motion is relative. The fly can fly 12mph (or whatever) relative to the air it's flying through. The air it's flying through in this case is inside the car, and moving along with it, at 75mph relative to the ground or the air outside the car. But that doesn't concern the fly, does it? Just like if you get up and walk around on an airplane, that doesn't mean you can walk at 600mph. Nor does it matter that the Earth is spinning, or that it's going around the Sun, or that the Sun is part of a galaxy that is itself spinning and moving relative to other galaxies, and so on and so on.

[A Childs Mind]

so wut your saying is its not the fly thats moving its the car. everything in the car is relly not moving at all

[Sisyphus]

No, I'm saying that what matters to the fly is how fast it's moving relative to the air that it's in. How fast that air is moving relative to something else doesn't make any difference. There's no such thing as "really moving." All motion has to be compared to something else.

[A Childs Mind]

ahhhh. now i get it. so wut your saying is were are moving compared to the earths rotation.

[mooeypoo]

This gets even better, what would happen if you stop your car suddenly? Would the fly stay in place, or maybe it would be thrown forward to splat on the front window? After all, your body would (which is why you should wear a seat belt).

 

The answer has to do with moment of inertia which is dependent upon mass.

[math]\text{Moment of Inertia I} = \int r^2 dm[/math]

Where dm is a small unit of mass.

 

The Kinetic energy of the object is dependent upon its moment of inertia, too, so the object will continue its motion relative to the moving/stopping environment in relation to how MASSIVE it is.

 

Here's a nice visual experiment to show how different masses are affected: http://users.hubwest.com/hubert/mrscience/science18.html

 

Your body's mass is large enough to get you to continue the motion of the car even if it stops rapidly. The fly's mass is tiny, so it would be barely affected.

 

On top of that, the air in the car is standing still compared to the car itself. It's trapped in the "box" of the car and moves along with it. If you would open a window, allowing for the air to move *through* the car the fly would likely to be pushed backwards towards the rear window (or outside with the air flow).

 

It's a neat way of looking at momentum and inertia, btw

 

~moo

[A Childs Mind]

WOW that makes so much more scence thank you

[swansont]

This gets even better, what would happen if you stop your car suddenly? Would the fly stay in place, or maybe it would be thrown forward to splat on the front window? After all, your body would (which is why you should wear a seat belt).

 

The answer has to do with moment of inertia which is dependent upon mass.

[math]\text{Moment of Inertia I} = \int r^2 dm[/math]

Where dm is a small unit of mass.

 

The Kinetic energy of the object is dependent upon its moment of inertia, too, so the object will continue its motion relative to the moving/stopping environment in relation to how MASSIVE it is.

 

Moment of inertia has to do with rotating systems, though, not linear ones.

[Vortigon]

But what about the inertia of the air within the car?

[mooeypoo]

The air is trapped in the car, it's moving along with it. On top of that, as I said before moment of inertia is dependent on mass. Air molecules have very little mass.

[Vortigon]

Ah I thought perhaps they would be act like a fluid rather than discrete molecules.

[Airbrush]

The fly will feel the car's acceleration very little compared to the wind resistance of stationary air inside the car. If you stand in a high wind, along with a fly, the wind will blow the fly away but the wind will not budge you.

[mooeypoo]

Ah I thought perhaps they would be act like a fluid rather than discrete molecules.

Not if they have nowhere to flow to.. if the windows are closed, the air is trapped. If the windows are open, the fly will be blown out.

[swansont]

But what about the inertia of the air within the car?

 

Then you have to look at density, which is the point of the golf/ping-pong ball demo in moo's link. Things that are more dense than air will continue moving and push the air out of the way. This is why a helium-filled balloon will move backward when you hit the brakes — the air continues to move forward, since it's more dense.

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Merged post follows:

Consecutive posts merged

The air is trapped in the car, it's moving along with it. On top of that, as I said before moment of inertia is dependent on mass. Air molecules have very little mass.

 

You shouldn't be using moment of inertia here — there's nothing rotating. Mass and density are sufficient to explain the phenomena.

[hermanntrude]

swanson, my textbook seems to be using inertia in linear systems... is it because it doesn't use the word "moment" that it's OK, or is my textbook wrong?

[mooeypoo]

You shouldn't be using moment of inertia here — there's nothing rotating. Mass and density are sufficient to explain the phenomena.

Well, in the case of the closed windows, and the air carried along with the car, there's nothing with inertia, true. I didn't notice I'm mixing the two subejcts -- I started my thread about the scenario of the car stopping rapidly- the human body will be thrown out of the car while the fly will not be too affected. Those two are related to their masses, aren't they? Did I (err, again! ) confuse "moment of inertia" with "momentum" and "inertia" ?

 

That won't be my first time confusing these, so, appologies if I did. The main concept remains the same, though, depending upon the masses of the two objects.