Thought Experiment to argue about.....

[PhantomWolf]

I came up with this while driving home yesterday and being annoyed by a fly.

You are driving down on a straight country highway at 100km/h (60mph) relative to the ground. A fly on the back window takes off and starts to fly towards the windscreen at about 10km/h (6mph) relative to the car.

Suddenly a deer runs out ahead of you and you slam on the brakes, stopping in 4 seconds.

What happens to the fly?

I'm not going to make it a poll, so feel free to give and answer and show your working.

 

[Meadmaker]

Nothing.

As soon as the fly takes off from the back window, it isn't attached to the car. It did start out with some 60 mph forward velocity, but is immediately hit by a wind that slows it down almost instantly to normal fly speed. Even if the car started decelerating the instant the fly left the window, the slowing down of the fly due to air resistance would be much faster than the slowing down of the car, so the fly wouldn't catch up to the car.

 

[pzkpfw]

I read this as the fly being inside the car.

 

[Trebuchet]

I came up with this while driving home yesterday and being annoyed by a fly.

You are driving down on a straight country highway at 100km/h (60mph) relative to the ground. A fly on the back window takes off and starts to fly towards the windscreen at about 10km/h (6mph) relative to the car.

Suddenly a deer runs out ahead of you and you slam on the brakes, stopping in 4 seconds.

What happens to the fly?

I'm not going to make it a poll, so feel free to give and answer and show your working.

Depends on whether the fly is on the inside or outside of the window, of course.
If on the outside, you left him behind as soon as he left the quiescent air immediately behind the window.
On the inside, the classic puzzle is a helium balloon. When you brake it moves back, because it's less dense than the air around it.
The fly is more dense, and already has a little forward inertia, so it'll pick up a little speed. But not much.

 

[Robin]

I came up with this while driving home yesterday and being annoyed by a fly.

You are driving down on a straight country highway at 100km/h (60mph) relative to the ground. A fly on the back window takes off and starts to fly towards the windscreen at about 10km/h (6mph) relative to the car.

Suddenly a deer runs out ahead of you and you slam on the brakes, stopping in 4 seconds.

What happens to the fly?

I'm not going to make it a poll, so feel free to give and answer and show your working.

From the road's frame of reference it travels at the same speed, slowed down a little by the air in the car accelerating towards it.

From the car's frame of reference it accelerates towards the front windscreen at approximately 7 m/s^2 less the pressure from the air in the car

If we know how far the windscreen is from the fly's initial position then we could calculate at what time and velocity it hits the front windscreen.

 

[Dave Rogers]

From the car's frame of reference it accelerates towards the front windscreen at approximately 7 m/s^2 less the pressure from the air in the car.

Ah, but there's the rub. As the car decelerates, the fly accelerates in the non-inertial frame of the car; but as it accelerates, it experiences drag. Flies are pretty badly streamlined, so I would expect it'll take less than 4 seconds for it to lose the vast majority of its 66mph relative to the car. I don't know the drag coefficient of a fly, but I suspect it won't be going fast enough to cause it significant problems in simply landong on the windscreen.

Dave

 

[Meadmaker]

I read this as the fly being inside the car.

Oh. That makes more sense.

 

[Dragon]

Isn’t air very viscous from a fly’s perspective ? I seem to remember something in one of Richard Dawkins’ books that it’s actually quite hard for tiny critters not to fly, or at least be carried off on an air current.
I’m with Dave; air resistance would win over momentum.

 

[alfaniner]

I think much depends on the mass of the fly.

 

[Darat]

I suspect the Brundelfly would make quite a mess.

 

[theprestige]

You're sitting on the back seat of the car. The car suddenly stops moving. What happens to you?

There's an apple balanced on your knee. What happens to the apple?

You're bouncing the apple in your hand. The car stops moving. What happens to the apple?

There's a fly buzzing around the apple. What happens to the fly?

 

[jadebox]

You're sitting on the back seat of the car. The car suddenly stops moving. What happens to you?

There's an apple balanced on your knee. What happens to the apple?

You're bouncing the apple in your hand. The car stops moving. What happens to the apple?

There's a fly buzzing around the apple. What happens to the fly?

The fly is much smaller and lighter in relationship to the air. Relative to the fly, the air is very dense and isn't very easily compressed. So, think what would happen to you or the apple when the car stopped if the inside of the car were filled with water. Rather than being moved forward, you would be moved around a bit as the water sloshes around. It would be sort of the same for the fly in air.

Builders of model aircraft deal with something called the "Reynolds Number." A small model of an aircraft will have different flight characteristics than the full size aircraft even if you carefully scale both the size and weight of the model. This is basically because the air stays the same size even though the model is smaller.

 

[pzkpfw]

Are we assuming a spherical fly?

 

[Matthew Best]

In my calculations I assumed a cubular fly.

 

[Dave Rogers]

Are we assuming a spherical fly?

I'm a physicist so I was going to make some comment about a spherical fly in a vacuum, but the latter condition would clearly be significantly different to what I suspect is implied in the question.

Dave

 

[theprestige]

The fly is much smaller and lighter in relationship to the air. Relative to the fly, the air is very dense and isn't very easily compressed. So, think what would happen to you or the apple when the car stopped if the inside of the car were filled with water. Rather than being moved forward, you would be moved around a bit as the water sloshes around. It would be sort of the same for the fly in air.

Builders of model aircraft deal with something called the "Reynolds Number." A small model of an aircraft will have different flight characteristics than the full size aircraft even if you carefully scale both the size and weight of the model. This is basically because the air stays the same size even though the model is smaller.

Reasonable enough, but beyond the scope of the question, I think. How the fly handles the entrained air in the car follows from the fact that the fly - like everything else in the car - will keep moving forward when the car stops. Regardless of whether the fly is in the air or perched somewhere when the stop happens.

Probably the fly will be slapped out of existence by an airbag.

 

[lomiller]

The fly “falls” towards the front windshield at a terminal velocity based on it’s acceleration (the de-acceleration of the care) and air resistance. Flight speed may or may not add to this based on the actions of the fly. Impact with the front window would be a little less than if it fell/flew towards the ground.

There could be some air movement helping it towards the front of the car, but my assumption is that this is small enough to ignore.

 

[lomiller]

Probably the fly will be slapped out of existence by an airbag.

The de-acceleration in the OP is less than 1G, so the airbag shouldn’t trigger.

 

[The Atheist]

It's not a thought experiment, but a science experiment.

Would the fly act like a helium balloon?

 

[Lothian]

The fly would probably lay it's eggs in the dead deer.