# Does a fly move relevant to your head?

#### Lars Laborious

Just a simple question: Does a fly that buzz around your head while you travel on an open boat (with some sort of wind screen in front) move relevant to you, and therefore travel at higher speed with no more effort than it would if your boat stood still?

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#### HallsofIvy

Science Advisor
Homework Helper
Do you mean "relative"? Yes, a fly does move "relative" to you but that doesn't mean it is going "at higher speed with no more effort".

If for example, a you are traveling in a closed boat at 20 knots (relative to the shore) and the fly, inside the boat, flies past you toward the bow at 0.5 knots (relative to you) then the fly is moving at 20.5 knots relative to the shore.
That has nothing to do with "effort", just the difference in the ways the flies speed is being measured. If you calculate the flies speed relative to the 747 that is passing over, you'd get a really huge number!
You might get a more interesting result if you calculate the flies kinetic energy using each of those speeds. Where did that enormous change in kinetic energy come from?
(Remember that kinetic energy is also always measured relative to something. Energy itself has no physical significance. Energy difference[\b] has physical significance.

#### Jelfish

It all depends on the amount of wind resistance that the fly has to encounter. For example, the reason why you don't really feel like you're moving if you're in a steadily moving car is that everything around you is static. This is only the case if the windows are up. Now imagine if you were in a car with the windows down and you were moving at a pretty fast speed (say, 60mph) and you try to toss and catch a crumpled piece of paper. You'd expect that paper ball would fly toward the back of the car. If the windows were all up, you'd be able to do it as though your car were not moving.

The idea is that for you to be in an enclosed steadily-moving 'box' and experience it as though it were at rest, the entire contents of the box must move with it. This includes the air inside. When air escapes or enters in the form of wind, it brings with it an external force into your system and makes you aware of the universe outside your box.

For your fly and boat example, it may be that the windshield reduces the wind near the person enough that the fly can still follow the person's head without feeling like it has to compensate for the entirety of the speed of the boat, but so long as there is some wind breezing past your boat, the fly will have to exert more effort to stay around the person's head than if your boat were at rest.

#### Lars Laborious

HallsofIvy said:
Do you mean "relative"?
Yes. Sorry.

HallsofIvy said:
Yes, a fly does move "relative" to you but that doesn't mean it is going "at higher speed with no more effort". [...]
You might get a more interesting result if you calculate the flies kinetic energy using each of those speeds. Where did that enormous change in kinetic energy come from?
(Remember that kinetic energy is also always measured relative to something. Energy itself has no physical significance. Energy difference[\b] has physical significance

What I mean is, will the fly get more exhausted keeping up with the boat, even though the windshield bounce off all the wind? If not, why?

#### Lars Laborious

Jelfish said:
... For your fly and boat example, it may be that the windshield reduces the wind near the person enough that the fly can still follow the person's head without feeling like it has to compensate for the entirety of the speed of the boat ...
Ok, how come? Is the fly some how gaining the boats kinetic energy?

#### russ_watters

Mentor
Lars Laborious said:
What I mean is, will the fly get more exhausted keeping up with the boat, even though the windshield bounce off all the wind? If not, why?
However much wind the fly feels is how fast he has to go to keep up with you. If you're talking about dragonflies, they will fly alongside boats, in the airstream, at up to 40mph. But any insect can fly along with you if they are not in the airstream.
Ok, how come? Is the fly some how gaining the boats kinetic energy?
If the fly is behind the windshield, the boat pulls a bubble of air behind it and the fly is pulled along with the bubble of air.

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#### Jelfish

Lars Laborious said:
Ok, how come? Is the fly some how gaining the boats kinetic energy?
Oh no. Perhaps I was assuming a bit too much. The fly's kinetic energy and the boat's are seperate. My explanation was assuming that the fly was buzzing around the person as though the boat were at rest. If the fly had no desire to stay in the boat, the wind shield wouldn't be enough to keep the fly in the boat (though that really depends on how large the wind shield is, how the fly is flying around, and the fluid mechanics of the system). The fly would only tag along if it were inside an enclosed space, where it wouldn't feel the effect of the boat moving away from the fly's air space.

Sorry if this getting more confusing.

#### Danger

Gold Member
Assuming total efficiency of the windscreen (let's simplify that by making it a completely enclosed cabin), the fly is moving relative to the air mass around you under its own power. Relative to anything else, the combined velocities of the boat (and captured air), the Earth, the solar system etc.. are all externally supplied and take no effort on the part of said bug. Speaking as one who got hit by a bee at 140kph when it flew in my car window... ouch!

#### DaveC426913

Gold Member
The air in the boat is moving to some extent with the boat; it is not stationary with respect to the lake. It may feel quite windy to you, but if you stood on the bow of the boat and stuck your head out of the air bubble, you would have difficulty breathing in the blast of wind. In the boat, both you and the fly are greatly protected from the wind even with an open cabin.

Yes, the fly is carried in the bubble of air created by the boat.

#### Lars Laborious

Ok, thanks! This all helped a lot.

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