Relative Wind

Date: Tue, 08 Nov 94 21:34:59 pdt
From: Anonymous
Subject: math question

Hi Dr. Math,

My name is J.P. and I am a 5th grader from Springfield Or.

I have a question about relative wind. If you know the speed of an 
object can you determine the speed of the relative wind?

Does the shape or weight of an object affect the relative wind?

We will look forward to your reply.

Date: Sat, 12 Nov 1994 00:21:19 -0500
From: Dr. Math
Subject: Re: Math Question

Hi J.P.!

Sorry it has taken us so long to get back to you. You had a great 
question:

>I have a question about relative wind. If you know the speed of an 
>object can you determine the speed of the relative wind? Does the 
>shape or weight of an object affect the relative wind?

To tell you the truth J.P., I'm not 100% positive that this is the answer
you're looking for, but here goes:

You asked "can you determine the speed of the relative wind?" This 
is the example that I thought of when I read that.  Say you throw a 
rock and you know how fast you threw the rock.  In addition, you 
also know how fast the rock was going after it was thrown.  Is it 
possible to determine the speed of the wind?  (Is this the question 
that you were asking??) Well yes, it is possible to determine the speed 
of the wind if you have this information.

For example, say you throw the rock in the direction that the wind is
blowing and when it leaves your hand it is going 30 mph.  But then 
you measure the speed that the rock is traveling through the air to be 
45 mph.  You can then calculate the wind speed to be 15 mph.  Do you 
see how??  In other words, the fact that the wind is blowing 15 mph 
(in the same direction that the rock is going) makes the rock go 15 mph 
faster than if there was no wind.

Here's something for you to think about: say that the wind is blowing 
15 mph in your face when you throw the rock, and then you throw it 
INTO the wind.  If the rock is going 30 mph when it leaves your hand, 
but the wind is blowing AGAINST it, how fast will the rock be going? 
(take a look at the picture I tried to draw)
       ____
      /    \         30 mph                       15 mph
     / rock \  ---------------->     <-------------------------- WIND
     \______/
        YOU

You also asked if the shape of the rock makes a difference in "the 
relative wind." Well, the shape of the rock makes no difference in how 
fast the wind will blow, but it can affect how far the rock will travel 
through the air.  Have you ever heard of "air resistance"? 

Think about it this way.  Say you're walking outside on a windy day 
holding a big sheet of cardboard out in front of you, and your friend 
is holding only a small book out in front of him.  Who has more 
problems walking into the wind?  You do, because the wind is blowing 
on the cardboard that you're holding and makes it harder for you to 
walk into the wind.  This is called air resistance.  So if you throw 
something wide and flat (like cardboard) into the wind, and at the same 
time you throw a small round object (like a rock) in the same direction, 
which will go further? The rock will because the wind will slow down 
the cardboard faster because it is bigger.  Does this make sense?  I
realize that this is pretty confusing.

You also asked about the weight of the object.  The weight does not 
affect how fast that you throw an object, but it does affect how far it 
will go.  For example, say you had a feather.  And let's say that you also 
happen to have another feather, only this one is made out of metal. 
If you threw both of these feathers as hard as you could with one hand, 
they have to be going the same speed right after you threw them since 
you threw them out of the same hand, right?  Which one do you think 
would go further?  The metal feather.  But the important thing to realize 
is that you threw both of the feathers at the same speed, say 30 mph. 
It's just that the metal feather will go farther than a plain old bird 
feather, because the bird feather slows down faster. Does that make sense?

I realize that this answer probably got into some areas that you weren't
even asking about, and that it might have been kind of confusing, but it
was also a pretty hefty question that you were asking! If you have more
questions, or this wasn't what you were asking, please write back!! 
I hope this helped at least a little bit...

-Vanessa, M.D.