# Air resistance?

1. May 18, 2007

### Tomm42

I've never worked with air resistance before so any help just getting me started would be much appreciated.

1. The problem statement, all variables and given/known data
A 20 kg object falls free from a height of 15 m. Including air resistance, what will its position be after 0.6 seconds? Air density is 1.11 kg/m^3.

2. Relevant equations
The only equation I know for this is d = 0.5 * g * t^2
and I don't know if there are any equations for air resistance. And does air density have anything to do with this at all?

Well, I don't really know how to deal with air resistance since we haven't gone through that and still our teacher wants us to solve this. :uhh:

2. May 18, 2007

### Kurdt

Staff Emeritus
It might be helpful to know what kind of class you're taking.If this is just a question thats been assigned and you're expected to know how to deal with air resistance then I suggest you look through some of your recommended reading or class notes because I doubt they'd let you go unequipped. If its a problem solving class then they probably want you to derive something for air resistance yourself.

3. May 18, 2007

### husky88

I don't think you can solve this. You need to find out the friction force with the air. Density is directly proportional to the friction force, but it depends what shape the object has. If it is aerodynamic (or not), the friction force varies from shape to shape.

I think there is a formula relating friction force due to the air to V^2.
Have you ever used that?

Last edited: May 18, 2007
4. May 18, 2007

### cristo

Staff Emeritus
Well, I don't think that is the case here, since the question is clearly a homework/coursework type question!

Kurdt's advice is the best; look through your textbook to see if there are any cases of air resistance being included in a projectile question. There are many models for air resistance, ranging from the relatively simple, to the highly complex, and thus it would help us to know your background before suggesting one for you to work with.

5. May 18, 2007

### husky88

Sometimes it happens that problems are formulated wrong in a textbook.

I don't know, from my point of view, there are too many variables.
The force of air resistance increases with time until it becomes equal to Fg and the object reaches its terminal speed.
The only formula I found was
D = Cd * .5 * r * V^2 * A
where
D = drag
Cd = drag coefficient
r = air density
v = velocity
A = reference area on which drag coefficient is based
And again too many variables.

Last edited: May 18, 2007
6. May 18, 2007

### cristo

Staff Emeritus
:uhh: I don't think that one should automatically assume that the textbook is incorrect!

As I said earlier, there are many ways in which one can incorporate air resistance into a problem. I think we should ask the original poster to look in his book for such a discussion, rather than try and deduce which equation he should use! I'd say that, more often than not, if a problem is in a textbook, then it is soluble!

7. May 19, 2007

### husky88

Ok, ok I give up...