# Simple Dynamics

1. Sep 28, 2010

### Superanden

Hi All.

Im trying to make a computer program that calculates the movement of a ball in the air.

i'll need information as:

- the air density
- the ball's crosssection
- the ball's current acceleration
- the balls current speed

is this enough to calculate the frictional force?

I'll also need:

- Ball's mass
- Ball's current location

Is this enough to completely determine the ball's acceleration.?

There will be bo colisions, nor both a static and a dynamic frictional force from the air, and the air density will be the same no matter how high the ball is.

Please explain clearly as it's been a LONG time since my math and physics.
Maybe for simplicity, explain the dynamics in one axis to avoid using vector math.

/Andreas

2. Oct 1, 2010

### K^2

Technically, yes, it is enough, but computations are very complex.

You have two forces acting on the ball:

$$W = g * M$$

$$F_D = \frac{1}{2}\rho v^2 C_D A$$

Where M is mass, g is acceleration due to gravity, ρ is air density, v is velocity, CD is drag coefficient, and A is cross section area.

First is straight forward. Second is mostly straight forward, except for CD. Problem is that it depends on shape of the object and its Reynolds number.

So what you want to do is the following.
1) Read Wikipedia article on Drag Equation.
2) Read Wikipedia article on Reynolds Number.
3) Search Google for experimental/computed values of CD for object that matches your parameters in the Reynolds number ranges you are interested in.

3. Oct 19, 2010

### Superanden

Thanks a lot... I got it from here..

You're the best using your time to help us "physics ignorants".

PS: The cross-section of a ball is pretty simple when i neglitiate the airodynamics properties... so thanks a lot.