Magnus Force Calculations returning null.

Question

Hey i didnt know whether to come here or stack overflow as i have a programming problem for a mathematics equation. Im getting a null value back when calculating the magnus force(spin). Im using XNA so that i can represent the data graphically and i have a few debugs to show the values. The value that this function is returning is NaN. Here is what i have got:

private Vector3 getMagnusForce()
        {
            float VapparentLength = (float)Math.Sqrt((double)Vector3.Dot(Vapparent,Vapparent));
            float radiusSquared = (float)(Math.Pow(radius, 2));
            float VapparentSquared = (float)(Math.Pow(VapparentLength, 2));
            //Find omega(Angular Velocity) by getting the length of R(Rate of Spin).
            float omega = (float)Math.Sqrt((double)Vector3.Dot(rateOfSpin,rateOfSpin));
            //Find cl by multiplying radius times omega then dividing it by the VapparentLength.
            float cl = (float)radius * omega / VapparentLength;
            Vector3 crossProduct = Vector3.Cross(rateOfSpin, rateOfSpin);
            float dotProduct = Vector3.Dot(crossProduct, crossProduct);
            float lengthOfDotProduct = (float)Math.Sqrt(dotProduct);
            Vector3 magnusVector = crossProduct/lengthOfDotProduct;
            float magnusPI = (float)Math.PI / 2 ;
            float magnusEffect = magnusPI *density * radiusSquared *  cl * VapparentSquared;
            this.forceOfMagnusEffect = magnusEffect * magnusVector;//forceOfMagnus is a Vector3
            return forceOfMagnusEffect;
        }

The formula that i am using is PI/2 * row(density) * r^2(radius of a sphere squared) * CL * the apparent velocity squared * by the cross procduct of R*Va / the length of the cross product of R*Va.

I hope i explained it correctly if not dont hesitate to ask as im happy to give more information.

Answer 1

I don't understand the overall picture, but here are some possible sources of problems:

The cross product of a vector with itself is always the zero vector. So this line of code

Vector3 crossProduct = Vector3.Cross(rateOfSpin, rateOfSpin);

will give you $\text{crossProduct} = (0,0,0)$.

Then dotProduct and lengthOfDotProduct will also be zero. Then, this line of code

Vector3 magnusVector = crossProduct/lengthOfDotProduct;

will be calculating $0/0$, which will give you NaN.

By the way, NaN and null are two very different things.