Originally Posted by

**kevinchang**
Yes. it's a non-linear problem. I want to research on the shape of a phenomenon about liquid. So I put the shpae onto the coordinate system with x-axis and y-axis, Then, I derived respectively the position on the x-axis and y-axis and use the function of t (time) to express it. The shape is a curve so I need cosɵ and sinɵ.

cosɵ= x'(t)/[(x'(t)^2+y'(t)^2)^1/2]

sinɵ=y'(t)/[(x'(t)^2+y'(t)^2)^1/2]

That's why I got x'(t) and y'(t) in my equations. ......

(I will PM you the details if you are available. Thank you!)