Question: [![Windmill](https://i.stack.imgur.com/txg2H.jpg)](https://i.stack.imgur.com/txg2H.jpg)

I am trying to design the windmill blades (the green part) in Solidworks but am having trouble since the radius of curvature for the blades varies at different points. I tried bending it as a sheet metal, but am having a lot of difficulty. Any ideas?

Answer: 
The classic example of a frame of reference in which gravitational forces *disappear* is called a *freely-falling* reference frame.

This is what you get when in deep space, or in orbit around the earth, or when falling off a tall building or down a mine shaft on earth.

In a freely-falling reference frame, pendulums do not swing because all the (mg) terms vanish.
In the so called **center of mass frame** the gravitational forces of the constituents sums to zero per definition. That's the reason why we describe two body problems as (1) motion of the center of mass, and (2) relative motion with respect to the cantor of mass.