Stevin: geometry, physics and trigonometry

Stevin was the first to show how to model regular and semiregular polyhedra by delineating their frames in a plane. 

He distinguished stable from unstable equilibria.

In The Principal Works of Simon Stevin, Vol. I, Ch II, Bk I, Th XI, he derived the condition for the balance of forces on inclined planes using an ingenious and intuitive diagram with a "wreath" containing evenly spaced round masses resting on the planes of a triangular prism (see the illustration). He concluded that the weights required were proportional to the lengths of the sides on which they rested assuming the third side was horizontal and that the effect of a weight was reduced in a similar manner. It's implicit that the reduction factor is the height of the triangle divided by the side (the sine of the angle of the side with respect to the horizontal).

Stevin also made contributions to trigonometry. His book, De Driehouckhandel, included Plane Trigonometry.

He demonstrated the resolution of forces before Pierre Varignon, which had not been remarked previously, even though it is a simple consequence of the law of their composition.

Stevin discovered the hydrostatic paradox, which states that the downward pressure of a liquid is independent of the shape of the vessel, the area of the base, and depends solely on its height. He also gave the measure for the pressure on any given portion of the side of a vessel.

He was the first to explain the tides using the attraction of the moon.

In 1586, he demonstrated that two objects of different weight fall down with exactly the same acceleration.

• en.wikipedia.org/wiki/Simon_Stevin