The Second and Third Laws of Motion

39. The Second law of nature: Every motion in itself is straight. Things which move in a circular manner always tend to move away from the center of the circle which they describe.

The second law of nature is: each particle of matter, considered separately, moves only along straight lines.

Many are often forced to deflect due to encounters with others.

All the matter moving together forms a circular pattern.

This has the same cause as the First Law – the immutability and simplicity of how God preserves motion in matter.

He preserves it precisely every time, without any consideration of what it might have been earlier.

No motion happens in an instant. Yet everything which is moved, in each instant while it is being moved, continues its motion in a straight line, never along a curved line.

For example, let A be a stone rotating in the sling DA through the circle ABF.

When it is at the point A, it moves along line towards C, so that the straight line AC is a tangent to the circle.

But it cannot be supposed to be determined to any curved motion. Although it came along a curved line, this curvature does not remain in it while it is at the point A.

This is also confirmed by experience, because if it then leaves the sling, it will not continue to move toward B, but towards C.

From which it follows that every body which is moved circularly tends perpetually to recede from the center of the circle which it describes.

We experience this by the very sense of touch in a stone while we whirl it around in the sling.

40. The Third Law: A body does not lose its motion when it encounters a more powerful body. But it transfers some of its motion when it encounters a less powerful body

Body weak has less force than body strong. If weak, moving in a straight line, hits strong, then weak will be deflected, retaining its motion but losing its direction.*

But if strong were moving and hits weak, then strong will move weak with it. strong will lose as much of its own motion as it gives to weak.

Thus, when hard bodies are projected and strike another hard body, they do not stop moving. Instead, they are reflected to the opposite direction.

On the contrary, when they encounter a soft body, they can easily transfer all their motion to it. This stops them.

All the changes which happen to corporeal bodies are contained in this Third Law.

I do not look here into what force human or angelic minds have for moving bodies. That will be discussed in the treatise on Man.