Believe it or not gravity is not really a force, it is an acceleration and on Earth it is about 32 feet/sec2. This same science principle applies in decelerating devices like parachutes and parafoils. The parachute decelerates or slows down the fall of the object.
What is a .DS_Store or ._ AppleDouble file? .DS_Store files are created by OS X to store metadata about folders and files including viewing options, icon positions and color labels. ._ AppleDouble files are created by the OS X when an application adds resource data or metadata to a file on a disk whose format doesn't support resources or metadata.
The BT and the WS folders are the same thing, the feedstock for the Windows upgrade. If your copy of Win10 is activated, then you can run disk cleaner to get rid of the windows.old and the $windows --BT folders. The $windows---WS folder does NOT clean up by the disk cleaner.