This persnickety number determines the strength of magnetic fields. It figures in everything from motors and generators to audio speakers. Oh, and without it we’d live in eternal darkness.

I see this problem all the time. The idea that if you apply a constant force on an object, it should move at a constant speed. Really, this is what Aristotle would say, right? There are two course ...

Our motion perception is remarkably well tuned to detect small changes in speed and direction. For example, soccer goalkeepers need to precisely judge the speed, direction, and curvature of an ...

The little spheres represent water molecules. Molecules are in constant motion. The attractions water molecules have for each other keep them close together. They can move past each other, but their ...