Field
Gravitation
Two bodies pulling on each other, and everything that goes wrong at three.
Neither body is still, and the wobble is how planets are found
A planet does not orbit its star. Both orbit a point between them, and the star's share of that motion is small, measurable, and the reason thousands of planets are known.
The tide is a difference, which is why there are two of them
The Moon pulls the ocean toward it. That explains one bulge. The second one, on the far side, is the whole of the physics — and it comes from subtracting.
Five places that keep station, in a problem with no solution
Three bodies under gravity cannot be solved. Restrict the problem slightly and five exact answers fall out anyway — three of them roots of a quintic, two of them perfect equilateral triangles.