Clearly, the plane must also exert a force on the block to keep the block from falling through. This force must be exactly enough to keep the block from going through the plane, no more, and no less. So, the force that the plane exerts on the block is equal to the component of the gravitational force perpendicular to the plane, and pointing against it. This force is called N in the figure. The perpindicular force is exactly cancelled by N, so that there is no motion perpendicular to the plane.
G is the universal constant of gravity.
This force acts directly between the objects, so they are attracted towards each other. Each of Kepler's laws can be shown to be part of Newton's gravity. This is analgous to the conservation of momentum and states that objects with are rotating then to continue rotating unless acted on by external forces. Kepler's Third Law, that strange thing about the proportionality of periods and radii, tells us that the same law of gravity holds for all planets. Circular Orbits To demonstrate how all this fits together, we'll find the period of an orbit for a perfectly circular orbit, and show that it matches with Kepler's third law.