Planets don’t have enough energy to escape the Sun, and so their orbits are ellipses, to a good approximation; because the planets pull on one another, actual orbits are not exactly conic sections. A rigid physique is an object whose measurement is merely too massive to neglect and which maintains the same shape over time. In Newtonian mechanics, the movement of a inflexible physique is usually understood by separating it into motion of the physique’s middle of mass and motion around the center of mass. A free physique diagram for a block on an inclined airplane, illustrating the traditional force …