Universe Today
Physics by 17th Century:
Planets move around the Sun in ellipses, with the Sun at one focus
The line connecting the Sun to a planet sweeps equal areas in equal times.
The square of the orbital period of a planet is proportional to the cube (3rd power) of the mean distance from the Sun in (or in other words--of the"semi-major axis" of the ellipse, half the sum of smallest and greatest distance from the Sun).
Newton's Three Laws:
When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.
The vector sum of the external forces (F) on an object is equal to the mass (m) of that object multiplied by the acceleration vector (a) of the object. In mathematical form, this is expressed as: F=ma
When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.
Newton and the “Apple Incident”:
“…we went into the garden, & drank thea under the shade of some appletrees; only he, & my self. amidst other discourse, he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. “why should that apple always descend perpendicularly to the ground,” thought he to himself; occasion’d by the fall of an apple…”
The Physics Classroom - Newton's Law of Universal Gravitation
