It's not accurate to say that any specific "law" gave Newton the idea of gravitational force. It was a combination of observations, existing knowledge, and his own genius that led him to formulate the law of universal gravitation.

Here's a breakdown:

* Observations: Newton observed the motion of the planets, particularly the Moon's orbit around Earth. He knew that the Moon was constantly falling towards Earth but didn't crash into it, meaning a force must be acting upon it. He also observed the falling of objects on Earth.

* Existing Knowledge: He built upon the work of earlier scientists like Johannes Kepler, who formulated laws describing planetary motion. Kepler's laws showed that planets move in elliptical orbits with varying speeds, suggesting a force influencing their motion.

* Newton's Genius: He realized that the same force that pulls an apple to the ground must also be responsible for keeping the Moon in orbit around Earth. He then formulated the law of universal gravitation, which states that every particle of matter in the universe attracts every other particle with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

In summary:

* Newton did not discover a specific law that gave him the idea of gravitation.

* He observed the world around him, built upon existing knowledge, and used his own unique insights to develop the law of universal gravitation.

The law itself is the culmination of his understanding of the relationship between force, mass, and distance, not the source of the idea.