Galileo had supplied the observational evidence. Many questions remained that Newton answered. Newton had a long vacation and set out to study the force that holds the planets captive to the sun. Every object remains at rest or moves at a constant speed in a straight line unless an unbalanced external force acts on it. Newton invented calculus to prove his ideas.
Planets orbit the sun in curved paths.
Newton deduced that a force was acting onthe planets. If no force was present, planets would travel in straight lines. The more mass an object has, the harder it is to move. More force is needed to give the same acceleration to a more massive object. Twice the mass requires twice the force.