He connected the
concepts of force and acceleration with a new concept: mass. He
found that the acceleration of a body by a force is inversely
proportional to its mass, and formulated the equation that force
equals mass time acceleration. Another law was his principle of
inertia that any body, in so far as it is able, continues its
state either of rest or in uniform, rectilinear motion. His next
law was that when a body A exerts a force on a body B, then B also
exerts a force on A which is equal in amount but opposite in
direction.
Newton had a radically novel idea that equated instantaneous
acceleration to the gravity force which provoked it. He theorized
that the same gravity force that pulled an apple down from a tree
extended out to the moon hold it in its orbit around the earth. He
connected these movements by imagining a cannon on a mountain
shooting a series of cannonballs parallel to the earth's surface.
The first shot had only a tiny charge of explosive, and the
cannonball barely makes it out of the muzzle before falling to the
ground. The second shot is propelled by a larger charge, and
follows a parabolic arc as it falls, The next shots, fired with
increasingly more propellant, eventually disappear over the
horizon as they fall. Lastly, with enough gunpowder, a speeding
cannonball would completely circle the earth without hitting it.
He combined the inductive and deductive methods of inquiry, first
making observations, and then generalizing them into a theory, and
finally deducing consequences from the theory which could be
tested by observation.
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