5. what is required to cause acceleration?

Acceleration is the rate of velocity of an object changing within time.

An object's acceleration is the result of all forces acting on the object, as described by Newton's Second Law.
It states that the acceleration of an object is dependent upon two variables: the net force acting upon the object and the mass of the object.

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.