The time rate of change of velocity. Since velocity is a directed or vector quantity involving both magnitude and direction, a velocity may change by a change of magnitude (speed) or by a change of direction or both. It follows that acceleration is also a directed, or vector, quantity. If the magnitude of the velocity of a body changes from v1 ft/s to v2 ft/s in t seconds, then the average acceleration a has a magnitude given by Eq. (1): (see above)
A minor disappointment not to find
angels pushing the planets around their courses
as Leibnitz believed. A shame, but not a great one,
that the universe seemed less and less to hang
glimmering from God's chain like a golden fob,
although a pendent weight shaped Newton's thought.
Sitting alone there in that storied orchard,
he'd seen the apples drooping from their boughs;
until one formed, unplucked, a grand conclusion.
The apple fell because it had to fall,
as objects move toward objects, in accord.
It struck a dizzying tune into his head.
The clockwork of the heavens may make music,
but it was a grave music that he heard,
the whirl of mass, the hum of centrifuge,
and calculations on the page would prove
such motion both a falling and a flight.
Thus bodies spin each other round in space.
And gravity, too, becomes a kind of grace.
-from "Some Notes on Grace and Gravity", from the collection Field Knowledge