[PatrickHenry]

Gotcha. Thanks. I was temporarily confused because an orbiting object, like the moon, is in free fall. But of course, a falling object is always in free fall, and it undergoes acceleration, so the one doesn't preclude the other. I'm often confused, it seems.

[longshadow]

I'm often confused, it seems.

The thing to remember is that, as per Newton, things in motion tend to stay in motion unless acted upon by an outside force. In particular, any CHANGE in EITHER speed or direction requires a force acting upon the object, as

dv(t)/dt = a(t)

and

f = ma

hence

f = mdv(t)/dt

Thus, for the velocity to change (either speed or direction), a force must act upon the object, producing an acceleration of the object. The acceleration is, by definition, the change in velocity with respect to time.

If the earth's gravitational attraction ("force") were to cease to act on the moon, the moon would go sailing off in a straight line; it would not continue to "orbit" the earth. Likewise the electrostatic attraction between an electron and the nucleus of the atom around which it wanders.

Absent a force, producing an acceleration that changes the direction the object travels, objects would only travel in straight lines.

(geek alert: this explanation is based on the simplified Newtonian view; in the GR version, the "force" of gravity is replaced by a curvature of space-time, but the results are the same.)