On Wed, 24 Oct 2001, talking about an oscillating pendulum,
Ludwik Kowalski wrote:
"In other words, the NET FORCE, and thus the acceleration,
is zero at the bottom and non-zero at an angle, large or small."
On seeing this, an student of mine asked: the pendulum bob is
describing an arc of a circunference, isn't it? So, at the bottom
there must be a net force pointing to the center of the arc, isn't it ?
I said: I agree with you and now your job is to evaluate that
force in terms of the initial release angle.