Large auditorium.

Big pendulum hangs from the celing over the lecturer's bench.  Weight on the end is a disk of copper.  On the desk is a very strong magnet (perhaps salvaged from a moderate-to-large magnetron tube.)  One of those buggers with a couple quarter-donuts of metal as thick as your neck, growing up out of a strong, flat, metal plate, so the ends face each other across a few inches of gap.  Magnet is positioned where the disk would swing through the gap at the bottom of its travel (if it could).  Pendulum is held up at one end of its travel by a rope.

Comes demo time.  The prof passes his hand through the gap ("Nothing up my sleeve - especially no mechanical watch!")  Then he yanks the rope, untying the knot and letting the pendulum swing free.  It starts toward the magnet, picking up a bunch of speed in its three-story drop.  The leading edge starts to enter the gap.

WHANG!

The disk rings like a gong and stops nearly dead with the leading edge inside the magnet gap.  Then it gradually eases into the gap until it's at the bottom of its travel, where it stops.