The idea behind the chain we
built. Other ideas for chains that pull-in are in the paper. If one lets a tilted rod fall onto a table, its
other end speeds up on hitting (by 50% for a uniform rod with
small θ having plastic impact: VB+ =
(That's why things break on the 2nd bounce, see the cover
story in the Wall Street Journal, Friday Dec 17, 1993).
Now if we attach something to the end B
of rod, that will speed up after A hits. This is the basic idea
we use for our chain. We make a chain whose links are tilted
rods arranged in a zig-zag pattern vertically. As each link hits at
one end its other end pulls down on the chain above.
Chain: we made the chain with tilted rods as its links.
Release mechanism that drops the chain
simulaneously. High speed camera. Phantom V7.1 camera at
Making the Chain
We used two essentially identical chains made of dowels tied
together with Vectran. The weights of the two chains were 218g
(plus or minus 2 g).
Originally an electromegnetic release was used. It didn't work well.
The magnetic field stayed on briefly even after turning off the
current, and this was hard to control between the two magnets.
Finally a mechanical release was devised and built as shown below:
1. Connect Zip-tie to the mechanism so
the posts A are held protruding..
2. Hang the chains from the release posts (A).
3. Cut the zip-tie (like pulling a trigger).
4. Spring retracts, releasing the chain and object.
This trigger releases 'simultaneously' within 2-3ms. Video
of trigger (25sec @ 6006 fps)
Chain falls faster than gravity!
The chain on the table falls faster than the one in air. In the
leftmost frames, at release, the bottoms of the chains were
75cm above the table. In the rightmost frames the chain on
the left has pulled ahead by about 6-7 cm.
chain on the table (on left) falls faster than chain in air
To make sure the chains were essentially the same in behavior,
the experiments were repeated with the chains swapped. Again the
chain on table ended up ahead by 6-7cm. Videos:
friction. Drag is different
between a chain and, say, an apple. So we compared two
contraction. When a chain is released the tension
in the chain drops nominally to zero. Because the chain
always has some elasticity, this drop to zero
starts an overall chain contraction that continues as the
chain falls. This elastic contraction pulls the top of the
chain down, making it go faster. Again, using identical
chains made this effect the same in both.
anecdote: Earlier on, we were hoping to be dramatic and
drop an apple simultaneously with the chain, hoping that
the chain would win. Later, whatever we dropped with the chain
(a bearing, a nut, a metal bar) we always called it an apple.