Tension Puzzler
tension - forces in equilibrium - spring balance
What it shows:
The two ends of a dial-type spring balance are each connected to strings
which run over pulleys. With equal weights attached to the ends of the
strings, the spring balance indicates the value of one of the weights.
How it works:
The demonstration is presented to the class as a puzzler: the spring balance
is turned around so that the class can't see the dial. Students are invited
to guess what it is reading. Invariably they guess the sum of the two
weights. The lecturer then turns the face of the dial gauge around showing
them the error of their ways. This naturally leads into a discussion of what
it is that the spring balance is measuring - tension.
Setting it up:
The apparatus is assembled from standard lab clamps, bars, pulleys, etc. It's best
to preassemble it on a small lecture cart so that it can be set up off to one side
and not block the view of the blackboard. Also it can be quickly moved in and out
of the lecture hall if time is an issue. Although slightly different every time,
it typically ends up looking something like this:
Comments:
A puzzler is always a fun way of introducing a subject or emphasizing a concept. Rating **