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"Yesterday someone asked
in class what happens to conservation of momentum when you have a pendulum
swinging. I find it easier to consider first a rigid earth wobbling back
and forth with the pendulum. The pedagogical point is that there must be at
least two bodies interacting before there can be a pendulum. Realistic
modelling of the earth can be added as a footnote."
One of my more favoured demos. is the pocket watch on a platform hanging
by three threads suspended by a single one. Glued (tacky wax) to the
watch is a mirror. A pocket diode laser's beam reflecting off the
mirror and onto a, not so nearby, wall clearly shews the watch's
"wobbling".
bc
Mark Sylvester wrote:
>Actually, I was just indulging in a little pun over breakfast coffee...
>
>...but in my experience, far from faltering, the juvenile mind thoroughly
>enjoys following through the logic that leads to the conclusion that "the
>earth moves" when I jump into the air. And this is not an article of faith.
>It's the logical exploration of a model. In this case the model requires a
>perfectly rigid earth, so it's not realistic, but physics is full of such
>creatures and it's often useful to talk about them. Yesterday someone asked
>in class what happens to conservation of momentum when you have a pendulum
>swinging. I find it easier to consider first a rigid earth wobbling back
>and forth with the pendulum. The pedagogical point is that there must be at
>least two bodies interacting before there can be a pendulum. Realistic
>modelling of the earth can be added as a footnote.
>
>Mark
>
>PS looking back over the thread, I see Justin Parke asks "Any suggestions
>
>
cut