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Re: impulse/momentum



"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