Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Joe Dweck on 13/09/2008 11:51:15
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Joe Dweck asked the Naked Scientists:
Love the show, I catch it on podcasts. (http://www.thenakedscientists.com/HTML/podcasts/)
Ya know the Newton's Cradle desktop toy with steel balls? When you pull two away, they snap back and two bounce out the other way. I understand conservation of energy, but why doesn't one come out wih twice the energy?
What do you think?
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This has been discussed many times on this board, I am sure a reference can be found.
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This has been discussed many times on this board, I am sure a reference can be found.
Here are two:
http://www.thenakedscientists.com/forum/index.php?topic=8225.0
http://www.thenakedscientists.com/forum/index.php?topic=11548.0
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"why doesn't one come out wih twice the energy?"
Essentially, because both momentum and energy have to be conserved.
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It all boils down to what happens when a single ball hits an identical ball - if there is no energy loss, the parting speed will be the same as the speed of approach - they will 'exchange velocities. Total momentum and energy remain the same. You can extend this to a whole row of balls.
We did the sums in sixth form A level mechanics and it all came out with some very simple calculations.
You can end up with an increase in speed when you have different masses and with both objects moving towards each other - e.g firing a ball bearing at the front of an express locomotive, when the ball bearing will bounce back with (almost) the sum of the two initial speeds.
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Here is a simple explanation, by smiley :
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.freesmileys.org%2Fsmileys%2Fmerv%2Foffice.gif&hash=dffb216e09776c8507653d3b161010fd) (http://www.freesmileys.org)
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Here is a simple explanation, by smiley .
Funniest thing I've seen for ages.