Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Brian on 19/07/2008 10:54:52

Title: How does a Newton's Cradle work?
Post by: Brian on 19/07/2008 10:54:52

Brian asked the Naked Scientists:

In high school I was taught that mass times velocity equals mass times velocity.  The teacher showed an example by showing us a device with 5 balls hanging from strings in a row.  The teacher pulled back 2 balls and let go.  The 2 balls at the other end of the contraption went up about the same distance the the first 2 balls.  

So why didn't one ball fly off at twice the speed? What physics law explains why 2 balls flew up.  The equation mv=mv would still
work if only one ball flew up at twice the speed.  

Hope you understand what I mean.  thanks

What do you think?

Title: How does a Newton's Cradle work?
Post by: lyner on 19/07/2008 14:30:21

If you had  started off with all the balls separated by a bit, you could analyse each collision in turn. When identical (ideal) balls strike each other, they 'exchange velocities'  (mv=mv and using a coefficient of restitution of 1 implies their relative speed of parting is the same as the relative speed of approach) so, if balls 1 and 2 are moving, the first collision would be between balls 2 and 3. 3 would go off at the original speed of 2, leaving 2 stationary. Then 1 would strike 2 and 3 would strike 4, leaving 1 and 3 stationary with 2 and 4 moving. This would carry on to the end until, first ball n would leave, then ball n-1 - no ball ever goes faster than the original velocity.
The same thing will happen if the balls are touching, to start with.

Title: How does a Newton's Cradle work?
Post by: paul.fr on 19/07/2008 15:57:36

Brian,
A rather dashing young chap asked two similar questions
http://www.thenakedscientists.com/forum/index.php?topic=11548.0
http://www.thenakedscientists.com/forum/index.php?topic=8225.0