Re: Momentum conservation

On Aug 20, 7:26 pm, Timo Nieminen <t...@xxxxxxxxxxxxxxxxx> wrote:
On Mon, 20 Aug 2007, Peter wrote:
Thanks. I think you are completely right. Would you know if there
exists experimental verification that corroborate what you say? I
realize the amounts to be measured are extremely small.

Build yourself a 2-ball Newton's cradle. Maybe use golf balls? You'll be
able to screw in an eye to hang them from. Wrap the balls in some material
that'll cause energy to be lost. Try open-cell foam, clay or plasticine,
whatever you want. Use the same amount on each ball so that the two masses
are the same.

Stick a force sensor between the balls and get all the details!

A related lab exercise is done in many introductory physics courses, with
a trolley/cart and a stationary force sensor. The data analysis usually
just involves integrating the force over the whole collision, but one
could separately integrate the force over the compressing half and the
restoring half. The boundary between the two halves of the collision might
be when the force is maximum (since that's when the compression is
greatest), but you check by integrating since the impulse up to that point
must equal the initialmomentum.

Sometime a similar thing is done with 2 carts, but that seems to be much
less common (probably because it needs twice as much equipment). This is
usually done to demonstrate Newton's 3rd law, but it can show the loss of
energy in collisions as well if, again, the time of maximum compression
can be seen/found.

Google with terms like force, collision, cart, and you'll find various
descriptions.

--
Timo Nieminen - Home page:http://www.physics.uq.edu.au/people/nieminen/
E-prints:http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits:http://www.users.bigpond.com/timo_nieminen/spirits.html

I tried to do the experiment using two of the balls of a Newton's
cradle and two photogates, but I couldn't: the photogates are just not
accurate enough. Every time I did the test, I got different results.
If the incident steel ball does not stop, but keeps going, when it
collides head-on with another steel ball of the same mass, I suppose
it will stop completely if it has a little less mass than the target
ball. Would you know if there is a way to find out, analytically, what
is the mass required for the incident ball to stop?

Peter

.

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