av84fun said:
That's because so far as I know nobody in the history of the universe has claimed that.
Well, Bob did. Read the thread.
And besides, what constitutes an authority on this subject?
How about one of the co-authors of the white paper entitled:
Reality and Theory in a Collision by Norman Derby (Bennington College) and Robert Fuller (University of Nebraska)
Below is a copy of my e-mail to Mr. Derby and his reply.
I have stated multiple times and will again here that I agree that whether an object stops for a millisecond or for a zero length of time before reversing direction has nothing to do with the pool stroke. It is just an interesting topic in my view as your responses and several others show.
But I think it is time, for me at least, to turn my attentions elsewhere so as not to exhume a dead horse in order to keep beating on it!
With kindest regards,
Jim
NOTE: The article mentioned happens do deal with collisions in pool balls and is a VERY interesting. Here is a link.
http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1002&context=physicsfuller
-----Original Message-----
From:
AV84fun@aol.com [mailto:AV84fun@aol.com]
Sent: Sat 11/3/2007 10:13 PM
To: Derby, Norman
Subject: Reality and Theory in a Collision
Hello Mr. Derby,
I just happened upon your article in collaboration with Mr. Fuller. I am a
pool player so the findings had more than academic interest for me.
I am curious about one issue about which you wrote. At the moment of
collision, you state that:
"The solution of this model is straightforward. Immediately after the
collision, the cue ball has lost all
its linear momentum..."
Is it your view, in other words, that upon impact with the object ball, the
cue ball stops dead before continuing forward?
And similarly, if there was draw imparted to the cue ball such that upon
object ball impact it would reverse directions, is it still true that however
briefly, the cue ball would stop for a measurable increment of time before
reversing direction?
In fact, do ALL objects in one dimensional motion come to a complete stop
before reversing direction?
There seems to be some debate on that topic and I would greatly appreciate
your views...or a link to some reference on point.
THANKS!
Jim
Hi Jim,
Let me see if I can shed light on your 2 questions.
One seems to be philosophical in nature. If an object reverses direction, must it come to a halt at some point? Yes. Our understanding of physics says that any object with momentum can change its momentum ONLY by interacting with something else (that's the essence of the law of inertia). We describe that interaction by saying that these 2 things exert forces on each other.
To produce a finite change in momentum in zero time would require an infinite force, so this does not happen in the real world. At fast enough frame rates you'd always see the object slow down, stop and then begin to speed up in the other direction.
Did our incoming ball stop completely when it collided with the stationary ball? Within the limits of the video we used, it did. If you've seen the kid's top known as Newton's Cradle (with colliding steel balls) then you've seen the dropped ball hit one (or more) and the one other ball take off with nearly all the momentum. While pool balls aren't quite as hard as ball bearings, they would be expected to behave in much the same way. The behavior of hardball collisions is fairly simple because they remain in contact for such a short time period (there is so little deformation of the balls) that the forces they exert on each other are HUGE for a short time during which all other forces (like the tabletop) are comparatively insignificant.
Hope that's helpful,
Norm Derby