"John Knight" <johnknight at usa.com> writes:
<
<
<"Parse Tree" <parsetree at hotmail.com> wrote in message
<news:Wfp%8.6912$DN4.1074154 at news20.bellglobal.com...
<> "Cary Kittrell" <cary at afone.as.arizona.edu> wrote in message
<> news:ahkjf2$rvp$1 at oasis.ccit.arizona.edu...
<> >
<> > In article "Parse Tree" <parsetree at hotmail.com> writes:
<> > <
<> > <"Cary Kittrell" <cary at afone.as.arizona.edu> wrote in message
<> > <news:ahkcm8$og6$1 at oasis.ccit.arizona.edu...
<> > <> In article "Parse Tree" <parsetree at hotmail.com> writes:
<> > <> <
<> > <> <"Cary Kittrell" <cary at afone.as.arizona.edu> wrote in message
<> > <> <news:ahk14h$ib0$1 at oasis.ccit.arizona.edu...
<> > <> <> In article "John Knight" <johnknight at usa.com> writes:
<> > <> <>
<> > <> <> <Sqr means square-root of the equation in the parenthesis ().
<> > <> <> <So, the resulting velocity would be the same, as the same time is
<> > <spent
<> > <> <on
<> > <> <> <the fall, and the tension would be zero.
<> > <> <>
<> > <> <> The "resulting velocity would be the same" if both masses were
<> > <> <experiencing
<> > <> <> the same acceleration the instant of release, but they were not.
<> The
<> > <> <> bottom mass was experiencing -2mg downards due to gravity and +2mg
<> > <upwards
<> > <> <> due to the tension in the spring. The upper mass is experiencing
<a
<> > <> <> now unopposed -mg downwards due to gravity and a -2mg downwards
<due
<> > <> <> to the same spring tension. You figure it out.
<> > <> <
<> > <> <But they really are experiencing the same acceleration at the
<instant
<> of
<> > <> <release.
<> > <>
<> > <> That is right as far is acceleration due to gravity is concerned,
<> > <> as is implied in my statement above. But each body is experiencing
<> > <> additional forces due to the spring, so they will no be subject
<> > <> to the same accelerations. If you mentally switch off gravity,
<> > <> the two bodies will move towards one another with an acceleration
<> > <> proportional to the 2mg tension in the spring. If you now switch
<> > <> gravity back on, the whole system will accelerate downards at
<> > <> 1 g, but this acts equally on the whole system, so you're back
<> > <> to considering things in the frame of referrence of the system
<> > <> itself -- as Jet implied.
<> > <
<> > <This is not true. If you switch off gravity, then each sphere will
<stay
<> at
<> > <rest. Firstly, you're assuming tension again, and secondly, the
<tension
<> you
<> > <assume exists only because of gravity.
<> > <
<> >
<> > This suddenly grows more interesting. I've been reading "spring"; it
<> > in fact says "string".
<>
<> Yes, I noticed that. But it's true even with a real spring.
<>
<> > <Regardless, you can simulate this using two balls and a string. Just
<put
<> > <them on a table and attach them with some string. Then pull on them
<and
<> > <release. They don't move together with a force proprotionational to
<how
<> > <much you pulled them apart.
<> > <
<> >
<> > If you assume an infinitely strong string, then you are correct.
<Otherwise
<> > they will indeed move, unless you've stretched the string inelastically.
<> > However, I'm being picky, and you're on to the intention of the
<question,
<> > I think.
<>
<> They will move, but they won't move the same amount that they are pulled
<on.
<> Even a spring won't, unless it is perfectly elastic.
<>
<> > <> <Also, you're assuming the value of the unknown.
<> > <> <
<> > <>
<> > <> Um, beg pardon? Assuming the value of what unknown? If you mean
<> > <> the spring tension, I simply said that the /initial/ spring
<> > <> tension is 2mg, because the lower mass is being pulled downwards
<> > <> by a force of 2mg due to gravity. Since it isn't moving initially,
<> > <> there must be an equal and opposite force: 2mg of tension in the
<> spring.
<> > <
<> > <The initial spring tension is unknown. You're assuming that the bottom
<> > <sphere is suspended from the top one. It simply says that it's
<suspended
<> at
<> > <rest. Which could simply mean that the system is suspended at rest.
<Who
<> > <knows? Actually, I find many of these questions to be very imprecise.
<> > <
<> > <Regardless, the acceleration of the system is g. And the acceleration
<of
<> > <all of the parts are g. Thus the string's tension should be 0.
<> >
<> > Assuming an infinitely strong string -- one whose relaxation is zero --
<> then
<> > you are correct.
<>
<> Yes. There are too many assumptions in these questions though. I can see
<> why they're difficult. There was another question about probability which
<> didn't even seem to specify if the two values involved were independent or
<> not.
<>
<>
<
<You "can see why they're difficult"?
<
<To whom are they difficult? To the 29% of American boys (after correcting
<for guesses), it obviously wasn't difficult.
<
<Maybe to the 47.7% of Norwegian boys who got it wrong it was difficult, but
<you can't claim that the other half found it to be difficult, can you?
<http://christianparty.net/timssh04.htm
<
<It's notable that the international average for girls who got it correct,
<after correcting for guesses, was only 1.7%, which is lower than the 3%
<standard error, which suggests that most women in the world probably agree
<that with you that "they're difficult". But is that a fact? No.
<
<What's truly awesome about this forum is that you've already been given the
<answer, and you *still* find it "difficult".
Quoth John, who did not DARE attempt to discuss the physics involved, in
spite of endless requests by Jet and myself that he do so.
<It's like teaching a pig to sing. It frustrates the teacher and
<irritates the pig.
Nice aphorism. Come up with it yourself? [hint: don't even think about it]
-- cary
[absurdly long References: header generated by John's newsreader trimmed
to the three most recent]
