Motion
in [Physics]
|
Prev: Free fall
Next: 50% OF POPULATION BELOW AVG IQ!
From: platopes on 8 Oct 2005 20:48 PD wrote: > > One of the great things about Newton's laws (or, equivalently, the law > of conservation of momentum) is that they apply separately in each > dimension. So in the classical (3 spatial dimensions) case, Newton's > 2nd law is really three equalities. Since motion in a circle is planar, > this really reduces to two equalities. I'm certainly free to choose any > coordinate system that spans this 2-space, so I'll choose radial and > tangential coordinates. > > Now we have two statements of Newton's 2nd law, one for the radial > direction and one for the tangential motion. In the tangential > direction there is no net force on the orbiting object, and so Newton's > second law and first law both say there will be no change in the speed > in that direction. > > All of this could also be said using the conservation of mechanical > energy. The orbiting body has a constant radius in a central field, so > its potential energy is constant. The work-energy theorem then says > that any other force acting in the direction of the displacement will > contribute (perhaps positively, perhaps negatively) to the kinetic > energy. However, there is no force acting in the direction of the > displacement; the two are perpendicular to each other. Thus the kinetic > energy remains the same. > > PD > A pleasure to read as usual. Like listening to classical music, reading about science, in the precise language thereof, makes one smarter. Happened upon this the other day - http://hyperphysics.phy-astr.gsu.edu/hbase/hoocyl2.html#c1 Which would you have bet on? Any comments? p
From: TomGee on 9 Oct 2005 05:35 Yes, all that is true, Don1, but it is also true that the _KE_ of the object is not changed by the right angle force doing all that you say. Any change in KE would be caused by something else.
From: TomGee on 9 Oct 2005 05:39 Too bad he gave no credit to whomever he copied all that from, p.
From: TomGee on 9 Oct 2005 05:53 Randy Poe wrote: > Don1 wrote: > > Randy Poe wrote: > > > Randy Poe wrote: > > > > TomGee wrote: > > > > > Aren't you the one who said KE is not changed by an external force? > > > > > > > > No, I never said such a thing. Can you provide the quote where > > > > you garbled that inference from? > > > > > > > > > > I think I can save you some work. I said that in the case of > > > circular motion, the force does not do any work on the object, > > > and it also does not change the KE. > > > > > Are you saying that a force acting at right angles to a moving body is > > not doing any work. > > I'm saying that work is done by the component of force > in the direction of motion. > > If the force is ALWAYS at right angles to the motion, no > work is done. > > > Ah yes, so then work is being done by the component of force which you claim is a non-existent force? What force makes the work in moving the body? Is it KE? Could that be the body's inherent force? But if it is, that force could not be one of your unmentioned perpetual motion systems, could it, since it is doing work? The answer is of course that the KE of the body is exhibited by its momentum force in moving the body, and that is the inherent force which Newton noted. > > > > or changing the Kinetic energy as it changes the > > directin? I find this hard to believe. > > KE = 0.5*mv^2 > > If v doesn't change, KE doesn't change. Do you think that > the speed ever changes for constant-speed circular motion? > > > I think we need to study this > > situation with a fine tooth comb. > > There's nothing to study. > > Ah but there is Randy, you need to study physics a little more generally to get out of the narrow hole you've gotten yourself into. That's what's wrong with education - the more you get, the narrower a hole it puts you into while the world in general goes on without you (TomGee).
From: donstockbauer on 9 Oct 2005 06:42
Ah but there is Randy, you need to study physics a little more generally to get out of the narrow hole you've gotten yourself into. That's what's wrong with education - the more you get, the narrower a hole it puts you into while the world in general goes on without you (TomGee). ***************** Yep. Need a system of constuctivistically self-organized/educated General Systems Thinkers all connected by the internet into a Global .......you know. |