It's amazing how we can get wound up on some hot topics around here. (My
personal sore spot tends to be on getting the laws of physics right.)
I have no qualms about you telling your doctor that you weigh 104 kg.
You have NIST SP 811 on your side regarding the use of "weigh' as a verb.
Of course, some folks here have a vision of SI that is much more pure
than NIST's or even CGPM/CIPM's.
Jim
Carleton MacDonald wrote:
Holy moley, what I started by just telling the doctor that I "weighed" 104
kg ...
Carleton
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of James Frysinger
Sent: Monday, January 26, 2009 14:36
To: U.S. Metric Association
Subject: [USMA:42607] Re: The real physics (was Small item seen on TV)
Thank you, Gene. But I continue to disagree with you.
You state, "There is no pull on the orbiter by the weight of the
orbiter, which is tangential to the path of the orbiter!" That is
patently false. The orbiter's weight vector points from the orbiter
towards the center of mass of the Earth (essentially perpendicular to
the orbiter's near-circular path). The weight vector is not tangential
to the orbiter's path.
I know that you worked with NASA in earlier days. As I've said, NASA is
fond of spacecraft-centric frames of reference. It simplifies many of
the engineering calculations they make. It also confounds the physics of
the situation. Recall, these are the folks who created the slinch.
That view, of course, fails to explain what keeps orbiting bodies from
traveling off into space in a straight line (per Newton's first law of
motion, Galileo's principle of inertia) and instead accelerates them
into elliptical orbits. Acceleration is the rate of change in velocity,
which implies a change in speed, direction, or both. This requires the
application of an unbalanced external force --- Newton's second law of
motion. Centripetal acceleration is not a "magic event".
Jim
[email protected] wrote:
Jim,
Ah! But your classic physics is in error as detailed below for the
approximate inertial frame with origin at the center of the earth. There is
no pull on the orbiter by the weight of the orbiter, which is tangential to
the path of the orbiter! Even the earth frame is non-inertial to some degree
as the earth accelerates in its orbit about the sun.
Certainly, the free falling elevator is more obviously a non-inertial
frame.
Inertial frames exist only in approximation when motions within the solar
system, motion of the solar system within the milky way galaxy, motion of
the milky way within its cluster, etc. are included in increasingly more
cosmic cases.
Gene.
---- Original message ----
Date: Mon, 26 Jan 2009 09:57:31 -0600
From: James Frysinger <[email protected]>
Subject: [USMA:42600] Re: The real physics (was Small item seen on TV)
To: "U.S. Metric Association" <[email protected]>
Gene,
You are using non-inertial reference frames, which can give rise to
pseudoforces (such as centrifugal force). I know that NASA is fond of
this.
I'll stick by classical physics, thank you.
Jim
[email protected] wrote:
Jim,
Here are several corrections of your statements. See below.
---- Original message ----
Date: Sun, 25 Jan 2009 18:38:55 -0600
From: James Frysinger <[email protected]>
Subject: [USMA:42574] Re: The real physics (was Small item seen on TV)
To: "U.S. Metric Association" <[email protected]>
...
The astronauts orbiting the Earth are not weightless.
False. There weights are nearly zero with respect to their immediate
surroundings. They are "weightless" in first approximation in the reference
frame of their spacecraft.
Their gravitational weight is the centripetal force that pulls them
around in orbit.
False. The central force accelerates them toward the center of the
earth, but it is perpendicular to their direction of motion in the orbit.
Likewise, their orbiter's weight pulls it around in orbit.
False. There is no tangential pull (thrust) and little drag on the
spacecraft in parking orbit.
...
*With the earth as the frame of reference*:
The weight of an object at Shuttle Orbiter altitudes is >roughly 92 %
of its weight on the surface of the Earth. At >ISS altitudes it's roughly 91
%.
*With the orbiter as the frame of reference*:
...the astronauts are in orbit with their orbiter and since >both
experience the same centripetal acceleration, the >astronauts feel
"weightless" and float around inside the >orbiter.
A similar phenomenon can be experienced (briefly) in an >elevator whose
cable has been cut...
I have no quibble with your elevator and other examples.
Gene.
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
(F) 931.657.3108
--
James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(C) 931.212.0267
(H) 931.657.3107
(F) 931.657.3108
