eliptical orbits

[Rob Campbell]

Hi!

Newton's explanation remains the simplest one that fully explains this, but
I won't attempt maths in a text-only email!

So, here's the hand-waving version.

Earth satellites like the Moon or the ISS 'fall' toward Earth.  They never
arrive because there's a sideways component to their velocity.  If the ISS
really does move toward Earth, it loses PE and gains sideways KE.  The
sideways motion tends to draw it tangentially away from Earth - which
doesn't prevent it coming closer (it does.)

As it approaches Earth, tangential velocity builds up, slowing then
reversing the Space Station's fall.  It starts to rise, gaining PE as it
does so, and losing KE.  At its furthest point from Earth, its orbital
velocity is lowest; at the nearest point, its orbital velocity is highest.
This generates an ellipse, though at 10pm I can't illustrate this in words!
A moment's thought will show that an ellipse is reasonable; 1/2 hour's
thought with pen and paper will prove it.

There's no difference in principle between a satellite orbiting Earth and a
planet orbiting the Sun - just one of scale.  Planets tend to follow
elliptical orbits approximating to circles, maybe on account of their fiery
origins.

An ellipse is a squashed circle (apologies, mathematicians) with a major and
minor axis rather than a constant radius.  Really, a circle is the special
case ellipse where the two axes are equal.

Hope that helps,

Rob Campbell


-----Original Message-----
From: phys-teach-talk-bounces at nzip.org.nz
[mailto:phys-teach-talk-bounces at nzip.org.nz] On Behalf Of Terry Moffat
Sent: Monday, 7 April 2008 8:22 a.m.
To: phys-teach-talk at nzip.org.nz
Subject: eliptical orbits

Hi,

can someone give me a simple reason why planets move in eliptical orbits.


does it have something to do with the fact that the system is moving through
space?


ta  Terry




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