Re: [Phys-l] gravitational force

["Krishna Chowdary" <professor.head@gmail.com>]

On 10/31/06, Mike VanAntwerp <mvanantw@hpsk12.net> wrote:I'm looking for
ways to get students to

> work with Kepler's Laws and F= G (m1m2/d^2) in a lab setting.  This is
> for a high school class.


I agree with JD that working with the G (m1 m2/d^2) is tough to do in a high
school lab setting.  However, I have some thoughts about Kepler's laws.

(1)  If you're fine with simulations, there are tons of great applets out
there.  I've worked with some where you can adjust the semi-major axis and
measure the period, or show equal times in equal areas, etc.

(2)  If you're fine with numerical stuff like on a spreadsheet, have the
students use an Euler step method and plot orbits around a gravitational
force center (essentially suggestion (1), but making their own simulation).
This is tricky because of rounding; it is hard to get closed orbits.  The
advantage is that students can change the force law and see what happens.
(aside:  several people have mentioned Interactive Physics on some of the
listservs I'm on.  Is this the kind of thing you can do easily in IP?)

(3)  We do an end run.  We do orbits using a spring force center:  attach a
mass to a spring (we shove a ball in a slinky which works well enough) and
twirl the slinky around your head in a reasonably constant circle.  Measure
the radius and the period.  Do it again, but this time twirl it faster so
that it is in a larger orbit; again measure radius and period.  You can show
that the period is reasonably independent of the radius, and then we do a
derivation to show why (essentially Newton's argument, except using a spring
force instead of gravity).  You can then show as an exercise that the
inverse square force is the way to get the observed period to radius
relation.


--
regards
-Krishna

Krishna Chowdary
Department of Physics & Astronomy
Bucknell University