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I'm not really sure what they are getting at. Are we doing calculations with Kepler's 2 and third laws? Are we setting gravitational force (Gmm/r^2 = mv^2/r) to solve for velocity of orbits? Both? Neither? Can anyone shed some light on what they really want?

When you look at the clarification statement it says: "[Clarification Statement: Emphasis is on Newtonian gravitational laws governing orbital motions, which apply to human-made satellites as well as planets and moons.] " So I would guess its focusing on Newton's Law of Gravitation. This looks like a good webpage about it :) http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html '' target="_blank"> http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html ' target="_blank"> http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html

Anyone have interesting activities to teach this [to freshman - many with low math skills]?

Looking at the website from the previous post simplifies things quite a bit, because HS-ESS1-4 limits our mathematical model to "the solar system". In which case, the sun's mass is so overwhelmingly bigger than any of the planets, that it is essentially the center of mass. I think we could create a table with columns--Distance between sun and planets, Mass of sun, Mass of planet, Force of Gravity, Speed of Planet. We could give them the formulas for Force of Gravity and Speed of Planet, as well as a bunch of values for distance already in the table. They use calculators to fill in values for Fg and V in the table. Then they can explain how Fg and V change as distance changes. Fg = G*M1*M2/R^2 V = SquareRoot(Fg*R/M2) I think its also important to explain that the planet stays in orbit because of Fg pulling on it. But the planet doesn't crash into the sun because of the centrifugal force (caused by velocity) pulling it in the opposite direction. Thoughts? -Gregg Draper, Utah

There are also 3 free SciObjects on Gravity and Orbits (Orbits, Gravitational Force & Universal Gravitation), so you can access the teacher content and animations/simulations that Tina mentioned in these SciObjects.