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Name: John
Status: educator
Grade: N/A
Location: Md
Country: N/A
Date: 12/25/2004
Question:
My question is about orbital paths and projectile motion. A diagram in my Astronomy text (Astronomy Today, Chaisson & McMillan, 4th edition) shows the elliptical shape of all orbits, including those that are "interrupted" by the Earth's surface. From this diagram, one would conclude that projectile motion should take an elliptical path, contrary to what we see in every physics text. Further, the diagram shows that objects that achieve escape velocity have hyperbolic paths. Shouldn't a parabolic curve be possible for speeds in between ellipses and hyperbolas? Is the diagram wrong?

Replies:
The various conic sections taken by a projectile depends upon whether the kinetic energy of the projectile is less than that accounted for by gravity (a parabola), equal to i.e. in "free fall" (an ellipse), or greater than (a hyperbola).

http://en.wikipedia.org/wiki/Parabolic_trajectory

gives a more quantitative algebraic treatment. For those wanting even more details and are willing to navigate a large physics web site try:

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Vince Calder


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