Phys 100
Spring 2009
Concepts of Physics: Homework 1
Due: 28 January 2009
1Hobson, Physics: Concepts and Connections, Ch. 1 Conceptual Exercises 3, page 27.
2Hobson, Physics: Concepts and Connections, Ch. 1 Conceptual Exercises 4, page 28. A
complete solution includes a description of which feature of a planet you observe, what the
results of this observation could be, and how these results support or disprove the possibility
of simple circular motion.
3Hobson, Physics: Concepts and Connections, Ch. 1 Conceptual Exercises 8, page 28.
4 Indirect vs. direct evidence for circular planetary motion.
Consider the motion of the planet Saturn. How do we know that it moves in a nearly circular
orbit around the sun rather than about the earth? A direct method for doing this would be
to observe the position of Saturn, the Earth and the Sun from a distance during one entire
orbit of Saturn. This would require observing the three from a space probe at a significant
distance from the Sun. The only candidates for this are the Voyager and Pioneer planetary
space probes which reached suitable distances from the sun in the late 1990’s. Suppose that
these were used to photograph Saturn, the Earth and the Sun, once per week and that these
photographs were played back as a movie.
a) Sketch the paths of Saturn, the Earth and the Sun according to the ancient Greek
notions of the solar system.
b) Sketch the paths of Saturn, the Earth and the Sun according to Copernicus’ notion of
the solar system.
c) Look up the time taken for Saturn to complete one orbit (a “Saturn year” or the “orbital
period”) and write this down together with the source of your information. What portion
of Saturn’s orbit could the probes conceivably have observed? Could a hypothetical
movie produced by these probes be used to support either the ancient Greek notions or
those of Copernicus?
A movie of this type could be regarded as “direct” evidence for Copernicus’ notion of the
solar system. Such observations were not actually done by these or any other space probes.
In fact, the orbits of the planets needed to be known in advance so that the probes could
approach various planets closely (they did this successfully).
d) Based on discussions in the class and the text describe evidence for the paths of Saturn,
the Earth and the Sun according to Copernicus’ notion of the solar system. Would you
consider this as “direct” or “indirect” evidence (i.e. does the evidence actually show the
paths of the planets or do you have to deduce the possible path from the evidence?
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