Physics Review Question
Unit: Motion in a Plane
Year: 1990 Question#: 68
Question: In the diagrams below, P represents a planet and S represents
the sun. Which best represents the path of planet P as it orbits the Sun? [The diagrams
are not drawn to scale.]
What is this question really asking?
Explanation by: Simon
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Answer 1
INCORRECT
Answer 1 is incorrect because bodies of mass do not travel in circles by themselves. To
travel in a circle, a body of mass needs something that applies a force to it that is
always perpendicular to its velocity. There is nothing in the center of the planet's orbit
to provide such a force, so this scenario is impossible.
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Answer 2
INCORRECT
Answer 2 in incorrect for the same reason that answer 1 is wrong, bodies of mass do not
travel in circles by themselves. Also, it is safe to assume on this test that the sun is
more massive than a planet, so the sun would not be orbiting the planet. return
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Answer 3
INCORRECT
Answer 3 is incorrect because the sun is orbiting the planet, which could not happen
because the sun is more massive than the planet. return to top
Answer 4
CORRECT
Answer D is correct because all the elements fit. The planet is orbiting the sun like in
our solar system, and it is moving in a simple elliptical orbit like all planets do. Note
that the sun sits at one of the foci of the ellipse. return to top
What's this question really asking?
This question is really asking if you know how our solar system works. Apply Kepler's law
of ellipses. return to top