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Applications of circular motion and gravitation

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On a planet with no atmosphere and a mass twice that of Earth's, a 12.5kg rock falls off of a cliff. Beginning at an arbitrary time during free fall, the resulting vertical velocity of the rock as a function of time is shown in the graph.
A line graph shows velocity over time. On the y axis, velocity ranges from negative forty five meters per second on the bottom to zero meters per second on the top. The x axis shows time, ranging from zero to eight seconds. A diagonal line begins at time zero and a velocity of negative ten meters per second. The line has a constant downward slope and ends at eight seconds and negative forty meters per second.
Determine the radius of this planet in terms of Earth radii, RE.
Write your answer to two significant figures.
  • Your answer should be
  • an integer, like 6
  • a simplified proper fraction, like 3/5
  • a simplified improper fraction, like 7/4
  • a mixed number, like 1 3/4
  • an exact decimal, like 0.75
  • a multiple of pi, like 12 pi or 2/3 pi