time rising = 20.5/2 = 10.25 seconds
v = Vi + g t
at top v = 0
so
0 = 13 + g (10.25)
since I chose acceleration up, g will be a negative acceleration.
An astronaut on a distant planet wants to determine its acceleration due to gravity. The astronaut throws a rock straight up with a velocity of +13 m/s and measures a time of 20.5 s before the rock returns to his hand. What is the acceleration (magnitude and direction) due to gravity on this planet? (Indicate direction by the sign of the acceleration.)
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