A stone is dropped from a height of 180 m (g = 10 m/s^2, no air resistance). With what speed does it hit the ground?

Answer and explanation

Correct answer: D. 60 m/s

  1. Step 1: Energy conservation gives m g h = (1/2) m v^2, so v = sqrt(2 g h).
  2. Step 2: v = sqrt(2 x 10 x 180) = sqrt(3600) = 60 m/s.
Answer
60 m/s.
Why not the others: '1800 m/s' multiplies g by the height; '3600 m/s' gives v^2 and forgets the square root; '30 m/s' forgets the factor 2 inside the square root.

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