A stone is dropped from a height of 45 m (g = 10 m/s^2, no air resistance). With what speed does it hit the ground?
Answer and explanation
Correct answer: D. 30 m/s
- Step 1: Energy conservation gives m g h = (1/2) m v^2, so v = sqrt(2 g h).
- Step 2: v = sqrt(2 x 10 x 45) = sqrt(900) = 30 m/s.
Answer
30 m/s.
Why not the others: '450 m/s' multiplies g by the height; '900 m/s' gives v^2 and forgets the square root; '15 m/s' forgets the factor 2 inside the square root.
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