A 1 kg object is dropped from rest from a height of 20 m. What is its speed just before hitting the ground? (Ignore air resistance, g = 10 m/s^2)
Show the answer and explanation
Correct answer
B. 20 m/s
Principle or equation
Conservation of mechanical energy: mgh = (1/2)mv^2, so v = sqrt(2gh).
Why this answer is correct
Substitute g = 10 m/s^2 and h = 20 m: v = sqrt(2*10*20) = sqrt(400) = 20 m/s.
Example
If an object falls from 5 m, v = sqrt(2*10*5) = sqrt(100) = 10 m/s.
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