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Question

An object dropped from a height of 20 m is undergoing free fall. What is its velocity just before it touches the ground? (Take g=10 ms−2)

A
10 ms1
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B
20 ms1
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C
15 ms1
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D
17 ms1
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Solution

The correct option is B 20 ms1
Given Height h=20 m
Acceleration due to gravity g=10 ms2.
Initially the body is dropped, so initial velocity u=0 ms1.
According to the conservation of energy, energy will remain constant throughout. So, potential energy at top is equal to kinetic energy at bottom
mgh=12mv2
or v2=2gh=2×10×20
or v2=400
or v=20 ms1

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