A body of mass $20 k g$ is dropped from a height of $2 m$ . If $g$ is taken to be equal to $10 m s^{- 2}$ , the kinetic energy of the body just before striking the ground will be:

400 J

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4 J

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40 J

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None of these

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Solution

The correct option is A $400 J$
In the case of a body falling towards the earth, potential energy changes into kinetic energy.
In the given case we need to calculate $K . E$ while the body just hits the ground and hence we consider that all the $P . E$ gets converted to kinetic energy near the ground.
$K . E = P . E = m g h$
$= 20 \times 10 \times 2$
$= 400 J$
Thus, the kinetic energy of the mass is $400 J$ before it hits the ground.

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