A mass of 3 kg is dropped from a tower of 125m high. After 3s its K.E. will be :-

1300 J

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1050 J

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750J

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550J

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Solution

The correct option is A 1300 J
Checking if the ball has reached the ground before or at $t = 3 s$ .
So,
Applying $2^{n d}$ equation of motion
$h = (u) (t) + \frac{g}{2} t^{2}$
$h = 0 (3) + 4.905 (3)^{2} = 44.145$ m
As $h < 125$ m, the mass is under free fall
So, till $t = 3 s$ work done by gravity
$= m g h = 3 \times 10 \times (44.145)$
$= 1300$ J
As only gravity is acting on the mass under free fall
By work-Energy theorem,
W gravity $= Δ$ KE
$K E_{f} - K E_{i} = 1300$ J
As, $4 = 0$
$K E_{i} = 0$
So, $K E (3 s) = 1300$ J
Option A is correct

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