First slide

Rectilinear Motion

Question

A body falls freely from the top of a tower. It covers 36% of the total height in the last second before striking the ground level. The height of the tower is

Moderate

A

50 m
B

75 m
C

100 m
D

125 m

Solution

Let height of tower is h and body takes t time to reach to ground when it fall freely.
$∴ h = \frac{1}{2} g t^{2} . . . . . (i)$
In last second, i.e., t^th sec body travels=0.36h
It means in rest of the time i.e. in (t - 1) sec it travels
$= h - 0.36 h = 0.64 h$
Now applying equation of motion for (t - 1) sec, we get
$0.64 h = \frac{1}{2} g (t - 1)^{2} . . . . . . (i i)$
From (i) and (ii), we get t=5 sec and h=125 m.

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