Question

A particle is dropped under gravity from rest from a height $h \left( g = 9.8 m / s ^ { 2 } \right)$ and it travels a distance 9h/25 in the last second, the height h is

• A. 100 m
• B. 122.5 m
• C. 145 m
• D. 167.5 m

Answer 1

Answer: (B)

122.5 m

Answer 2

Distance travelled in $n$ sec = $\frac { 1 } { 2 } g n ^ { 2 }$ = h .....(i)

Distance travelled in $n ^ { \text {th } } \mathrm { sec }$ $= \frac { g } { 2 } ( 2 n - 1 ) = \frac { 9 h } { 25 }$ .....(ii)

Solving (i) and (ii) we get. $h = 122.5 \mathrm {~m}$ .