Question

A body falling freely under gravity passes two points 30 m apart in 1 s. From what point above the upper point it began to fall? (Take g = 9.8 m s^-2)

• A. 32.1 m
• B. 16.0 m
• C. 8.6 m
• D. 4.0 m

Answer 1

Answer: (A)

32.1 m

Answer 2

Suppose the body passes the upper point at second and lower point at (t + 1) s then

$S _ { 2 } - S _ { 1 } = \frac { 1 } { 2 } g ( t + 1 ) ^ { 2 } - \frac { 1 } { 2 } g t ^ { 2 } = \frac { 1 } { 2 } g ( 2 t + 1 )$

Or 30m $= \frac { 1 } { 2 } \times 9.8 ( 2 t + 1 )$

$\therefore \mathrm { t } = 2.56 \mathrm {~s}$

$\mathrm { S } _ { 1 } = \frac { 1 } { 2 } \mathrm { gt } ^ { 2 } = \frac { 1 } { 2 } \times 9.8 \times ( 2.56 ) ^ { 2 } = 32.1 \mathrm {~m}$.