Question

Two balls are dropped to the ground from different heights. One ball is dropped $2\, s$ after the other but they both strike the ground at the same time. If the first ball takes $5\, s$ to reach the ground, then the difference in initial heights is $(g=10m{{s}^{-2}})$

• A. 20 m
• B. 80 m
• C. 170 m
• D. 40 m

Answer 1

Answer: (B)

80 m

Answer 2

${{s}_{1}}=\frac{1}{2}gt_{1}^{2}=\frac{10}{2}\times {{(5)}^{2}}=125\,m$ ${{s}_{2}}=\frac{1}{2}gt_{2}^{2}=\frac{10}{2}\times {{(3)}^{2}}=45\,m$ $\therefore$ ${{s}_{1}}-{{s}_{2}}=125-45=80\,m$