• # question_answer A ball is projected upwards from the top of a tower with a velocity of 50 m/s making in angle of $30{}^\circ$ with the horizontal. The height of the tower is 70 m. After how many seconds from the instant of throwing, will the ball reach the ground? (Take$g=10\,m/{{s}^{2}}$) A)   2s                  B)  5 sC)   7s                  D)  9 s

Vertical component of initial velocity $=50\sin {{30}^{o}}$$=25\,m/s.$ $U\sin \,g\,s=ut+\frac{1}{2}a{{t}^{2}}$along the y-axis $-70=25t-\frac{1}{2}\times 10\times {{t}^{2}}$ or $5{{t}^{2}}-25t-70=0$ or ${{t}^{2}}-5t-14=0$ or $t(t-7)+2(t-7)=0$ or $t=7\,\sec .$ Hence, the correction option is [c].