question_answer

A body is projected vertically upwards with a velocity u. It crosses a point in its journey at a height h twice, just after 1 s and 7 s. The value of u in \[m{{s}^{-1}}\] is (Take \[g=10\,\,m{{s}^{-2}}\])

A)  50                                         

B)  40               

C)  30                                         

D)  20

Correct Answer: B

Solution :

 The situation is shown in the figure. In the upward journey, the body is at A at \[t=1\] s and in downward journey it is at A at \[t=7\] s. Hence, we can conclude that during upward journey, the body will be at 6 at \[t=4\] s.   Thus,     \[v=v-gt\] \[0=u-10\times 4\]          \[(\because \,\,v=0)\] \[\therefore \]  \[u=40\,m{{s}^{-1}}\]