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 and 7 s. The value of \[\mu \] is ms-1 is: \[(g=10{{m}^{-2}})\]

A)  50                                         

B)  40

C)  30                                         

D)  20

Correct Answer: B

Solution :

                 The projectile of the body 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 \[\beta \] at t = 4 s. Thus, \[v=u-gt\] \[0=u-10\times 4\]                          \[(\because \,v=0)\] \[\therefore \]  \[u=40\,m/s.\]