A point moves with uniform acceleration and $V_1,V_2$ and $V_3$ denote the average velocities in the threesuccessive intervals of time $t_1, t_2$ and $t_3$. Which of the following relation is correct ?
The two ends of a train moving with constant acceleration pass a certain point with velocities $u$ and $3 u$. The velocity with which the middle point of the train passes the same point is ........... $u$