Question

How many vibrations, a second's pendulum will complete in 52 second ?​

Answer 1

We know that the time period of a seconds pendulum is T=2 sec. One second for a swing in one direction and one second for the return swing.  

In 52 seconds it will swing 52 times, 26 swings up , 26 swings down

consider vibration(up-down) = swing here

total vibrations = 26(up) + 26(down) = 52