A student tries to measure the period of a pendulum that is already swinging left and right. At the moment when the pendulum is fully to the left, she
counts 'One' and starts a stopwatch. She counts successive swings each time
that the pendulum returns to the left. When she counts ‘Ten' she stops the
stopwatch, and sees that it reads 12.0 s.
a. What was her mistake?

b. What is the period of swing of this pendulum?

c. In this particular experiment, explain the likely effect of her reaction time
on her answer