A small ball is suspended by a thread 1 m long from the ceiling of the carriage. At what speed of the carriage

A small ball is suspended by a thread 1 m long from the ceiling of the carriage. At what speed of the carriage will the ball vibrate especially strongly under the impact of the impact of the wheels on the rail joints? Rail length 12.5 m.

Obviously, this will happen when the period of oscillation of the pendulum coincides with the period of impacts from impact on the rail. The period of the pendulum is determined by the formula:
T = 2π√l / g where l is the length of the pendulum g is the acceleration of gravity. In this case, we get:
T = 2π√1 / 9.8 = 2 s.
Then the train speed should be:
12.5: 2 = 6.25 m / s.