College Physics (4th Edition)

If a grandfather clock is running too fast, then the period of the pendulum is too small. We need to increase the period of the pendulum. $T = 2\pi~\sqrt{\frac{L}{g}}$ In the equation for the period of a pendulum, we can see that increasing the length of the pendulum will increase the period of the pendulum. Therefore, to fix the grandfather clock, the pendulum should be lengthened.