A compound pendulum is an apparatus used to measure the acceleration due to gravity. It consists of a rod suspended from a fixed point and a bob at the end of the rod. The bob is usually made of metal, and is attached to the rod by a pivot. The bob is allowed to swing freely, and its motion is measured using a ruler or protractor. The acceleration due to gravity can be calculated using the measured time period of the pendulum's swing. The compound pendulum is typically used in physics classrooms, laboratories, and amusement parks.