A ballistic pendulum is an old-fashioned device used to measure the speed of a bullet. A bullet of mass m is fired at a block of wood (mass M) hanging from a string. The bullet embeds itself in the block, and causes the combined block plus bullet system to swing up a height h. If you know the masses and measure the height h, then you can calculate the speed of the bullet.
Modified from Andrew Duffy's web page at Boston University. The animation uses the Physlet Java Applets of Wolfgang Christian and Mario Belloni.