A ballistic pendulum is a device which is used to arrive at the kinetic energy and velocity of a bullet. The development of the ballistic pendulum was a significant event in the history of ballistics, allowing this field of study to advance significantly. Today, other devices are used to study the movement of bullets, but the ballistic pendulum lives on in physics classrooms around the world, where it is used to provide a simple introduction to concepts like momentum. In classrooms, a springloaded “gun” is usually used to fire a mock bullet, for safety reasons.
The design of a ballistic pendulum is fairly simple. The device consists of a weight attached to an arm. The weight has a wooden block attached which is designed to catch a bullet when it is fired at the block. Knowing the properties of the pendulum and the properties of the bullet, someone can fire a bullet at the pendulum, take note of how far it travels, and use this information as the basis of several equations to determine velocity and collect other information about the bullet's movement.
This device appears to have been developed in England in 1700s. Benjamin Robins, a British mathematician and ballistics expert, is often credited with the development of the ballistic pendulum. His 1742 book New Principles In Gunnery introduced the ballistic pendulum and explained how it could be used. Robins noted that one could also use the device by attaching a gun to the pendulum, firing it, and noting the movement of the pendulum in the wake of the explosion.
With the 1800s came the development of ballistics devices which could directly measure velocity, making for more accurate and easy measurements. However, the basic principles behind the ballistic pendulum are still sound, and such devices can be used with a variety of projectiles, not just bullets. Historically, ballistic pendulums were used to measure the momentum of everything from bullets to cannon balls, providing a great deal of information which had not been available before to members of the ballistics community.
In physics classes, students are sometimes shown a ballistic pendulum demonstration or film to learn about conservation of momentum and conservation of energy. Students may be asked about what kinds of calculations they can use to gather information about the movement of the bullet, and they may be asked about the physics behind the movement of the bullet and the pendulum.