ballistic pendulum

noun Physics.
  1. a device consisting of a large mass hung from a horizontal bar by two rods, used to measure the velocity of an object, as a bullet, by retaining the object upon impact, its velocity being a function of the displacement of the mass.

Origin of ballistic pendulum

First recorded in 1770–80
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018