The Dual-Spring-Cannon-Propelled Cart

Carl E. Mungan

United States Naval Academy

Two identical spring-loaded cannons pointing backward are mounted on a frictionless railcart. The cannons are to be fired to propel the cart forward as rapidly as possible. Does it matter whether the two cannonballs are launched simultaneously or sequentially? The velocities and kinetic energies of the two balls and of the cart in various reference frames are analyzed in detail in order to explain the answer.