Two trajectories: a dynamic monkey tale

摘要

The “monkey drop” experiment is an engaging and instructive demonstration of basic kinematic principles. A toy stuffed monkey is suspended at height above the horizontal floor and at horizontal distance a from a toy cannon on the floor which is loaded with a golf ball projectile. The cannon is aimed directly at the monkey, that is at the angle α=arctan(b/a) relative to the floor. A trigger mechanism simultaneously fires the cannon and releases the monkey. To the amusement, not to say astonishment, of onlookers the projectile arches gracefully through the air and strikes the monkey before it reaches the floor [1]. A trial in which the launch speed of the projectile is increased, but the firing angle is unchanged,also results in a strike [2]. This behaviour is explained physically (air resistance being ignored) by three fundamental kinematic principles: Galileo's law of fall; the principle of inertia; and the synthesis of planar motion from its vertical and horizontal components. A dynamical analysis of the experiment via Newton's differential equations of motion is also well within the grasp of students in an introductory calculus course.