40. The Monkey and the POE

Attention can be called to the well-known Monkey and Hunter demonstration as an illustration of the Principle of Equivalence (POE). The impressive point is that no complicated ballistic calculation is needed. Consider a reference frame falling with the bullet and the monkey. (Imagine a frame released the instant the bullet leaves the muzzle and the monkey starts falling.) In this frame the bullet is initially directed at the monkey and travels uniformly in a straight line toward it. By the POE this falling frame is truly inertial and the fact that the bullet hits the monkey is an immediate consequence of Newton's First Law. (A. Huffman, A.J.P. 48, 314 (1980))