The complexity of much of the mechanical gear used for shock absorption and arresting in aircraft engineering is apt to obscure some of the broad generalizations that may be deduced from the elementary principles of classical mechanics. More particularly does this apply to the distance required to arrest the motion of an aircraft or falling body when the retarding force must not exceed a specified value. This note merely presents some more or less self-evident truths which may be of some value in dealing with new landing and arrester problems.