Flight of fancy
(a) A force diagram for an "old-rules" (pre-1984) javelin. The "pitching
moment" varied throughout the flight. (b) A "positive" pitching moment occurred
in the early stage of the flight, rotating the javelin about its centre of mass
so that the angle of attack increased, giving the javelin a low lift and low
drag. In the later stages of its flight the pitching moment acted in the
opposite sense a "negative" pitching moment. This rotated the javelin so
that the angle of attack decreased and the drag and lift increased
until the tip only just hit the ground first. (c) In the mid-1980s the IAAF
changed the rules so that the centre of mass had to be moved 4 cm further
forward. The "new-rules" javelin only has a negative pitching moment, which
helps to keep the nose down and reduces the distance travelled.