Fig. 7.

Three effects of water molecules (blue) on the motion of a large particle X (orange). I Water molecules collide on all sides of the large particle apparently at random (black arrows). If X is stationary, there is no directional bias to the random collisions. II If X is moving right, water molecules will collide more frequently on the particle’s right side and less frequently on its left. There will be a deterministic force opposing X’s velocity. III The arrows show how the water molecules might move in the time it would have taken them to collide with X had X been stationary (dotted circle). The water molecules on the left have not yet collided with anything, while the molecules on the right have collided and reflected away with higher speeds. They are all further to the right than they would have been otherwise, causing a net flow of water to the right