Observers saw a rectangle at three orientations along a path of rotation. They attempted to remember the third orientation and were then tested with a fourth orientation that was either the same as, or slightly different from, the third. As in previous representational momentum studies we find that memory for position is distorted in the direction of the implied motion, in analogy to physical momentum. We now report that memory shift increases with retention interval for small intervals, as predicted by the analogy. However, instead of reaching some asymptotic value, the memory shift then decreases with retention interval. The resulting U-shaped curve may be considered the result of two competing effects: a positive memory shift attributable to representational momentum, which dominates at short intervals, and a negative shift attributable to memory averaging effects, which dominates at longer intervals. The memory averaging effect increases with retention interval and is strongest for faster presentation rates. For very short retention intervals the rate of increase in memory shift is proportional to the implied velocity of the inducing display, as predicted from the analogy to physical momentum.