In designing rotating space stations to give for artificial-gravity environments, one of the constraints that must be considered is motion sickness. Studies have shown that the negative effects of motion sickness start to appear when the rotational motion is faster than approximately 1.5 revolutions per minute. On the other hand, the magnitude of the centripetal acceleration at the astronauts' feet should equal the magnitude of the acceleration due to gravity on earth. Therefore, to eliminate the difficulties with motion sickness, designers must choose the distance among the astronaut's feet and the axis about which the space station rotates to be greater than a certain minimum value. What is this minimum value?