Final answer:
A baby's rattle makes noise when moved back and forth because the fillings inside collide with the rattle's walls due to inertia, producing sound. If moved steadily, the fillings move with the rattle, preventing noise. This rattling sound is an auditory stimulus for the baby's development and differs from loud sounds that could frighten a newborn.
Step-by-step explanation:
The reason why a baby's rattle makes noise only when moved back and forth, and not when moved steadily in one direction, relates to the principles of momentum and inertia in physics. When the rattle is shaken back and forth, the fillings inside collide with the walls of the rattle due to their inertia, which causes them to lag behind the motion of the rattle's exterior. This lag creates a forceful impact that produces the rattling sound. Conversely, when the rattle is moved steadily, the fillings inside move along with the rattle's shell and do not strike against it, resulting in no sound being made.
Babies naturally react to sounds of potential danger, such as loud noises or the cries of other babies. In this context, the rattle provides an auditory stimulus that helps the baby develop sensorimotor skills; however, a consistently loud or jarring noise, unlike the gentle sound of a rattle, could startle and frighten a newborn.