Some objects fall over when they are pushed or pulled while other objects move freely. Think about a tree blowing in the wind. The upper branches of the tree move with the wind. Once the wind ends, the tree sways back and forth until it comes to rest at its original position. This type of repeated motion occurs often in other situations. While you are playing with a yo-yo, the toy moves up and down and up and down. When a car is stuck in the snow, the driver rocks the car forward and backward until it is freed. An expectant father pacing in a waiting room also shows repeated motion. Can you think of any more examples of repeated motion? Discuss this question with your group and write down some of your ideas to share with the class.

Bats fly back and forth in a cave at night to find their food. While flying at night, a bat emits squeaks that reflect off the walls of caves and flying insects. These squeaks return to the bat's nervous system and are used as clues to determine the location and shape of objects. The bat can tell where the next meal is and when to avoid other objects. A sonar ranger (also called an ultrasonic detector) uses technology for a similar purpose. This type of motion detector sends out high frequency sound waves out to a target object and waits for it to come back. Bat and sonar rangers "know" how fast sound travels. By timing how long the sound travels it can determine the distance of the object.

A sonar ranger uses a motion detector that sends out high frequency sound waves to a target object and waits for it to come back. The sonar ranger measures the time needed for the wave to leave and return to the detector. The software determines and displays the distance the pulse covered based on the speed of sound. Try it out for yourself with the sonar ranger! Go to the "Investigations" to find out how.