In preparation for this problem, review Conceptual Example 7. From the top of a cliff, a person uses a slingshot to fire a pebble straight downward, which is the negative direction. The initial speed of the pebble is 6.46 m/s. (a) What is the acceleration (magnitude and direction) of the pebble during the downward motion? (b) After 0.667 s, how far beneath the cliff top is the pebble?