Final answer:
Using the equation of motion for objects under gravity, the pebble hits the water with a velocity of 39.2 m/s after falling for 4 seconds with an initial velocity of 0 m/s.
Step-by-step explanation:
To determine how fast the pebble strikes the water after falling for 4 seconds, we can use the equation of motion for an object under gravitational acceleration. The equation is: v = u + at, where v is the final velocity, u is the initial velocity (which is 0 m/s since the pebble is dropped), a is the acceleration due to gravity (9.8 m/s2), and t is the time the object has been falling. Plugging in the values, we have v = 0 + (9.8 m/s2) (4 s), so v = 39.2 m/s. Therefore, the pebble hits the water with a velocity of 39.2 m/s.