Final answer:
The rock thrown downward at 10 meters per second falls vertically due to gravity, which consistently accelerates it at -9.80 m/s², making option B correct.
Step-by-step explanation:
When a rock is thrown downward at 10 meters per second, the correct option for what happens to it is B. It falls vertically. This is because objects thrown downward will continue to move in the direction of the initial force due to inertia, and gravity will accelerate them further toward the ground.
The acceleration due to gravity is a constant -9.80 m/s², regardless of whether an object is moving up or down. This means that even if a rock is thrown upward, once the force of the throw is overcome by gravity, the rock will decelerate, come to a momentary stop, and then start to accelerate back toward the Earth at 9.80 m/s².
When an object is thrown straight up and falls back to Earth, this is considered one-dimensional motion. At the top of the object's trajectory, its velocity is zero because the upward motion has ceased and it is about to reverse direction.