The horizontal velocity D. remains constant throughout the fall.
Explanation:
The motion of the rock off the cliff is a projectile motion, which consists of two independent motions:
- A horizontal uniform motion. Since there are no forces acting along the horizontal direction (we consider the air drag to be negligible), the acceleration along this direction is zero, so the horizontal component of the velocity is constant
- An accelerated motion with constant acceleration. There is one force acting along the vertical direction, the force of gravity, which is constant so the rock experiences a constant acceleration (downward) of magnitude
(acceleration of gravity). This means that the vertical component of the velocity keeps changing.
Here in this problem, we are only interested in the horizontal velocity. We said that the horizontal acceleration of the rock is zero: therefore, the horizontal velocity of the rock remains constant during the fall. (if the air drag was not negligible, then it would affect the horizontal velocity, which would decrease as the rock falls).
Learn more about projectile motion:
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