Final answer:
The question involves Physics, specifically mechanics and the influence of gravity on projectiles. A rock tossed upwards with an initial velocity will slow down due to gravity until it stops and starts falling back down, experiencing constant acceleration due to gravity in both upward and downward motion.
Step-by-step explanation:
The question pertains to the principles of kinematics and dynamics in classical mechanics, a sub-field of Physics. When a rock is thrown vertically with an upward velocity, gravity continuously exerts a downward acceleration of approximately -9.80 m/s² on the object, regardless of its motion direction. A rock maintaining a continuous upward velocity of 8 km/s would defy the usual influence of gravity, possibly suggesting a scenario where extraneous forces are acting on it, or it's not on Earth. However, in a general physics context related to Earth, such a continuous upward velocity cannot be sustained unless an equivalent continuous force is applied to counteract gravity. In the case of objects subject to Earth's gravity, their ascent will slow down due to gravitational pull until their velocity reaches zero at the peak of their trajectory, at which point they will start descending, accelerating downward at the same rate of -9.80 m/s².
It's important to distinguish between instantaneous velocity and average velocity in these scenarios. The instantaneous velocity of a projectile changes at every point of its trajectory, becoming zero at its peak, and then negative as it descends. Yet, the magnitude of its acceleration due to gravity remains constant throughout the motion. Additionally, free-fall motion applies to both upward and downward motion, with the same constant acceleration due to gravity.