You throw a rock upwards; it rises and then falls back down. we know that the acceleration due to gravity is 9.8 m/sec2 near the surface of earth. what is the rock’s acceleration at the very top of its trajectory (where its velocity is zero for an instant)? ignore air resistance.

Acceleration = 9.8 m/sec2

Explanation:

We know that the acceleration due to gravity is 9.8 m/sec2 near the surface of earth.

Remember  that the gravity is always affecting everything on earth at every instant, even the rock at the very top of it's trajectory. It doesn't mind if the velocity is zero for an instant or if there's air resistance, the gravity force is there, and always aims towards the center of the earth.

So for all the trajectory of the rock, gravity is 9.8 m/sec2

snell's law

refractve index = sin incidence / sin refraction