1. A shuttle bus slows down with an average acceleration of -1.8m/s^2. How long does it take the bus to slow from 9.0m/s to a complete stop? For this question, we can immediately say that the final velocity is 0m/s because it stops. Now let's take a look at the second question: A robot probe drops a camera off the rim of a 239 m high cliff on Mars, where the free-fall acceleration is -3.7 m/s^2. Find the velocity with which the camera hits the ground. Why here this final velocity is not '0' m/s because i think it hits the ground, so it stops? 2. A model rocket is launched straight upward with an initial speed of 50.0 m/s. It accelerates with a constant upward acceleration of 2.00 m/s^2 until its engines stop at an altitude of 150m. My question is its acceleration is 2.00m/s^2 upward, but do we need to consider about the gravity 9.81m/S^2 that pulls it down because that's the reason why later on it will slow down? Thanks a lot for explanations.