I'm a beginner studying mechanics for the first time. I am a homeschooler, stuck with an incomplete and vague course textbook, and largely self-taught, so please bear with me; these are painfully basic questions, but I need clarification of concepts.
I've attached an image of the question. It has multiple parts.
My questions are: For part (iii): The answer key uses one of the constant acceleration kinematics formulas to solve this, given intial velocity $5.5 m/s$ and time $4 seconds$ to find $s$. It keeps using the same acceleration (-0.25 m/s^2). Why? Doesn't acceleration change when the box is "struck"?
For part (iv): It stops using the acceleration -0.25 m/s^2. Why? Also, we can't use t = 4 for time any more. Why?
In part (v), we can't assume the speed of the rebound is the same as the speed of arrival at the boundary board. Why? In the mechanics sense of things, I mean; what is the force exerted by the boundary board upon impact equal to? Not the force at which the box hits the board?
If possible, can I get the "whys" for everything? I'm set to give a super difficult exam soon and need to know the whys.