Barron's 2nd Edition DT Q16

Barron's Book

16) Question 16 asks us to find the intervals where the magnitude of the object's velocity is decreasing. This also means that the object is slowing down. An object only slows down when its acceleration and the velocity are of opposite signs. For example, having a positive velocity and a negative acceleration.

Let's start with the velocity of the object. The velocity at any point will be the slope of the given position graph.

Where v is velocity

Here is where a lot of people get tricked. If we look at interval A, the velocity is constant at -15 m/s. This means there is no acceleration on interval A. If we look at interval B, the velocity is constant at 0 m/s. There is no acceleration on interval B either. In fact, if we look at all the intervals, the velocity is constant. The only time the velocity changes, is in the instantaneous moment between each interval. However, that isn't an answer choice, so our answer must be E(None of these).

Answer: E