(This is a photocopy of the page, no real books were defiled in the process of studying :P)
The Attempt at a Solution
In an example in my book, a problem reads that a car is moving in the positive x direction, the initial velocity is 15m/s and it takes 5 seconds to slow down to 5m/s. The example tells me the cars average acceleration is -2m/s^2. Then it says that in this case the acceleration is to the left, in the negative x direction evn though the velocity is pointing right. (I assume they mean instantaneous velocity) Now what I don't understand is how can the velocity be moving in the positive x direction but have a negative magnitude? If v1= 15 and vf=5, then the average v= -10. so if the signs tells us where the direction is, and this says it's negative, why is it moving in the positive x direction???