Answer
See the figures below.
Work Step by Step
In this problem, the author asks us for 3 sketches.
a) The motion diagram.
b) The force diagram.
c) The free body diagram.
See the figures below.
The groceries bag is having the same speed as the car and the same acceleration as it slows down.
But what makes the bag slows down if we are choosing it as our system?
We know that when you apply the brakes to slow your car down, the bag needs to maintain its initial state by continue moving forward.
So, it had to slide forward but that did not happen which means that it was on the verge of sliding forward and there is some force that prevents it from doing that.
This force is the static friction force, as you see below.
