On a tabletop rests a heavy block A, with a lighter block B on top of it (see figure at right). We will use T to represent the tabletop, and F to represent the finger pushing on the stack of blocks. Consider the forces NAB, fTA, fAB, NA, fB, and NBA.
In each of the three scenarios below, specify the directions (left, right, up, down) of the above forces. If any of the forces are zero in a given scenario, state so directly. Always ignore air resistance.
a) You start pushing on block A, but the block is too heavy and doesn't move. In this instant where you're pushing but block A isn't moving, what are the directions of each of the forces above? Support your argument with free body diagrams.
b) Now you add some oomph to your push and block A starts moving. Block B moves with it and doesn't slip. Both blocks are now speeding up in tandem. What are the directions of each of the forces above? Support your argument with free body diagrams.
c) Now you ease up a little so the blocks move at a constant velocity. As before, the blocks move together - block B isn't slipping. While the blocks are moving with constant speed, what are the directions of each of the forces above? Support your argument with free body diagrams.