When the two blocks in Question 1 are instead treated as a single combined system, the contact force between them is not shown on the system’s free-body diagram. Which of the following best explains why that force is omitted?
a) Because the surfaces are frictionless, the contact force is zero.
b) It is negligible compared with the applied force F, so it can be ignored.
c) It is internal to the chosen system and appears twice, cancelling in the net-force calculation.
d) Since the blocks move together without any relative motion, no force acts at their interface.