PHYSICS · Single correct · AP · Physics 1: Algebra-based · Force and Translational Dynamics · Force and Translational Dynamics
Problem
A block slides down a rough, fixed incline while still in contact with the surface. A student chooses one of two possible systems for analysis:
• System 1: the block alone
• System 2: the block plus Earth (which includes the incline)
Which statement best explains how the kinetic friction force on the block is classified in each system?
(a) It is internal in System 1 because only the block feels the force, but external in System 2 because Earth cannot exert a force on itself.
(b) It is external in both systems because any contact force with a surface is always outside the chosen system.
(c) It is external in System 1 since the incline is outside the boundary, but internal in System 2 because the interaction happens entirely between objects within the system.
(d) It is internal in both systems because the action–reaction pair between block and incline cancels out inside the boundary no matter how it is drawn.