Physics midterm question: proposed POF-OSO-WTF test for Newton's first law forces

Physics 205A Midterm 1, fall semester 2017
Cuesta College, San Luis Obispo, CA

A Physics 205A student proposes a three-part "POF-OSO-WTF" test for identifying a pair of forces related by Newton's first law:

__ POF = Pair of Opposite Forces
__ OSO = On the Same Object
__ WTF = With just Two Forces

Determine whether or not this "POF-OSO-WTF" test can correctly identify a pair of forces related by Newton's first law acting on a 600 kg load of cargo that rests on the bed of a stationary truck. Explain your reasoning using free-body diagram(s), the properties of forces, and Newton's laws.

Solution and grading rubric:

• p:
  Correct. Complete free-body diagram, and discusses/demonstrates:
  1. that there are two forces acting on the 600 kg cargo load:

     weight force w of Earth on cargo load (downwards),
     normal force N of truck on cargo load (upwards); and

  2. since the cargo load is stationary, then from Newton's first law these two forces are equal in magnitude in order for the net force to be zero; then
  3. tests each part of the presumptive "POF-OSO-WTF" test for the cargo load, where the weight force and the normal force are the "Pair of Opposite Forces," and both act "On the Same Object" (the cargo load), and this test applies to the cargo load "With just Two Forces." (Note that it cannot be applied to the truck, even though Newton's first law applies to it as well, as there are three forces acting on it.)
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. May apply the "POF-OST-WTF" test to forces on the truck (when instructed to identify a pair of forces acting on the cargo load), or to an interaction pair of forces (which are actually related by Newton's third law, despite being instructed to identify a pair of forces related by Newton's first law); but at least has (1)-(2) above complete and correct. Or may have (2)-(3) essentially complete/correct, but has minor errors in forces on free body diagram, such as involving forces that do not act on the cargo load, and/or forces exerted by the cargo load.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. Essentially only (1)-(2) complete/correct, but does not explicitly discuss relevance of "POF-OSO-WTF" test.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some substantive attempt at applying Newton's laws to a free-body diagram
• x:
  Implementation of ideas, but credit given for effort rather than merit. No systematic application of Newton's laws to the forces on a free-body diagram.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: midterm01mOoL
p: 32 students
r: 10 students
t: 8 students
v: 3 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 5102):