Physics final exam problem: frictionless box pushing box with sliding friction

Physics 205A Final Exam, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.63

A 7.0 kg box and a 3.0 kg box are in contact with each other on a horizontal floor. The 7.0 kg box slides without friction on the floor, but the 3.0 kg box has a kinetic friction coefficient of 0.15 with the floor. A horizontal force of 5.0 N is applied to the 7.0 kg box such that both boxes are sliding together to the right. Determine the magnitude of the acceleration of these boxes. Show your work and explain your reasoning.

Solution and grading rubric:
  • p:
    Correct. Draws free-body diagrams, and applies properties of forces and Newton's laws to determine the kinetic friction force on the 3.0 kg box, finds the net horizontal force on the system of both boxes, and implements Newton's second law to find the acceleration of both boxes.
  • r:
    Nearly correct, but includes minor math errors. One of the two points (1)-(2) correct, other is problematic/incomplete.
  • t:
    Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
  • v:
    Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. At least some attempt at applying properties of forces and Newton's laws.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. Approach does not substantively use properties of forces and Newton's laws.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
Grading distribution:
Sections 70854, 70855, 73320
Exam code: finaln0M3
p: 13 students
r: 4 students
t: 7 students
v: 17 students
x: 13 students
y: 2 students
z: 5 students

A sample "p" response (from student 0000):

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