20161015

Physics midterm question: net force on falling coffee filter stack

Physics 205A Midterm 1, fall semester 2016
Cuesta College, San Luis Obispo, CA

A stack of paper basket coffee filters released from rest increases its downwards speed until it reaches a constant downwards value. Air resistance is not negligible. Discuss why the net force on the stack of filters has a greater magnitude just after it is released from rest, compared to the magnitude of the net force on the stack of filters once it has attained a constant downwards speed. Explain your reasoning using free-body diagram(s), the properties of forces, and Newton's laws.

Solution and grading rubric:
  • p:
    Correct. Argues why the net force on the stack of paper basket coffee filters decreases as it falls by either:
    1. recognizing the two forces acting on the filters:

      wEarth on filters, downwards;
      Fair drag on filters, upwards.

      Then discusses that the magnitude of wEarth is constant, but the magnitude of Fair increases as the filters fall faster downwards, which results in the net force ΣF = −wEarth + Fair to have a smaller (negative) value; or
    2. appealing directly to Newton's laws as applied the motion of the filters, where increasing downwards speed means that from Newton's second law the net force ΣF has a non-zero value, then when at terminal velocity (constant speed), Newton's first law says that the net force ΣF must be zero.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying Newton's laws to free-body diagrams or motion.
  • 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 (or motion).
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: midterm01br1Q
p: 30 students
r: 14 students
t: 8 students
v: 4 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 1984):

Another sample "p" response (from student 3428):

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