Physics final exam question: loosening a jar lid

Physics 5A Final Exam, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Conceptual Question 13.6

[10 points.] One way to loosen the metal lid on a glass jar is to run it under very hot water. Explain how this works using the properties of heat transfer and thermal expansion.

Solution and grading rubric:

• p = 10/10:
  Correct. As heat is transferred to the metal lid and glass jar, both of them undergo thermal expansion. However, the material in the lid must have a greater coefficient of thermal expansion than glass, in order to expand more and loosen in relation to the opening of the jar.
  
• r = 8/10:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Does not take in account that the glass jar must expand less (or negligibly so) relative to the metal lid in order for it to loosen.
  
• t = 6/10:
  Nearly correct, but argument has conceptual errors, or is incomplete.
  
• v = 4/10:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Explanation may actually result in lid being tighter on jar, or motivates contents under higher pressure will loosen the lid.
  
• x = 2/10:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 1/10:
  Irrelevant discussion/effectively blank.
  
• z = 0/10:
  Blank.
  

Grading distribution:
p: 10 students
r: 16 students
t: 1 students
v: 5 students
x: 0 students
y: 1 student
z: 0 students

A sample of a "p" response (from student 5711) is shown below:
A somewhat strange "v" response (from student 1024) that does result in loosening the lid is claimed to be caused by the glass jar contracting in size: