Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Conceptual Question 4.10(b)
A freight train consists of an engine and a caboose on level ground. If the engine and caboose are both moving at the same constant speed, discuss whether or not the engine's pull on the caboose exceeds the caboose's backward pull on the engine. Explain your reasoning using free-body diagram(s), the properties of forces, and Newton's laws.
Solution and grading rubric:
- p:
Correct. The force of the engine pulling on the caboose must be equal and opposite to the force of the caboose pulling on the engine by Newton's third law, as they are an interaction pair. - 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. Has some understanding that Newton's third law is used to analyze these two forces. - v:
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Uses Newton's first law (which cannot be used on two forces that act on two different objects), or Newton's second law (which is only applied to accelerating objects). - x:
Implementation/application of ideas, but credit given for effort rather than merit. - y:
Irrelevant discussion/effectively blank. - z:
Blank.
Grading distribution:
Sections 70854, 70855
p: 8 students
r: 4 students
t: 3 students
v: 27 students
x: 2 students
y: 0 students
z: 1 student
A sample of an "r" response (from student 5880) commingling Newton's third law with Newton's first law:
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