Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.63
A rope pulls box 1, such that both box 1 and box 2 both move together to the right at constant speed. All surfaces are not frictionless. Discuss why the friction force of box 1 on box 2 is directed to the right. Explain your reasoning by using free body diagram(s), the properties of friction, forces, and Newton's laws.
Solution and grading rubric:
Correct. Discusses that the (static) friction force of box 1 on box 2 must point to the right either because of (a) Newton's first law, in order for box 2 to move at constant speed to the right, the leftwards kinetic friction force of the floor on box 2 must be countered by a rightwards static friction force of box 1 and box 2; or (b) Newton's third law, as the static friction force of box 2 on box 1 points to the left, opposing box 1 from sliding off of the top surface of box 2, then the static friction force of box 1 on box 2 must point to the right.
As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
Nearly correct, but argument has conceptual errors, or is incomplete. At least some application of Newton's laws in relating direction of static friction force of box 1 on box 2 with another horizontal force.
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least some application of drawing a free body diagram and discussing properties of forces.
Implementation/application of ideas, but credit given for effort rather than merit.
Irrelevant discussion/effectively blank.
Sections 70854, 70855
Exam code: midterm01sWFf
p: 6 students
r: 19 students
t: 15 students
v: 16 students
x: 1 student
y: 0 students
z: 0 students
A sample "p" response (from student 8921), using Newton's first law:
Another sample "p" response (from student 8821), using Newton's third law:
Another sample "p" response (from student 6377), drawing free body diagrams for both boxes, and establishing both Newton's first and third laws for the horizontal forces on the boxes: