Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 4.76
The coefficient of static friction between a block and a horizontal table is 0.35, while the coefficient of kinetic friction is 0.22. The mass of the block is 2.00 kg. A horizontal force is applied to the block and slowly increased, until the moment it starts to slide, in which case the horizontal force has a constant magnitude. Determine the magnitude of the acceleration of the block after it starts to slide. Show your work and explain your reasoning using a free-body diagram, the properties of forces, and Newton's laws.
Solution and grading rubric:
- p:
Correct. At the moment the block begins to slide, the applied force has the same magnitude as the static friction force. After the block has begun to slide, it is subject to both the applied force, and the force of kinetic friction, and uses both forces in Newton's second law to determine the magnitude of the acceleration. - r:
Nearly correct, but includes minor math errors. - t:
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Has zero kinetic friction, or infers that "constant magnitude" means zero acceleration, or some garbled attempt at calculating fs, fk, ΣF using a free-body diagram and Newton's laws. - v:
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Typically attempts to apply energy conservation. - x:
Implementation of ideas, but credit given for effort rather than merit. - y:
Irrelevant discussion/effectively blank. - z:
Blank.
Grading distribution:
Sections 70854, 70855
p: 9 students
r: 1 student
t: 21 students
v: 8 students
x: 0 students
y: 0 students
z: 0 students
A sample of a "p" response (from student 1123):
Another "p" response sample (from student 5215):
A sample of an "r" response (from student 1567):
And a sample of a "t" response (from student 1863):
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