Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 8.36
[20 points.] A meter stick pivoted at one end is held horizontally by a string attached to the other end of the meter stick, such that it pulls at an angle of 30.0° with respect to the normal. If the mass of the meter stick is 0.200 kg, what are the magnitudes and directions of the horizontal and vertical forces F_x and F_y exerted on the meter stick by the pivot? Show your work and explain your reasoning. Solution and grading rubric:
- p = 20/20:
Correct. Applies Newton's first law for rotations, such that the sum of the torque of the weight of the meter stick and the torque of the string is set to zero. Solves for the string tension, coincidentally is m*g* = 1.96 N. Then applies Newton's first law for both x- and y- directions, to find magnitudes and directions of F_x and F_y. - r = 16/20:
Nearly correct, but includes minor math errors. Some systematic application of Newton's first law for rotations and x- and y- directions. - t = 12/20:
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Applied Newton's first law to either rotations, or to x- and y- directions only. - v = 8/20:
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. May solve for F_x and F_y by exploiting F_x and F_y being x- and y- components of a vector with magnitude = m*g, with no insight as to how this was possible with the given parameters. - x = 4/20:
Implementation of ideas, but credit given for effort rather than merit. - y = 2/20:
Irrelevant discussion/effectively blank. - z = 0/20:
Blank.
Grading distribution:
Sections 70854, 70855
p: 1 student
r: 5 students
t: 8 students
v: 23 students
x: 2 students
y: 0 students
z: 0 students
The sole "p" response (from student 0125):
A sample "v" response (from student 1120):
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