Physics quiz question: suitcase with diagonally applied force

Physics 205A Quiz 3, fall semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Example 4.15

A 16 kg suitcase is pushed by an applied force of 210 N directed at angle of 15° below the horizontal, along a frictionless floor. The __________ has the greater magnitude.
(A) Normal force of the floor on the suitcase.
(B) Weight of the suitcase.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

The suitcase has three vertical forces acting on it:

Weight force of Earth on suitcase (downwards, magnitude w = m·g = 160 N).
Normal force of the floor on suitcase (upwards, magnitude N_floor = ?).
Vertical component of the applied force pushing on suitcase (downwards, magnitude F_applied = (210 N)·sin(15°) = 54 N).

Because the book is stationary in the vertical direction, the forces must sum to zero along the vertical direction due to Newton's first law, such that the upwards normal force of the floor on the suitcase must be equal to the two downward forces on the suitcase:

N_floor = 160 N + 54 N = 214 N.

Sections 70854, 70855
Exam code: quiz03Fn37
(A) : 18 students
(B) : 5 students
(C) : 31 students
(D) : 0 students

Success level: 33%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.28