Physics quiz question: accelerating crate pulled by falling block

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

A 2.0 kg crate is attached to a block (of unknown mass) by use of an ideal rope and pulley. As a result, the crate slides across the table to the right with an acceleration of 1.2 m/s² (while the block descends downwards). The table is not frictionless, and exerts a kinetic friction force of 5.9 N on the crate as it slides. Newton's __________ law tells you that these two forces are equal in magnitude and opposite in direction:

Tension force of block on the crate.
Kinetic friction force of table on the crate.

(A) first.
(B) second.
(C) third.
(D) (These forces are not equal in magnitude and/or opposite in direction.)

Correct answer (highlight to unhide): (D)

The crate has two vertical forces acting on it:

Normal force of table on the crate (upwards).
Weight force of Earth on the crate (m_box·g = 4.9 N, downwards).

Because the crate is stationary in the vertical direction, these two forces are equal in magnitude and opposite in direction, due to Newton's first law.

The crate has two horizontal forces acting on it:

Tension force of block on the crate (to the right).
Kinetic friction force of table on the crate (to the left).

Since the crate is accelerating at 1.2 m/s² to the right, then according to Newton's second law the forces acting on it cannot sum to a net force of zero, and the tension force must have a larger magnitude than the kinetic friction force. Thus the tension force and the kinetic friction force, while opposite in direction, are not equal in magnitude.

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 2 students
(B) : 10 students
(C) : 3 students
(D) : 39 students

Success level: 72%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.52