A small traffic light of negligible air resistance hangs motionless from the ceiling…
PHYSICS · Multi-part · AP · Physics 1: Algebra-based · Force and Translational Dynamics · Force and Translational Dynamics
Problem
A small traffic light of negligible air resistance hangs motionless from the ceiling of an underground parking garage by a single light, inextensible cable that makes an angle of 12° from the vertical due to a steady cross-draft of air. A student sketches the following five labeled force vectors on a free-body diagram (FBD) for the traffic light:
• Gravity, F1, directed straight downward.
• Tension, F2, directed along the cable toward the ceiling.
• "Buoyant force of air," F3, directed straight upward.
• "Wind force," F4, directed horizontally toward the east.
• "Push of the cable on the light," F5, directed along the cable toward the light.
The student claims that because five forces are drawn, the object may or may not be in equilibrium; further analysis is required.
Respond to the prompts below in the order given.
(a) For the physical situation described, list all forces that actually act on the traffic light as an isolated object. Identify each as external or internal to the chosen object.
(b) Compare your list from part (a) to the five forces F1–F5 drawn by the student. For each of the student’s labeled forces, state whether it is correctly included, should be omitted, or is incorrectly represented. Provide a brief justification in each case.
(c) The student omitted the magnitudes of the forces. Explain qualitatively whether the directions of the correctly included forces are sufficient to establish whether the traffic light is in translational equilibrium. If additional information is required, state what is needed and why.
(d) The student argues that the presence of a horizontal "wind force" automatically prevents equilibrium. Using Newton’s first law, articulate a clear argument that either supports or refutes the student’s claim.