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ERIC Number: EJ926919
Record Type: Journal
Publication Date: 2011-Apr
Pages: 2
Abstractor: As Provided
Reference Count: 8
ISSN: ISSN-0031-921X
Can a String's Tension Exert a Torque on a Pulley?
Krause, Dennis E.; Sun, Yifei
Physics Teacher, v49 n4 p234-235 Apr 2011
A typical textbook problem in rotational dynamics involves calculating the angular acceleration of a massive pulley due to a string, such as in the example shown in Fig. 1. The string is assumed to be massless and to move without slipping over the pulley, which is mounted on a frictionless axle. If T[subscript L] and T[subscript R] are the tensions pulling at the left and right edges of the pulley (see Fig. 1), respectively, the net torque on the pulley is then T[subscript net] = (T[subscript L] T[subscript R])R, where "R" is the radius of the pulley. (It is assumed that positive torque corresponds to the counterclockwise direction.) While this analysis, which is typical of what is found in many introductory physics texts, is correct, it should raise several questions in the mind of a student. First, since most texts argue that the tension everywhere in a massless string is constant, why is T[subscript L] [not equal] T[subscript R]? Second, since tension is an internal force (except at the ends of the string, which are obviously not tied to the pulley), how can tension exert a force and torque on a pulley? In this paper, we will address these questions, which are overlooked in most textbook treatments of this problem whose approach appears inconsistent with the concepts presented elsewhere in the text.
American Association of Physics Teachers. One Physics Ellipse, College Park, MD 20740. Tel: 301-209-3300; Fax: 301-209-0845; e-mail:; Web site:
Publication Type: Journal Articles; Reports - Evaluative
Education Level: N/A
Audience: Teachers
Language: English
Sponsor: N/A
Authoring Institution: N/A