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ERIC Number: EJ1024875
Record Type: Journal
Publication Date: 2013-Oct
Pages: 4
Abstractor: As Provided
ISSN: ISSN-0031-921X
A Progression of Static Equilibrium Laboratory Exercises
Kutzner, Mickey; Kutzner, Andrew
Physics Teacher, v51 n7 p430-433 Oct 2013
Although simple architectural structures like bridges, catwalks, cantilevers, and Stonehenge have been integral in human societies for millennia, as have levers and other simple tools, modern students of introductory physics continue to grapple with Newton's conditions for static equilibrium. As formulated in typical introductory physics textbooks, these two conditions appear as SF=0 (1) and St=0, (2) where each torque t is defined as the cross product between the lever arm vector r and the corresponding applied force F, t=r×F, (3) having magnitude, t=Frsin?. (4) The angle ? here is between the two vectors F and r. In Eq. (1), upward (downward) forces are considered positive (negative). In Eq. (2), counterclockwise (clockwise) torques are considered positive (negative). Equation (1) holds that the vector sum of the external forces acting on an object must be zero to prevent linear accelerations; Eq. (2) states that the vector sum of torques due to "external forces about any axis" must be zero to prevent angular accelerations. In our view these conditions can be problematic for students because a) the equations contain the unfamiliar summation notation S, b) students are uncertain of the role of torques in causing rotations, and c) it is not clear why the sum of torques is zero regardless of the choice of axis. Gianino describes an experiment using MBL and a force sensor to convey the meaning of torque as applied to a rigid-body lever system without exploring quantitative aspects of the conditions for static equilibrium.
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 - Descriptive
Education Level: N/A
Audience: N/A
Language: English
Sponsor: N/A
Authoring Institution: N/A