16 Nov, 2012
PRESENTATION: A circular equilibrium board lies on a vertical axis, and it has three or four strings attached to its center. The angle between the strings may be changed, and different masses may be suspended from each of them. The equilibrium of the system is obtained when the addition of all forces vanishes, illustrating their vectorial nature and the first Newton law.
- An Equilibrium Board to Demonstrate Both Torques and Translational Forces, Mark Kinsler and Evelyn Kinzel, Phys. Teach. 46, 502 (2008)
- Getting the feel for vector addition of forces, Christopher Bracikowski, P. Joseph Garcia, and David J. Harper, Phys. Teach. 36, 114 (1998)
INTRODUCTION: According to the first Newton law, any body standing still, or moving in a rectilinear trajectory at constant speed keeps the same state unless an external force acts on it, or alternatively a non zero addition of external forces. There are many examples of this: a person standing on a bus tries to keep moving forward when the bus decelerates, a standing table does not move unless any force is applied on it, etc.
OBJECTIVE: To bring an object to an equilibrium position by applying a number of forces that cancel each other.
EXPLANATION: By applying forces (different weights) on the disk border at well-selected positions, the addition of all forces will be zero and the disc will stay in equilibrium on its axe.
MATERIAL: a plastic disk of diameter 200 mm., a piece of string of 480 mm of length, an adhesive circular plastic pattern with an angular degree division, 100 mm long bolts, screws and washers, three small wooden pieces.
SETUP: The adhesive plastic pattern with the degree scale must be glued onto the plastic disk. A hole must be punched at the center of the disk, where the three strings will be attached. Using the bolts and screws, different masses (the wooden pieces) can be attached to the free ends of the strings, and the equilibrium position may be attached by equilibrating the forces exerted by each of the hanging masses.
CONCEPTS: vector sum, weight, equilibrium.
- Tipler P.A. Física. Barcelona: Reverté, 2010.
- De Juana J.M., Física General, Pearson, 2009.
- Serway R.A y J.W.Jewett. Física, Thomson-Paraninfo, 2010.
- R. Ehrlich, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, Princeton University Press, 1997.
STUDENTS 2010-2011: Brais Freiría, Miguel García, Brais García
LINK pdf STUDENTS (in Spanish):