Momentum
28 Apr, 2012
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PDFWith a can
PRESENTATION: A magnet is fixed inside a cylindrical can, in a position displaced from the geometric center. From an equilibrium position, an accelerated rotation starts either in a clockwise or counter-clockwise sense, depending of the position of the additional mass. The torque or momentum that produces this rotation can even make the can climb up a certain slope.
- Battle of the Merry-Go-Rounds: An Angular Momentum Demonstration, Stephen J. Van Hook, Phys. Teach. 44, 304 (2006)
- Conservation of angular momentum apparatus using magnetic bearings, Michael J. Pechan, Alexandra O’Brien, and Wesley A. Burgei, Phys. Teach. 39, 26 (2001)
INTRODUCTION: The center of gravity (CG) is the point where all the weight of a rigid body is supposed to act. Its position is coincident with the center of mass (CM) if the gravity acceleration is the same for all points of the system. In equilibrium (∑F=0, ∑M=0), the vertical of the center of mass must cross the standing point of the object. If not, the center of mass will descend, making the body roll until the center of mass is placed on the vertical of the standing point.
OBJECTIVE: To study the behaviour of the rolling movement of the unbalanced cylindrical can on an inclined surface, depending of the initial orientation and critical angle.
MATERIALS: a cylindrical can, a magnet, a wooden board and a stand, a towel.
SETUP: Use the wooden board and the stand to create different slopes. Cover the board with a towel to increase friction. Fix the magnet in the inside wall of the can. The critical angle can be tuned by changing the slope of the board.
EXPLANATION: The can climbs up the slope because the gravity center is not placed in the vertical of the standing point. Although the whole object ascends, the CG is descending so that this reaches the vertical of the standing point. If we increase the angle of the ramp, the can will not eventually be able to ascend it.
CONCEPTS: center of gravity, center of mass, equilibrium, stability of rigid objects, inertia momentum, torque of a force, friction.
MORE INFORMATION:
TEXTS:
- J.M. De Juana, Física general. Pearson, 2005.
- R. Ehrlich, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, Princeton University Press, 1997.
STUDENTS 2011-2012: Alejandro Cangas, Gabriel Carballido, Guido Cerqueira.
LINK pdf STUDENTS (in Spanish):
STUDENTS 2010-2011: Marcos Basdediós, Alfredo Iglesias, Antonio Larramendi, Ignacio Álava
LINK pdf STUDENTS (in Spanish):