Center of mass
21 Feb, 2013
Up or down?
PRESENTATION: A v-shape ramp presents the wider end higher than the narrow end. A double cone is placed at the lower end of the ramp and its centre of mass performs a descending movement, surprisingly making the double cone ascend the ramp.
- Unbalanced, Ed van den Berg, Phys. Teach. 39, 202 (2001)
INTRODUCTION: The center of mass of an object is the point where one may consider its whole mass is concentrated, if a uniform gravitational field is supposed. The movement of any body may be predicted by observing the displacement of its center of mass.
OBJECTIVE: To demonstrate that this principle is universal to any object, even if it does not seem evident at first sight.
MATERIALS: cardboard, a pair of scissors, a cutter, glue.
SETUP: Cut and build the necessary elements (ramp, double cone, cylinder) with cardboard. The angle of the ramp should be 5º, and it should be placed on a horizontal surface. Place the objects on the ramp and observe what happens. While the cylinder goes down, the double cylinder goes up.
EXPLANATION: On the higher end of the ramp we leave the cylinder, observing that it goes down, because of the action of its weight, perpendicular to the horizontal plane, which forces the center of mass to descend. Then we place the double cone at the lower end of the ramp, observing that it turns to go up up the ramp. This is also due to the weight of the object that forces the center of mass to descend again. With this configuration, the center of mass, coincident with the center of geometry, comes closer to the horizontal plane as the cones ascend the ramp.
CONCEPTS: center of mass definition, gravity, rotation and translation movement, forces acting on a body.
- J. Wilson, A. Bufa, Física, Pearson, 2003.
- J.L. Merian, L.G. Kraige, Mecánica para ingenieros, Reverté, 2004.
- R. Serway, J. Jewett, Física para ciencias e ingeniería, Thomson, 2005.
STUDENTS 2012-2013: Natalia Cid, Alba Conde, Sandra Correa, Charles De Araújo
LINK pdf STUDENTS (in Spanish):