06 May, 2012
Bid and small
PRESENTATION: A small sphere and a big sphere are linked by a string that goes through a plastic tube. If the tube is placed horizontally, the different masses of the spheres are made clear. If the small sphere makes a horizontal rotation with enough angular speed the tension of the string is enough to keep the larger sphere in equilibrium. The effect of the angular speed on the equilibrium and that of the radius of rotation on the angular speed may be analysed.
- Simple experimental setup to consolidate the angular momentum concept, M. Boutinguiza and F. Lusquiños, Phys. Teach. 49, 456 (2011)
- Angular momentum conservation with a twist, Paul Hewitt, Phys. Teach. 47, 6 (2009)
INTRODUCTION: The angular momentum or kinetic momentum is a physical magnitude of great importance in Mechanics, as it is related to rotational symmetries of physical systems. Under certain conditions of rotational symmetry, this magnitude is conserved. This happens for instance in the planetary orbital motion around the Sun.
OBJECTIVE: To show the Law of conservation of angular momentum.
MATERIALS: two metallic balls, the mass of one of them should be more or less three times larger than the other, a string, a plastic tube, a chronometer.
SETUP: One ball is attached to the string, which will pass through the plastic tube and then attached the second ball.
EXPLANATION: If friction and gravity is neglected, the tension that the string exerts on the rotating ball is radial and produces no momentum on it. If we pull the string the radius of rotation will decrease. If no external momentum is present, the angular momentum is conserved and the linear speed of the rotating sphere should increase, so it can be checked that v1R1 =v2R2 and, multiplying by the sphere mass, we obtain the angular momentum conservation::
CONCEPTS: angular momentum conservation law, centripetal acceleration, uniform circular movement, angular and tangential speed, period, frequency.
- 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: Raquel López, Daniel López, Mar López
LINK pdf STUDENTS (in Spanish):