23 Mar, 2012
In a wheel
PRESENTATION: A rotating wheel suspended from a pair of strings can be used to relate momentum of weight with the change in angular momentum. With a single string, the wheel precedes, keeping the angular momentum on its plane of rotation. By changing the string, the direction of spin around the vertical that defines the direction also changes.
- Non-Mathematical Explanation of Precession, John Cordell, Phys. Teach. 49, 572 (2011)
INTRODUCTION: All rotating bodies, including the Earth, have gyroscopic inertia and present resistance to changes in their orientation in order to conserve their angular momentum, which is intrinsic to rotating bodies and depends on the radius (the distance from the edge to the axis of spin), on the mass and the velocity of spin. This effect is present in such common events as a spinning top or a rugby ball that is thrown.
Likewise, rotating bodies present a movement called precession, which is the spin that appears with a particular inclination with respect to the axis perpendicular to the ground, given any force that tends to change the plane of rotation.
OBJECTIVE: To show how a gyroscope tends to conserve angular momentum.
MATERIALS: spoked wheel, axle for the spoked wheel, four metal bars, string.
SETUP: Place two metal bars on the stands, perpendicularly to the horizontal plane. The fourth bar is placed on the bars and one of the ends of the string is attached to its centre. The wheel is hung from the other end, using the axle as an attachment point to leave the wheel in a horizontal position.
EXPLANATION: By placing the axle in a horizontal position and making the now vertical wheel spin, we can observe how the wheel moves and, despite the action of gravity, does not fall. This is due to the spokes pushing its upper part outwards, which creates a component of velocity outwards. The same happens to the lower part, but inwards. In this way, the net result is not a downwards movement but a lateral one. When the wheel begins to spin like this, the movement is called precession. That is, it presents gyroscopic inertia, tending to resist changes in its orientation in order to conserve its angular momentum.
CONCEPTS: angular momentum, precession, nutation.
- R. Serway, Física, Mac Graw Hill, 2010.
- P. Tipler, Física para la Ciencia y la tecnología , Reverté, 2012.
- R. Ehrlich, Turning the World Inside Out and 174 Other Simple Physics Demonstrations, Princeton University Press, 1997.
STUDENTS 2011-2012: Adrián Devesa, Carlos Coroas, María Cotón
LINK pdf STUDENTS (in Spanish):