11.11
Conservation du moment angulaire : Application
Un moment angulaire total d'un système reste constant si le moment extérieur net agissant sur le système est nul. Des exemples de tels systèmes inclue…
Pour tout objet tournant autour d’un axe de rotation, la conservation du moment angulaire est maintenue si aucun couple externe n’agit sur celui-ci.
Par exemple, supposons que le Soleil, ayant une vitesse angulaire de deux virgule six fois dix à la puissance moins six radians par seconde, s’effondre en une naine blanche de sorte que son rayon diminue d’un facteur cinq cents. En supposant que la masse perdue n'emporte pas de moment angulaire, quelle sera l'énergie cinétique de rotation finale de la naine blanche ?
Ici, les quantités connues sont les rayons initiaux et finaux, les masses initiales et finales et la vitesse angulaire du Soleil. La quantité inconnue est l’énergie cinétique de rotation finale de la naine blanche.
Ici, la conservation du moment angulaire se maintient, et en supposant que le Soleil et la naine blanche ont chacun des densités sphériques uniformes, substituant à leur moment d’inertie, la vitesse angulaire finale de la naine blanche peut être calculée.
L’énergie cinétique de rotation de la naine blanche peut être calculée en substituant la valeur de la vitesse angulaire finale.
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Q1: When does conservation of angular momentum apply to a rotating system?
Conservation of angular momentum applies when no external torque acts on a rotating system. A system's total angular momentum remains constant if the net external torque is zero. Examples include freely spinning objects in space or systems where friction and other external forces are negligible, allowing the angular momentum to be preserved.
Q2: How does angular velocity change when a rotating object's moment of inertia decreases?
When a rotating system's moment of inertia decreases, angular velocity must increase to conserve angular momentum. This relationship follows from the conservation principle: if the radius of rotation decreases, the angular velocity increases proportionally. Tornadoes exemplify this—as rotating storm systems contract, their angular velocity increases dramatically.
Q3: What happens to a collapsing star's rotation rate according to angular momentum conservation?
When a star collapses, its radius decreases significantly while its mass remains essentially constant. As the moment of inertia decreases, the star's angular velocity increases substantially to conserve angular momentum. For example, if the Sun collapsed into a white dwarf with radius reduced by a factor of 500, its rotation rate would increase dramatically.
Q4: How can you calculate rotational kinetic energy after angular momentum is conserved?
After determining the final angular velocity using conservation of angular momentum, substitute this value into the rotational kinetic energy formula. For a spherical object with uniform density, calculate the moment of inertia, then apply the kinetic energy equation. This approach connects angular momentum conservation to energy calculations using the work energy theorem for rotational motion.
Q5: Why do astronauts in space maintain zero angular momentum while twisting their bodies?
Astronauts floating inside a spacecraft experience zero external torque when they don't push against the vessel walls. Without external torque, their angular momentum remains conserved at zero. They can twist and reorient their bodies through internal motions, but their total angular momentum relative to the spacecraft stays zero.
Q6: How does the solar system's formation demonstrate angular momentum conservation?
The solar system formed from a large rotating cloud of gas and dust. Gravitational forces caused the cloud to contract, decreasing its radius. As the cloud contracted, its angular velocity increased due to conservation of angular momentum, eventually forming the rotating solar system we observe today.
Q7: What role does moment of inertia play in angular momentum conservation problems?
Moment of inertia determines how angular velocity changes when angular momentum is conserved. For uniform spherical objects, moment of inertia depends on mass and radius. When solving conservation problems, calculate initial and final moments of inertia, then use the conservation equation to find the final angular velocity and subsequent rotational kinetic energy.