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Two skaters, each of mass 85 kg, approach each other along parallel paths separated by 5.5 m. They have equal and opposite velocities of 1.9 m/s. The first skater carries one end of a long pole with negligible mass, and the second skater grabs the other end of it as she passes; see the figure. Assume frictionless ice. Describe quantitatively the motion of the skaters after they have become connected by the pole. What is their angular speed?

Question

Two skaters, each of mass 85 kg, approach each other along parallel paths separated by 5.5 m. They have equal and opposite velocities of 1.9 m/s. The first skater carries one end of a long pole with negligible mass, and the second skater grabs the other end of it as she passes; see the figure. Assume frictionless ice. Describe quantitatively the motion of the skaters after they have become connected by the pole. What is their angular speed?

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Solution

Primero, consideremos el sistema de los dos patinadores y el poste. Dado que el poste tiene una masa despreciable, no afectará significativamente el movimiento del sistema. Los patinadores se mueven en direcciones opuestas con velocidades iguales y opuestas de 1.9 m/s.

Cuando los patinadores se agarran del poste, comenzarán a girar alrededor del centro de masa del sistema. Debido a que ambos patinadores tienen la misma masa (85 kg) y están separados por una distancia de 5.5 m, el centro de masa estará en el punto medio del poste, es decir, a 2.75 m de cada patinador.

Para encontrar la velocidad angular del sistema, utilizamos la conservación del momento angular. El momento angular inicial del sistema es la suma de los momentos angulares de cada patinador respecto al centro de masa.

El momento angular L L de un patinador respecto al centro de masa es dado por: L=mvr L = m \cdot v \cdot r donde:

  • m m es la masa del patinador (85 kg),
  • v v es la velocidad del patinador (1.9 m/s),
  • r r es la distancia del patinador al centro de masa (2.75 m).

Para un patinador: L1=85kg1.9m/s2.75m L_1 = 85 \, \text{kg} \cdot 1.9 \, \text{m/s} \cdot 2.75 \, \text{m}

Calculamos L1 L_1 : L1=851.92.75 L_1 = 85 \cdot 1.9 \cdot 2.75 L1=444.125kgm2/s L_1 = 444.125 \, \text{kg} \cdot \text{m}^2/\text{s}

Dado que los patinadores tienen momentos angulares iguales y opuestos, el momento angular total del sistema es: Ltotal=L1+(L1)=0 L_{\text{total}} = L_1 + (-L_1) = 0

Sin embargo, cuando se agarran del poste, el sistema comienza a girar alrededor del centro de masa. La conservación del momento angular implica que el momento angular total del sistema debe ser igual al momento angular inicial.

El momento de inercia I I del sistema respecto al centro de masa es: I=2mr2 I = 2 \cdot m \cdot r^2 donde:

  • m m es la masa de un patinador (85 kg),
  • r r es la distancia de un patinador al centro de masa (2.75 m).

Calculamos I I : I=285(2.75)2 I = 2 \cdot 85 \cdot (2.75)^2 [ I

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