A puck of mass 80.0 g and radius 3.70 cm slides along an air table at a speed of

Question

A puck of mass 80.0 g and radius 3.70 cm slides along an air table at a speed of 1.50 m/s as shown in the figure below. It makes a glancing collision with a second puck of radius 6.00 cm and mass 115.0 g (initially at rest) such that their rims just touch. Because their rims are coated with instant-acting glue, the pucks stick together and spin after the collision.


(a) What is the angular momentum of the system relative to the center of mass?


(b) What is the angular speed about the center of mass?

Explanation / Answer

The center of mass of the system is most definitely moving! If both pucks were stationary, then the center of mass would not move. But since the center of mass of the system depends on the position of both pucks, as one puck moves the center of mass also moves. Big hint: Horizontal position of center of mass: M_{total} x_{cm} = m_1 x_1 + m_2 x_2 Horizontal speed of center of mass: M_{total} v_{cm} = m_1 v_1 + m_2 v_2

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