Examples on Angular Momentum
1. A disk of moment of inertia 𝐼 = 0.015 𝑘𝑔. 𝑚2 is rotating at 3 rev/s. A trickle of sand falls onto the
disk at 20 cm from the axis and builds a 20 cm radius ring of sand on it. How much sand must fall on
the disk for it to slow to 2 rev/s?
before trickle of sand after trickle of sand
Hint: Note that moment of inertia of a ring of radius r is given by I = mr2.
Answer: 0.19 kg of sand.
2. Conceptual question: In ice skating, a skater spins slowly with both arms and one leg outstretched,
but when she pulls her arms and leg in toward the rotational axis, she spins faster. Why? Explain.
