Rotational momentum

  • #1

Homework Statement

A Texas cockroach of mass 0.17 kg runs counterclockwise around the rim of a lazy Susan (a circular disk mounted on a vertical axle) that has radius 15 cm,rotational inertia 4.9 ✕ 10^−3 kg · m2, and frictionless bearings. The cockroach's speed (relative to the ground) is 2.0 m/s, and the lazy Susan turns clockwise with angular speed ω0 = 3.9 rad/s. The cockroach finds a bread crumb on the rim and, of course, stops.

(a) What is the angular speed of the lazy Susan after the cockroach stops?

Homework Equations

L_i = L_f
L=Iw = mvr

The Attempt at a Solution

L_i = 4.9 ✕ 10^−3 * 3.9 + .17*2.0*.15
L_f = (4.9 ✕ 10^−3)w_f

But when I solve the equations putting L_i = L_f, the w_f I get is wrong. please help!

Answers and Replies

  • #2
The cockroach is running counterclockwise and the table is turning clockwise, yet you added theirangular momentum. Is that right?
And what happened to the mass of the cockroach after it stopped on the turntable?
  • #3
oh right.
are my equations right otherwise?

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