Conservation of angular momentum of an an engine flywheel

[henry3369]

Homework Statement

Figure 10.30 shows two disks: an engine flywheel (A) and a clutch plate (B) attached to a transmission shaft. Their moments of inertia are and initially I_A and I_B, they are rotating with constant angular speeds ω_A and ω_B, respectively. We push the disks together with forces acting along the axis, so as not to apply any torque on either disk. The disks rub against each other and eventually reach a common angular speed Derive an expression for ω

Homework Equations

Conservation of angular momentum

The Attempt at a Solution

I'm confused about why angular momentum is conserved here. When the flywheel comes in contact with the clutch, isn't there friction before the two begin moving with the same speed, which is an external force that contributes to the torque of the system; thus, momentum would not be conserved? Is friction ignored because I am only looking at two moments in time: when the clutch and flywheel aren't in contact and after the two are rotating together?

 

[gneill]

The friction is a force that's internal to the system under consideration, so no external torques are involved.

The linear analogy is the perfectly inelastic collision where energy is lost to heat, sound, etc., yet momentum is still conserved.