A thin disk rotates about its fixed central axis. The graph of its angular momentum L (in kg·m²/s) versus time t (in s) can be described verbally as follows.
• From t = 0 s to t = 1.5 s, L increases linearly from 0 to 6 kg·m²/s.
• From t = 1.5 s to t = 4.0 s, L remains constant at 6 kg·m²/s.
• From t = 4.0 s to t = 6.0 s, L decreases linearly back to 0 kg·m²/s.
• From t = 6.0 s to t = 8.0 s, L decreases linearly to −4 kg·m²/s.
Answer the following questions about the external torque acting on the disk.
(a) For each of the four time intervals listed, state whether the net external torque is positive, zero, or negative.
(b) Rank the magnitudes of the net external torque during the four intervals from greatest to least. If any are equal in magnitude, indicate that clearly.
(c) Provide qualitative reasoning that supports your answers in parts (a) and (b) based solely on features of the verbal L–t description.