
Answer:
(a) ωf = 1200 rad/s
(b) t  = 25 s
Explanation:
Kinematic Equation to the rotor
ωf= ω₀ + α*t  Formula (1)
Where:
α : angular acceleration (rad/s²)
t : time interval (s)
ω₀ : initial angular velocity ( rad/s)
ωf : final angular velocity  ( rad/s)
(a) Angular velocity after 10.0 s
Data
α = -80 rad/s²
t = 10 s
ω₀ = 2000 rad/s
We apply the formula (1):
ωf = ω₀ - α*t
ωf = 2000 -  80*(10)
ωf = 2000 - 800
ωf = 1200 rad/s
(b) Time the rotor stops
Data
α = -80 rad/s²
ω₀ = 2000 rad/s
ωf = 0
We apply the formula (1):
ωf = ω₀ - α*t
0 = 2000 - Â 80*(t)
80*(t) = 2000
t = 2000 / 80
t  = 25 s