w ' = w+βt & lt； 1 & gt； Substituting w =1200rpm =40π/s, w' = 2700r =90π/s, t= 12s, the angular acceleration β = 25π/s 2 is obtained.
The second question, first find the angular displacement (rotation angle), and then divide it by 2π, that is, the number of revolutions:
According to the angular displacement formula of uniform and variable speed rotation: 2β θ = w' 2-w 2.