# Physics torque question?

A disk of mass 6 kg and radius 25 cm is rotating around horizontal axis that goes through its center with n=360 revolutions/min. When the disk was being stopped it stopped right after t=30 s. Calculate the stopping torque exerted on flywheel and number of revolutions N.

Apologize for poor english

### 1 Answer

- Steve4PhysicsLv 71 month agoFavourite answer
Moment of inertia I = ½mr² = ½*6*0.25² = 0.1875 kg.m²

Initial angular vel.: ω = 360 rev/min = 6rev/s = 6*2π rad/s = 37.7 rad/s

Angular accel: α = Δω/Δt = (0-37.7)/30 = -1.257 rad/s²

Torque: τ = Iα = 0.1875 * (-1.257) = -0.236 N.m

(The negative sign indicates torque acts in opposite direction to intial angular velocity).

There are various ways to find the number of revs. Simplest is to use average speed (in rev/s) and time(in seconds):

N = (average angular speed) * time

. . = ((6 + 0)/2) * 30 = 90