Acylinder of mass m and radius r is rotating in a v-shaped groove with a constant angular velocity ω0. the coefficient of friction between the cylinder and the surface is μ. what external torque must be applied to the cylinder to keep it rotating at a constant angular velocity? express your answer using some or all of the following variables: m for the mass of the cylinder, g for the gravitational acceleration, r for the radius of the cylinder and μ for the coefficient of friction.