# Rotational motion, using torque to find power

1. Oct 26, 2009

### ba726

1. The problem statement, all variables and given/known data
An electric motor can accelerate a Ferris wheel of moment of inertia 19200kgm2 from rest to 9.11 rev/min in 10.7s. When the motor is turned off, friction causes the wheel to slow down from 9.11 rev/min to 7.36 rev/min in 9.55s. Determine the torque generated by the motor to bring the wheel to 9.11 rev/min. Answer in units of Nm. Also, determine the power needed to maintain the rotational speed at 9.11 rev/min. Answer in units of W.

2. Relevant equations

torquenet= Inertia x angular acceleration
angular velocity (final)= angular velocity (initial) + ang accel x time
Power=torque x ang velocity

3. The attempt at a solution

I already found part 1, the torque generated by the motor (2080.257403 Nm) and I'm completely stuck on part 2. I don't know if I'm doing it wrong or what. I originally used the torque from the motor times the ang velocity converted to radians/s from rev/min. I've used net torque times the same ang velocity. I forgot to convert the 9.11 ang velocity so I've used both torques with ang velocity in rev/min. What am I doing wrong?!?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 27, 2009

### Andrew Mason

You do not use the torque generated by the motor in accelerating the wheel to solve the second part. You have to determine how much power is needed to keep the wheel going. That is a function of the rate of energy loss due to friction on the wheel.

Use:

$$\Delta L = I\Delta \omega$$

$$\tau = \frac{\Delta L}{\Delta t}$$

$$\Delta E = \tau \theta$$

AM