# Homework Help: Torque generated by the motor of ferris wheel?

1. Oct 25, 2009

### jeneekim

1. The problem statement, all variables and given/known data
An electric motor can accelerate a Ferris
wheel of moment of inertia 11300 kg · m2 from
rest to 9.6 rev/min in 14.6 s. When the mo-
tor is turned off, friction causes the wheel to
slow down from 9.6 rev/min to 8.37 rev/min
in 7.23 s.
Determine the torque generated by the mo-
tor to bring the wheel to 9.6 rev/min.
Answer in units of N · m.

2. Relevant equations

$$\alpha$$ = $$\omega$$/t
$$\tau$$ = I$$\alpha$$

3. The attempt at a solution

given:
I = 11300 kgm2
$$\omega$$o = 0
$$\omega$$f = 9.6 rev/min = 1.005309649 rad/s
tf = 14.6 s

The question asks to determine the torque generated by the motor to bring the wheel to 9.6 rev/min so I think that means I don't have to worry about the motor beign turned off or the friction to slow it down... sooooo...

$$\alpha$$ = $$\omega$$/t

$$\tau$$ = I$$\alpha$$
= 778.0821257 Nm

When I enter this answer, it says it is wrong? What am I doing wrong here?

Thank you in advance for your help!

Last edited: Oct 25, 2009
2. Oct 26, 2009

### rl.bhat

In the first case
T = I*αo + I*αr
where I*αr is the torque required to overcome the friction and I*αo is the torque need to accelerate the wheel after overcoming the frictional force.
In the second case
Tf = I*αr.