# Homework Help: Angular Momentum and Torque

1. Jun 28, 2011

### falco451

1. The problem statement, all variables and given/known data

I was just doing some extra credit problems and couldn't figure this one out.
I'd appreciate any help! Thanks!

A mass of 5.6 kg is attached to a light cord, which is wound around a pulley. The pulley is a uniform solid cylinder of radius 7.3 cm and mass 2.7 kg.

1 - What is the magnitude of the net torque on the system about the point on the axis at the center of the pulley? ( in N m)

2 - When the mass has a speed of v = 49 m/s, the pulley has an angular speed of w = v/R
Determine the magnitude of the total angular momentum of the system about a point on the axis at the center of the pulley (kg m^2/s)

3 - using the fact that avg torque = dL/ (dt) and your result from the previous section, calculate the magnitude of the acceleration of the mass. (m/s^2)

2. Relevant equations

3. The attempt at a solution

I tried using torque = inertia x angular acceleration
(mr^2)
I also tried
F = mg - ma = 5.6*9.81 - 5.6*a
so τ = F*R = (mg - ma)*R
I = 1/2*mR^2 = 1/2*2.7*0..073^2 = 0.007194 kgm^2 but got stuck and this didn't seem right.

I am just really confused. Thanks for all your help

2. Jun 28, 2011

### Staff: Mentor

What forces act on the system? What torque do they exert about the given axis?