# Homework Help: Torque; Angular impulse; Systems with zero torque

1. Apr 6, 2009

### jchojnac

1. The problem statement, all variables and given/known data
A uniform solid disk with radius 6 cm has mass 0.8 kg (moment of inertia I = ½MR2). A constant force 12 N is applied as shown. At the instant shown, the angular velocity of the disk is 30 radians/s in the -z direction (where +x is to the right, +y is up, and +z is out of the page, toward you). The length of the string d is 14 cm.

At this instant, what are the magnitude and direction of the angular momentum about the center of the disk?

What are the magnitude and direction of the torque on the disk, about the center of mass of the disk?

The string is pulled for 0.3 s. What are the magnitude and direction of the angular impulse Delta t applied to the disk during this time?

After the torque has been applied for 0.3 s, what are the magnitude and direction of the angular momentum about the center of the disk?

At this later time, what are the magnitude and direction of the angular velocity of the disk?

2. Relevant equations
t = r * Fnet
L = r * p
v = omega * r
T = 2*pi / omega

3. The attempt at a solution
For the first part, I thought it would just be L = r * p...the distance from the location times it's momentum, but it's not. I found the magnitude of the torque so can I just multiply the torque by the time?