# Homework Help: Spinning disk torque?

1. Apr 7, 2009

### fball558

spinning disk torque???

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

A uniform solid disk with radius 10 cm has mass 0.7 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 40 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 15 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?

3. The attempt at a solution

i know both of these have a -z direction. for finding the angular momentum i used
(MR^2/2)W (.7*.1^2/2) * (2(pi)/40) = 5.498e-4
then to find torque i used RF where F is tension force got this by (M*F) = (.7*12) = 8.4
then R*(8.4) = .1*8.4 = .84
what did i do wrong??

2. Apr 7, 2009

### mrudkin

Re: spinning disk torque???

Do NOT multiply by the mass (.5kg). So, in short, multiply your final answer by 2 to get the correct one.

3. Apr 7, 2009

### fball558

Re: spinning disk torque???

where did you get the mass of .5kg?? the mass in the problem is .7kg

4. Apr 7, 2009

5. Apr 7, 2009

### fball558

Re: spinning disk torque???

i did that get the same answer as before. says im wrong

here is what i did

L(rot) = Iw
I = 1/2MR^2 so 1/2*.7*.1^2 = .0035
w = 2(pi)/T where T = 40 so w = .1570796
Iw = 5.497787e-4
do you see anything wrong with this??

6. Apr 7, 2009

### sriceb01

Re: spinning disk torque???

alrite for part a) L(rot) = Iw
I=.5*.7*.1^2=.0035
L=.0035*40=.14

7. Apr 7, 2009

### fball558

Re: spinning disk torque???

OH.. ok
what i was doing was taking the 40rad/s
and using w = 2(pi)/T
so 2(pi)/40
then solving for that.

8. Apr 7, 2009

### sriceb01

Re: spinning disk torque???

part b)

torque=abs(R)*abs(F)*sin(90)
=R*F*1=.1*12=1.2

should work

9. Apr 7, 2009

### sriceb01

Re: spinning disk torque???

Common mistake

10. Apr 7, 2009

### jchojnac

Re: spinning disk torque???

for part d.) isn't it Lf-Li where Li is part a.) and Lf is I * omega with the given time?

11. Apr 7, 2009

### sriceb01

Re: spinning disk torque???

for part d) you multiply torque calculated in part c) by the change in time