February 1, 2015

Homework Help: To DAMON.. (RE: Phys question u solved yesterday)

Posted by Stacu on Tuesday, December 16, 2008 at 6:19pm.


The tension in the string is the force exerted down by the falling disk and up by whatever is holding the disk. They are equal and opposite. If the disk is a yo-yo, that force is exerted by your finger on the string to give the tension.
Force up on disk = T
Force down on disk = m g
a = acceleration down = (mg -T)/m
Torque on disk = T r
Torque = I alpha
alpha = T r/I
but alpha = a/r by geometry
T r/I = (m g - T)/(m r)
m T r^2 = (m g-T) I
T (m r^2 + I) = m g I
but I = .5 m r^2
T (1.5 m r^2) = .5 m^2 g r^2
T = (.5/1.5) m g = (1/3) m g
now you should be able to do the rest

for the energy part
KE = .5 m v^2 + .5 I omega^2
but omega = v/r


for the part where u have

T r/I = (m g - T)/(m r)..

where did that come from?? (the r in "mr"?

I get that a = mg - T / m

but the r?

Answer this Question

First Name:
School Subject:

Related Questions

physics question - I have a question about a problem regarding Newton's third ...
Physics - what are the forces acting on a string that is wounded around a ...
College Physics - An 0.10 kg object is hanging from your rearview mirror. While...
Science-Enr. Phys. - A box of books weighing 325 N moves at a constant velocity ...
Physics - A guitar string has a tension of 100 N and is supposed to have a ...
physics - A string is wrapped around a uniform disk of mass M = 1.6 kg and ...
mechanics - A string is wound around a uniform disk of radius R and mass M. The ...
physics - THree blocks of masses 3m, 2m and m are connected to strings A,B, and...
phys - A disk with a rotational inertia of 2.0 kg.m^2 and a radius of 0.40 m ...
physics question for damon (a clarification) - Hi Damon! You helped me out in ...