I'm sure there is something ridiculously simple I'm missing here but this questions has been driving me crazy and now I'm sure I couldn't see the answer if it stared me right in the face.(adsbygoogle = window.adsbygoogle || []).push({});

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

A massless string is wrapped around a solid cylinder. A block of mass m=2.0kg hangs from the string. When released the block falls a distance of 82 cm in 2.0s. Starting with a freebody diagram calculate the mass of the cylinder.

*I can't use conservation of energy to solve this as the next question indicates that I am to solve the question again using conservation of mechanical energy.

2. Relevant equations

y = y_{0}+ v_{0}t + 1/2at^{2}

F=ma

torsion = rF

Net Torsion = Moment of Inertia x rotational acceleration

3. The attempt at a solution

r = ? **This is the part that has me stumped - all of the formulae I know need a radius.

m = 2.0kg

y = 0.82m

t = 2.0s

v_{0}= 0m/s

m_{c}= ?

Step One: Determine the force of inertia of the cylinder:

y = y_{0}+ v_{0}t + 1/2at^{2}

a=(2y)/(t^{2})

a = (2 x 0.82m) / (2.0s^{2})

a = 0.41 m/s^{2}

F_{a}= ma = 0.41m/s^{2}x 2.0kg = 0.82N

F_{g}= mg = 9.81m/s^{2}x 2.0kg = 19.62N

F_{I}= F_{g}- F_{a}= 19.62N - 0.82N = 18.8N

This is as far as I can get before I start running into brick walls. I've played with several different formulae but I always run into a point where I need the radius of the cylinder (which I don't have). Am I missing something really obvious here?

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# Homework Help: Rotational Dynamics Help Needed

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