Masses Over a Uniform Cylindrical Pulley

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1. Oct 28, 2016

Cdh0127

ηϖ1. The problem statement, all variables and given/known data

2. Relevant equations
I=½MR2
PE=mgh

3. The attempt at a solution

The first thing that jumped out at me was "uniform cylinder" so I went ahead and calculated the moment of inertia for the cylinder and got I=½(4.4)(.4)2 = .352 and held onto that.

Then, I calculated the forces due to gravity of each mass that is pulling down on the string.
Fmb = 48kg×9.8m/s2 = 470.4N
I'm not sure if I do the same for ma because it's resting on the table, so is there a force pulling on the string creating tension?

But the next thing I did was find the gravitational potential energy of mb:
PE = mgh = 48kg×9.8m/s2×2.5m = 1176 J.

I'm not sure if torque is needed, but I went ahead and calculated it anyways:

T=F×r = 470.4N×.4m = 188.16

And that is all I can think to do. I'm not sure how the radius, inertia, mass, and other properties of the pulley affect the masses A and B that move up and down via the string over that pulley.

Could I take the 1176 J of potential energy and set it equal to ½mv2? But what mass would I use? Mass of the system (A + B)? Solve for V? That seems too simple for this section, because we are learning about angular kinematics, torque...etc. I think I'm missing something.

2. Oct 28, 2016

BvU

Hi there,

The hint in the exercise is pretty clear: use energy considerations. So list off all the energies (my clue: there's more than you mentioned so far, but you are thinking in the right direction) at t=0 at at t = bump.

Extra tip: does the $\bf I$ you calculated have any influence ? (Imagine a huge R0 to decide)