# Homework Help: Masses and pulleys?

1. Dec 12, 2004

### hannibal

I have a question where theres two masses attached to a pully that has a radius of 5 cm. one mass is heavier than the other one and the first part of the question is asking you to find the acceleration of the mass thats falling (it falls 75 cm in 5 sec). Now the way that i figured it should be done is by using the tension formulas (Mg - T = Ma for the mass falling) however this give the wrong answer, the equation you have to use is Y = Yo +(1/2)at^2. I can;t seem to figure out why you can't use the first method to figure this one out.

2. Dec 13, 2004

### jdstokes

To use Newton's second law for such a problem you would have to consider the moment of inertia of the pulley. However, since the acceleration is uniform and you are given the time interval and the displacement, you can just use plain old kinematics $\Delta s = at^2/2$ and solve for a.

3. Dec 13, 2004

### hannibal

Ok I know what the moment of inertia is, however my phys prof had to skip over the last chapter in the book due to time constraints. If there was no moment of inertia then the tension/force method would work right?