# 4 rotation problems

1. Dec 11, 2006

### hannibalisfun

1. The problem statement, all variables and given/known data
1. A 8 cm radius disk with a rotational inertia of .12 kg * M(squared) is free to rotate on a horizontal axis. A string is fastened to the surface of the disk and a 10 kg mass hangs from the other end. The mass is raised by using a crank to apply a 9 N * m torque to the disk. The acceleration of the mass is:

2. A small disk of radius R1 is mounted coaxially with a larger disk of radius R2. The disks are securely fastened to each other and the combination is free to rotate on a fixed axle that is perpendicular to a horizontal frictionless table top. The rotational inertia of the combination is l. a string is wrapped around the larger disk and attached to a block of mass M, on the table. Another string is wrapped around the smaller disk and is pulled with a force F, parallel to the table. The acceleration of the block is:

3. A block of mass m is tied to a light cord that is wrapped around a spoked pulley which has most of its mass Mp, around its rim and which has a radius R. The other end of the cord is attached to a block of mass M resting on a rough horizontal surface. There is no friction in the pulley. Find an expression for the acceleration of the block and the tension in the cord in terms of m, M, Mp, R, ad g as needed.

4. i know that the constant angular acceleration, 5, the intial angular volocity, 45, the angular distance it travel, 360 rad, and the time it took to travel that, 6 secs. i need to find the work it does.

2. Relevant equations

3. The attempt at a solution

1. the way i tried to solve it was i set up this equation and tried to solve for T. (9, torque from the crank, - T * .08, radius in meters,) = .12, rotiational intertia, * A, acceleration of the mass,/.08, radius in meters. then i subsituted the T value i got for T into T - m * g= m * A

2. i not sure how to do this one but we did this one in class and i think the answer might be ( R1 * R2 * F)/(1 + (M * R1 * R2))

3. I know I = Mp * R * R. I could work this problem if there was not a second mass because i would just solve for -T * R = Mp * R * A. But with the second mass i'm not sure how you take that into account.

4. i have no clue how to work this one because all the equation i have need rotational intertia to find the work.

2. Dec 11, 2006

### OlderDan

Posting multiple problems in one thread often leads to confusion as people respond to different parts of the set. Please post your problems separately unless the solutions to one problem is used in another.

Paraphrasing, as you have done in number 4 usually obscures the problem you are trying to solve. I have only a cloudy idea what the "it" is in "i need to find the work it does". Please state the problem exactly the way it was given to you.