# Homework Help: Acceleration Problem due to Rotational Inertia [Exam in 9hrs!]

1. Dec 17, 2007

### maxxed

[URGENT] Acceleration Problem due to Rotational Inertia [Exam in 9hrs!]

yes i know i might be late but i hope somebody can help me out with this problem because i cant figure it out. Exact word for word:

A block of mass m2= 12kg hangs from a rope. The rope wraps around a pulley and then attaches to a second block of mass m1=8kg, which sits on a frictionless table. The radius of the pulley is .10m and its rotational inertia is I=.040 kg-m2. What is the acceleration of the blocks when they are released.

here is how it looks:

http://spiff.rit.edu/classes/phys311/workshops/w5b/level_atwood/cart_and_weight.gif

a modified atwood machine.

2. Dec 17, 2007

### hotcommodity

You essentially need to draw 3 free body diagrams. Use Newton's Second Law to set up the equations for the two blocks, and Newton's Second Law for rotational motion, $$\Sigma \tau = I \alpha$$ to set up an equation for the net torque on the massive pulley. Also note, you'll want to use the rolling constraint $$a = r \alpha$$ to get the angular acceleration in terms of "a." Does this help?

3. Dec 17, 2007

### maxxed

sort of i was using
a=F/m
F=m2g->12*9.80=118
m=12kg + 8kg
a=5.9

but its not one of the choices which are:
a. 3.5
b. 3.9
c. 2.5
d. less than 2.5
e. cannot be determined