# Acceleration of an object rollling down a incline.

1. Apr 24, 2010

### jwu

1. The problem statement, all variables and given/known data
A cylinder of mass M and radius R rolls(without slipping) down an inclined plane whose incline angle with the horizontal is Θ. Determine the acceleration of the cylinder's venter of mass.(Derive the formula)

2. Relevant equations
I have the answer, but I just don't understand:

∑τ=Iα, use the parallel-axis theorem and a=Rα.
(Mg sin Θ)R=(1/2MR²+MR²)(a/R)
Rg sin Θ=3/2Ra
a=2/3g sinΘ

3. The attempt at a solution
Here's my question. Why do we need to use the parallel-axis theorem? isn't that the torque is just 1/2MR² ? Maybe I am confused with the concept of Torque. I don't know...Please help!!!

2. Apr 24, 2010

### rl.bhat

When the cylinder is rolling with out slipping, the relative motion of point of contact is zero.
So it acts as the axis of rotation of the center of mass. That is why you have to find the moment of inertia about this axis by using parallel-axis theorem.