Homework Help: Rotational Kinematics - Cylinder down a slope.

1. Nov 2, 2008

CaptainSFS

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

A certain non-uniform but cylindrically symmetric cylinder has mass 9 kg, radius 1.2 m, and moment of inertia about the center of mass 7.6 kg m2. It rolls without slipping down a rough 20° incline.

What is the acceleration of the cylinder's center-of-mass?

2. Relevant equations

kinematics and the rotational equivalents

3. The attempt at a solution

The hints I was given was to find/use 3 equations. I believe the equations I found are wrong. I seem to have some difficulty with kinematics, and now especially this rotational kinematics.

I found:
m*g*sin(20) - F(friction) = m*a
T(orque) = I * alpha
a = alpha * r

Any help to whether these are the useful equations, and how I may go about using these? Thx. :)

2. Nov 2, 2008

Astronuc

Staff Emeritus
What is the relationship between torque on the cylinder and the frictional force at the circumference?

3. Nov 2, 2008

CaptainSFS

Are they the same?

b/c the frictional force is a force not coming out from the CM; therefore it helps it spin? I believe it is the only other force, so they must be the same?

Last edited: Nov 2, 2008
4. Nov 2, 2008

CaptainSFS

I also tried using the gravitational force as the Torque.
I used m*g*sin(20)=I*alpha
m*g*sin(20)/I=alpha and alpha*R = a
so... m*g*sin(20)*R/I=a,
but this didn't work either.

-------------------
I just solved this problem.

Last edited: Nov 3, 2008
5. Nov 20, 2008

llamaworm

Use these equations:
mgsin(20) - F(friction) = ma
I(alpha) = F(friction)*r
a=(alpha)*r

The only force that applies a torque is friction, because the normal force and gravity both act on the center of mass.