Torque of cyclinder

1. Jun 18, 2004

ke2cool

I really need help with the following problem.

1) A 2kg mass is attached to a string which is wrapped several times around a uniform solid cyclinder of radius 3m and moment of inertia of I = 5kg m/s^2. Assume: The cyclinder can rotate freely. The acceleration of gravity is 9.8m/s. Find the torque on the cyclinder. Answer in units of N.m

2. Jun 18, 2004

Pergatory

F x L = T
F (Force) is the force of gravity pulling on the 2kg mass, which converts to 19.6N.
L (Length) is the distance to the focal point which is 3m (the radius of the cylinder).

Moment of inertia is not used in this problem.

3. Jun 18, 2004

NateTG

Since the acceleration of the cylinder downward affects the torque, moment of inertia is important, but isn't moment of intertia supposed to be in units of $$kg m^2$$?

ke2cool:
List the forces that are acting on the cylinder.
List the net torque on the cylinder in terms of those forces.
See what equations apply to the situation.

OT: Any idea what that cylinder is made of? A 6m diameter and it only weighs 2kg.

4. Jun 18, 2004

I think Nate's jumped off the deep end...

5. Jun 19, 2004

Staff: Mentor

find the tension in the string

I presume that the cylinder rotates about a fixed horizontal axis? And that you just messed up the units of I? (As NateTG pointed out.)

The only force exerting a torque on the cylinder is the tension in the string.

Identify all the forces acting on the mass and all the forces acting on the cylinder. Then apply Newton's 2nd law to both objects to get two equations so you can solve for the tension.

Hint: You'll have to relate the angular acceleration of the cylinder to the linear acceleration of the mass.