Find Forces Over a Disk - 65 Characters

[oreosama]

Homework Statement

http://i.imgur.com/EhPmE.png

The pulley(disk) has a radius R and a mass m the rope does not slip over the pulley and the pulley spins on a frictionless axle. the coefficient of kinetic friction between block A and the surface is μk. the system is released from rest and block B descends. black A has mass 2m and block B has mass m

given [R m h μk]

find the speed of block B after falling distance h
the time to drop distance h

Homework Equations

τ = I α

Idisk = mR^2

α = a/R

The Attempt at a Solution

I can do the basic non rotational forces no problem. I don't have any experience messing with rotational force but have seen the torque = I alpha formula.

τ = mR^2 (a / R)
= mRa

i don't know how to relate this torque(assuming it's even right :p) back into x y forces in the form of tensions to tie the entire problem together. I am fairly comfortable with using energy but I need to be comfortable with using forces as well and this question is asking for time anyways, so it will likely make things easier in the long run... Thanks for any help.

 

[frogjg2003]

To find the speed, you don't need to think about the forces, just the energy. What is the energy of a disk spinning about it's axis?

 

[oreosama]

1/2 I omega^2

I want to do it by forces for the sake of knowing how. need to know how to solve both ways for tests.

 

[PhanthomJay]

Moment of inertia of a disk is mr^2/2.
For the force method, draw free body diagrams of each block and the pulley, and use Newton's 2nd law on each. For the pulley, its the 2nd law of rotational motion.

 

[Nickie]

I would recommend utilizing the equations provided and solving for the speed of block B and the time to drop distance h. This can be done by setting up equations for the rotational and translational motion of the system, taking into account the forces acting on each block and the torque on the pulley. Then, solving for the unknown variables using algebraic manipulation. It may also be helpful to draw a free body diagram to visualize the forces and their directions. Additionally, it may be useful to review the concept of friction and how it affects the motion of the blocks. Overall, understanding the concepts and equations involved in rotational and translational motion will allow for a successful solution to this problem.