Block-pulley system and kinetic energy

[Kennedy]

Homework Statement

A pulley with a radius of 3.0 cm and a rotational inertia of 4.5×10^-3 kg·m2 is suspended from the ceiling. A rope passes over it with a 2.0-kg block attached to one end and a 4.0-kg block attached to the other. The rope does not slip on the pulley. At any instant after the blocks start moving, the object with the greatest kinetic energy is: (a) the heavier block (b) the lighter block (c) the pulley (d) either block (the two blocks have the same kinetic energy) (e) none (all three objects have the same kinetic energy)

Homework Equations

I know that the kinetic energy of the pulley is calculated by KE = 1/2(I)(w^2), and the kinetic energy of the blocks will be KE = 1/2(m)(v^2), but how do I go about finding the angular speed of the pulley and the speed of the blocks? Do I have to somehow use the fact that the tangential speed/radius is equal to the angular speed?

The Attempt at a Solution

[/B]
I'm thinking that the tangential acceleration will be the same for all of the objects in the system, which means that their speeds will be the same at any given moment (with the exception of the pulley which will have an angular velocity of tangential speed/radius). Is this right, and where do I go from here?

 

[ehild]

Homework Statement

A pulley with a radius of 3.0 cm and a rotational inertia of 4.5×10^-3 kg·m2 is suspended from the ceiling. A rope passes over it with a 2.0-kg block attached to one end and a 4.0-kg block attached to the other. The rope does not slip on the pulley. At any instant after the blocks start moving, the object with the greatest kinetic energy is: (a) the heavier block (b) the lighter block (c) the pulley (d) either block (the two blocks have the same kinetic energy) (e) none (all three objects have the same kinetic energy)

Homework Equations

I know that the kinetic energy of the pulley is calculated by KE = 1/2(I)(w^2), and the kinetic energy of the blocks will be KE = 1/2(m)(v^2), but how do I go about finding the angular speed of the pulley and the speed of the blocks? Do I have to somehow use the fact that the tangential speed/radius is equal to the angular speed?

The Attempt at a Solution

[/B]
I'm thinking that the tangential acceleration will be the same for all of the objects in the system, which means that their speeds will be the same at any given moment (with the exception of the pulley which will have an angular velocity of tangential speed/radius). Is this right, and where do I go from here?

You are right. The points on the circumference of the pulley move with the same speed v as the blocks do, so the angular speed of the pulley is equal v/r.
Write all kinetic energies in terms of v. Which is greatest?