Pulley problem with definite mass rope

  • #1
Hi, i've been working on this problem for more than 4 hours now. and I haven't got a clue on how to finish the problem. ANY help would be tremendously appreciated!
Thanks!!

Homework Statement


a pulley has a circumference of 1.20m and mass of 2.25kg. pulley is a solid uniform disk. a heavy rope, 8.00m in length with a mass of 4.80kg hangs over the pulley with one end of the rope 0.600m lower than the other end of the rope. the pulley is initially at rest. when the pulley is released, the pulley turns as the lower end of the rope accelerates downward. assume no slippage. what is the angualr velocity of the pulley at the moment the end of the rope leaves pulley?


Homework Equations


v=wr
I=mR^2
L1+L2+1/2(circumference)=8.00m
PEi=KE(translation)+KE(Rotation)


The Attempt at a Solution



ok so we know L1+L2+1/2(circumference)=8.00m; therefore, L1=3.4m, and L2=4.0m.
the initial potential energy is mg(4.00m) because the rope will leave pulley after it moves 4.00m more. The problem is that I don't know which mass to use. Do i use the mass of the whole rope? the mass changes as the rope falls over the pulley, as well as its acceleration.

ignoring the ambiguity of mass, our KE(trans)+KE(rotat)=1/2(mass1)(v^2)+1/2(mass of pulley)R^2)(w^2)=1/2(m)(rw)^2+1/2(mR^2)(w^2). In this equation, I don't know what is mass1. is it the mass of entire rope?

I am not even sure if my PE is right. So any help would be truly appreciated! Thanks in advance, Physics Forum members :)

Homework Statement





Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
tiny-tim
Science Advisor
Homework Helper
25,832
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Welcome to PF!

Hi Hopelessmin! Welcome to PF! :smile:

(have an omega: ω and a squared: ² :wink:)
a pulley has a circumference of 1.20m and mass of 2.25kg. pulley is a solid uniform disk. a heavy rope, 8.00m in length with a mass of 4.80kg hangs over the pulley with one end of the rope 0.600m lower than the other end of the rope. the pulley is initially at rest. when the pulley is released, the pulley turns as the lower end of the rope accelerates downward. assume no slippage. what is the angualr velocity of the pulley at the moment the end of the rope leaves pulley?

ok so we know L1+L2+1/2(circumference)=8.00m; therefore, L1=3.4m, and L2=4.0m.
the initial potential energy is mg(4.00m) because the rope will leave pulley after it moves 4.00m more. The problem is that I don't know which mass to use. Do i use the mass of the whole rope? the mass changes as the rope falls over the pulley, as well as its acceleration.
All PE is relative!!

So starting with "the initial potential energy is …" is wrong … just state what your "base" of PE is (in this case, choose "base" PE as being when the two ends are of equal length), and just use the difference in PE from that. :smile:

(and once you do that, it is obvious how much of the rope you use! :wink:)
ignoring the ambiguity of mass, our KE(trans)+KE(rotat)=1/2(mass1)(v^2)+1/2(mass of pulley)R^2)(w^2)=1/2(m)(rw)^2+1/2(mR^2)(w^2). In this equation, I don't know what is mass1. is it the mass of entire rope?
How much of the rope is moving? That's what has KE. :wink:

Then solve the equation KE + PE = constant. :smile:
 

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