Finding speed with a pulley?

[physics10189]

Finding speed with a pulley?

Homework Statement

Two objects are connected by a light string
passing over a light frictionless pulley as
shown in the figure. The 5.9 kg object is
released from rest at a point 4.53 m above the
floor.
The acceleration of gravity is 9.8 m/s2 .

What is the speed of the each object at
the instant before the 5.9 kg hits the floor?
Answer in units of m/s.

Homework Equations

The Attempt at a Solution

I use the energy equation where
PE(initial)=KE(final)
and solve for v but it does not seem to work

 

[davieddy]

decrease in PE = increase in KE

 

[tiny-tim]

Welcome to PF!

Hi physics10189! Welcome to PF!

I use the energy equation where
PE(initial)=KE(final)
and solve for v but it does not seem to work

Show us your full calculations, and then we can see what went wrong, and we'll know how to help!

 

[physics10189]

well here is what i did

mgh=.5mv^2
h=Lcos(theta)
mass cancel out
so this is what i came up with
v=2gLcos(theta)
where L=10m of the string's length

 

[physics10189]

whoops wrong problem
scrath the last reply

ok its the same way kinda
so here is what i did

PE(initial)=KE(final)
mgh=.5mv^2

masses cancel out
h=4.53m
there for
v=sqrt(2gh)

 

[davieddy]

Is the "final" PE of the system zero?
See my earlier post.

 

[physics10189]

yes because just before the blocks hit the ground can we assume that the height is zero therefore the PE final is zero?

 

[Santorican]

What answer are you getting?

 

[PhanthomJay]

A figure, or description, showing what is theta, the mass of the other object, etc., sure would help.