Posted by Luke on .
A small block on a frictionless surface has a mass
of 70 g. It is attached to a massless string passing
through a hole in a horizontal surface (see diagram).
The block is originally rotating in a circle of radius 45
cm with angular speed 0.80 rad/s. The string is then
pulled from below until the radius of the circle is 25
cm. You may treat the block as a point particle.
(a) Is the angular momentum of the block conserved?
Why or why not? (b) What is the final angular speed?
(c) What are the initial and final tensions in the string?
(d) What was the change in kinetic energy of the block?
(e) How much work was done in pulling the string?

Physics 
Marina,
Based on the fact that this is the exact same problem I am doing at the same time of the year I am guessing you might have been in physics 221 at Iowa state. If you are also in this class this may help you Paul, I have not figured out the tension part but
a) momentum is conserved
b) Paul already answered that one
c)?????
d)KE1=1/2*m(r1*w1)^2
KE2=1/2*m(r2*w2)^2
deltaKE=KE2KE1
for this exact problem I got 101606.4 for the answer
e)work=change in KE
hopefully this helps others in this class of this and future semesters 
Physics 
Marina,
I might be wrong but I may have just figured out how to find the tension even though it is not the way they wanted since the order is wrong but using the work we found we can probably use
W=Fd
and say the distance is the change in radius to get the force on the string. I hope this is the right answer and you don't have to calculate any forces caused by the blocks acceleration 
Physics 
Marina,
just realized I forgot to turn 70g into kg so ignore the answer I gave for work It should be 101.61 (just moved the decimal over three places)