# Simple Pulley System with Two Masses

## Homework Statement

http://imgur.com/Y1Dua

r = 0.2m
I = 1.4kg m^2
m1 = 2kg
m2 = 5kg
h = 4m

## Homework Equations

Δ(Gravitational potential energy) = m1*g*h - m2*g*h
Δ(Kinetic energy) = (1/2)*m1*v^2 + (1/2)*m2*v^2 + (1/2)*I*(v^2/r^2)

## The Attempt at a Solution

System is isolated; no change in the internal energy of the system so all the lost gravitational potential energy must go into increasing the kinetic energy of the system.

Therefore, ΔKE = -ΔGPE → (1/2)*v^2(m1 + m2 + I*r^(-2)) = -ΔGPE
→ v = √((-2ΔGPE)/(m1 + m2 + I*r^(-2))) = 2.36643191m/s is what the final answer works out to be.

Problem is, the answer sheet says that the answer should be 0.014. I'm kind of rusty with rotational motion, but even after analyzing this problem to death I still have no clue why my answer is nearly 170 times too big... I must be making a stupid mistake somewhere.

As always, any help is much appreciated.

P.S. sorry for the awful formatting, but these university computers are still running IE7 for some reason and the only part of the submission form that's actually responsive is the text box.

## Answers and Replies

PhanthomJay
Science Advisor
Homework Helper
Gold Member
I get the same answer as you. ehild
Homework Helper
Your solution is correct. The answer sheets do have mistakes.

ehild

Huh. Well then. Thanks for the confirmation.