# Flywheel with weight - Rotational Inertia

#### Bryon

Hey everyone! I have a small problem with part b. Im wondering if i miscalculated something. Any ideas? Thanks!

1. The problem statement, all variables and given/known data

A 15 kg flywheel has all its mass around its outer rim. A string is wrapped around it and a m = 4.7 kg weight is hanging on the string. The flywheel has radius R = 0.25 m.

--------------------------------------------------------------------------------
a) When the 4.7 kg weight is dropping with a speed of 1.1 m/s, what is the angular velocity of the flywheel?
4.4 OK

--------------------------------------------------------------------------------
b) When the 4.7 kg weight is moving with a speed of 1.1 m/s, what is the kinetic energy of the entire system? This one seems really easy, but for some reason I keep getting the wrong answer.

2. Relevant equations

K(rotational) = 0.5Iw^2
I = 0.5m(wheel)r^2
KE = 0.5m(block)v^2

3. The attempt at a solution

KE(system) = 0.5[0.5m(wheel)r^2]w^2 + 0.5m(block)v^2

0.5*[0.5*15*0.25^2]*(4.4^2) + 0.5*4.7*(1.1^2)
KE(system) = 4.5375 + 2.8435 = 7.381 J

For some reason 7.381 is not correct.

Last edited:
Related Introductory Physics Homework News on Phys.org

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving