# Homework Help: Kinetic Energy in a CD

1. Mar 30, 2012

### eagles12

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

A 12g CD with a radius of 6cm rotates with an angular speed of 38 rad/s. What is kinetic energy?

2. Relevant equations

k=1/2mv^2
v=rw

3. The attempt at a solution

v=(6)(38)=228
K=1/2(12)(228^2)
K=311904
but its saying this is the incorrect answer

2. Mar 30, 2012

### Sefrez

It says that the CD rotates, so you are much better off modeling under angular speeds rather than constantly changing translational speeds (e.g. particles x distance from center of rotating disk.) When deriving these equations a term shows up which is called the moment of inertia. Have you gone over this?

3. Mar 30, 2012

### Staff: Mentor

You are dealing with rotational motion, so you want rotational kinetic energy. This will involve the mass moment of inertia of the CD and angular velocity. Be careful about units, they have to be consistent; Work with meters and kilograms.

4. Mar 30, 2012

### eagles12

using this i got k=.0156 but this seems too small

5. Mar 30, 2012

### Staff: Mentor

Too small for what? And what are the units. Everything in physics has units.

6. Mar 30, 2012

### eagles12

the units are joules

7. Mar 30, 2012

### Staff: Mentor

Well then, the result looks reasonable

8. Mar 30, 2012

### eagles12

i had to use two significant digits but it said my answer was incorrect

9. Mar 30, 2012

### Staff: Mentor

10. Mar 30, 2012

### eagles12

11. Mar 30, 2012

### Staff: Mentor

The leading zero is not a significant digit.

12. Mar 30, 2012

### Sefrez

Rounded to the nearest hundredth, that should be correct. But as gneill stated, that is only one significant digit.

Bot/Troll?

Last edited by a moderator: May 5, 2017
13. Mar 30, 2012

### eagles12

got it thanks!

14. Mar 30, 2012

### Staff: Mentor

Spammer was banned. Please hit the "report" button to report a post if it looks fishy.