# Homework Help: Rotational Kinetic Energy

1. Oct 28, 2008

### dsptl

A 300 g ball and a 500 g ball are connected by a 60-cm-long massless rigid rod. The structure rotates about the center of the rod with angular velocity 5 rad/s. What is the rotational kinetic energy?

Attempt:
the rotational kinetic energy is given by
KEr = (1 / 2) I ω^2
where I is the moment of inertia if the ball is considered as a solid sphere then we get that
I = (2 / 5) M R^2 for a solid sphere

but i have a question what should i use for M and R?

2. Oct 28, 2008

### christensen

you dont have a solid sphere here.

0------X------0

where the "0"s are the balls and the "X" is the axis of rotation.

3. Oct 28, 2008

### dsptl

so wht should i do?

4. Oct 28, 2008

### christensen

you want to calculate the moment of inertia of the system here and continue as you were planning on with E = 1/2 * I * w^2

5. Oct 28, 2008

### dsptl

for I its just .5MR^2

and what would be my M and R since there r two mass given?

6. Oct 28, 2008

### christensen

Moment of Inertia is the sum of all of (mass times the sqaure of the distance to the axis of rotation)

In other words I =$$\sum$$ miri2

7. Oct 28, 2008

### dsptl

so I = (.5 + .3 ) (.60)^2

T = Ia

and solve for a, right?

8. Oct 28, 2008

### christensen

if the rotational axis is in the center of the rod, and masses are on the ends, and the rod is 60cm long, is the distance between the axis of rotation and the mass 60cms?

9. Oct 28, 2008

### dsptl

so I = (.5 + .3 ) (.30)^2

T = Ia

and solve for a, right?

10. Oct 28, 2008

### christensen

im not sure what you mean by "T = Ia"

11. Oct 28, 2008

### dsptl

oh sorry its suppose to E = .5Iw^2

and solve for E