Easy question that I'm not getting?

[dominus96]

Easy question that I'm not getting??

Homework Statement

A small ball with mass 1.40 kg is mounted on one end of a rod 0.850 m long and of negligible mass. The system rotates in a horizontal circle about the other end of the rod at 5020 rev/min.

Calculate the rotational inertia of the system about the axis of rotation.

Homework Equations

I=MR²/3

The Attempt at a Solution

Inertia for a rod spinning at one end = MR²/3, so I did that and got .337, but that wasn't the correct answer. What am I doing wrong?

 

[dominus96]

Come on, SOMEBODY has to know this =/

 

[alphysicist]

Homework Statement

A small ball with mass 1.40 kg is mounted on one end of a rod 0.850 m long and of negligible mass. The system rotates in a horizontal circle about the other end of the rod at 5020 rev/min.

Calculate the rotational inertia of the system about the axis of rotation.

Homework Equations

I=MR²/3

The Attempt at a Solution

Inertia for a rod spinning at one end = MR²/3, so I did that and got .337, but that wasn't the correct answer. What am I doing wrong?

The rod has (essentially) zero mass, so when you apply your equation you will find that the rotational inertia of the rod is zero.

But the system consists of more than the rod. What is the rotational inertia of this small ball about the axis of rotation?

 

[dominus96]

Not quite sure what you mean. There is one rod and it has a mass attached to it, so don't you just use that as the mass in the inertia equation?

 

[dominus96]

Wait, i think I get it. So do you do MR²/3 + MR²?
The inertia of the rod is 0, so the answer is just MR²? That would be 1.0115, am I right?