# Homework Help: Find torque of a ball on the end of a light rod

1. Oct 27, 2007

### mathcrzy

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

A small 1.25 kg ball on the end of a light rod is rotated in a horizontal circle of radius 0.890 m.
(a) Calculate the moment of inertia of the system about the axis of rotation.
____kg*m^2
(b) Calculate the torque needed to keep the ball rotating at constant angular velocity if air resistance exerts a force of 0.0800 N on the ball.
____m*N

2. Relevant equations

a) kg*m^2
b) kg*m^2-air resistance (dont know the formula; just a guess)

3. The attempt at a solution

a) 1.25*.89^2=.99kg*m^2 (i was able to figure out this answer)
b) 1.25*.89^2-.08=.91m*N

2. Oct 27, 2007

### saket

Hint: air resistance is a force, you are required to find torque. how do u corelate them?

3. Oct 27, 2007

### FedEx

Do you know how to find find the Moment of inertia?

4. Oct 28, 2007

### GTrax

You got the moment of inertia right.
Carefully consider what forces it faces when moving at constant angular velocity.

5. Oct 28, 2007

### FedEx

Find the torque which is applied by the air on the object.The same amount of torque should be applied in the opposite direction in the opposite direction.