# Rotational Motion change angular rotational

1. Oct 31, 2011

### physgrl

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

9. A force can change angular rotational if it acts through:

a. the axis of rotation

*b. a lever arm tangential to the direction of rotation

c. a moment arm parallel to the axis of rotation

d. none of these

2. Relevant equations

n/a

3. The attempt at a solution

I would like to understand why b is the correct answer.

2. Oct 31, 2011

### dacruick

I guess we could start with why it isn't a or c. Angular momentum is changed when Torque is applied. If you apply a force through the axis of rotation, then r is 0. if you apply force through a moment arm parallel to the axis of rotation then torque is also zero since

τ = F x r
|τ| = Frsinσ

since F and r are parallel, the angle is 0, therefore the torque is 0. If you applied it tangentially, then the σ value will be 90 and sinσ will be 1.

3. Oct 31, 2011

### physgrl

Got it...thanks!!

4. Oct 31, 2011

### dacruick

Awesome. youre welcome