Rotational Motion change angular rotational

[physgrl]

Homework Statement

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

Homework Equations

n/a

The Attempt at a Solution

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

 

[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.

 

[physgrl]

Got it...thanks!

 

[dacruick]

Got it...thanks!

Awesome. youre welcome