Why Does Static Friction Cause Torque in a Rolling Ball?

[raisatantuico]

Homework Statement

a solid metal ball of mass M and radius R is rolling without slipping down and incline. A static friction force of magnitude f is acting on the ball. What is the magnitude of the torque due to static friction?
a.) zero b.) rfcos theta c.) rfsin theta d.)Rf e. MRF

Homework Equations

ki+ui=kf=uf (conservation of energy)

The Attempt at a Solution

since the ball is rolling without slipping, why is there friction involved? do we just simply ignore friction, so the answer will just be the definition of torque: rfsin theta ?

 

[Doc Al]

since the ball is rolling without slipping, why is there friction involved?

Without friction, the ball would slide not roll.

do we just simply ignore friction, so the answer will just be the definition of torque: rfsin theta ?

No, you cannot ignore friction. (And how is that answer ignoring friction anyway? It includes f!)

 

[raisatantuico]

Without friction, the ball would slide not roll.

No, you cannot ignore friction. (And how is that answer ignoring friction anyway? It includes f!)

f in the equation is force. torque= radius x force sin theta. net torque is also equal to angular acceleration x moment of inertia

where does friction come in this definition of torque?

 

[Doc Al]

Here 'f' is the friction force. (What direction does friction act? What angle does it make with the radius?)