Why Doesn't the Energy Method Require Friction for Rolling Objects?

[RRachev]

An object (for example cylinder) is rolling down an incline, with no slip. We know there is some friction and we have to find the velocity of the objects. There are 2 ways of doing so with calculating resultant force and using Newton's laws and the energy method. Why is it that when we are using the energy method we don't have to take into account the friction and our answer is still right?

Mass - m
Radius of the cylinder - r
Moment of Inertia - I=(mr^2)/2
Distance traveled - s
Angle of Incline - θ
Acceleration due to gravity- g

I know the answer is v= sqrt(4*s*g*sinθ/3)

Thanks in advance

 

[szimmy]

What type of friction is present, kinetic or static?

 

[RRachev]

The question did not say. But when you compute the velocity using the force method or the energy method (while using the second one assume no friction force) and the answer is the same. I do not quite understand the solution probably that's the problem itself.

 

[szimmy]

The problems inadvertently says it, I'm asking what your interpretation of the problem is, do you think the friction keeping the cylinder from slipping is kinetic or static?

 

[RRachev]

Ohh now I see what you mean. Should be static?

 

[szimmy]

Correct. Can static friction do work on an object?

 

[RRachev]

No.
Yeah thanks a lot for the help.