# A pure rolling question

1. Jun 22, 2013

### andyrk

1. The problem statement, all variables and given/known data
A solid sphere of radius R is given a translation velocity vo on a rough surface of coefficient of kinetic friction=coefficient of static friction=μ. After what time does the sphere begin pure rolling? Also find the the angular velocity and linear velocity of the sphere at this time.

This was was found out and calculated by me. My question/doubt was something other which is not in the question. If once the sphere begins pure rolling does its angular velocity and linear velocity become constant? Or will it stop due to rolling friction? It is the same as saying that will the linear acceleration and angular acceleration of the sphere be the same once it has begun pure rolling? I think it should not have constant velocity and angular acceleration because even if it is pure rolling static friction is acting. Does that slow or speed up the sphere?

Also, can a sphere which is executing pure rolling motion ever slow down? If yes then how?

Last edited: Jun 22, 2013
2. Jun 22, 2013

### darkxponent

Pure rolling is an ideal. In real situations rolling friction is always present. SO the sphere is going to slow down. When we say pure rolling there is only static friction acting which does no work and the sphere never slows down.

3. Jun 22, 2013

### rcgldr

It will eventually stop due to rolling resistance, which is mostly related to energy lost during deformation and recovery at the point of contact (not friction).

Most these rolling problems are idealized and assume no losses due to rolling resistance, but do account for the energy converted to heat by sliding (kinetic, dynamic) friction.

4. Jun 22, 2013

### andyrk

So why is it said that rolling friction is very less than kinetic and static friction? Isn't it a resistance and not a frictional force? And How exactly is it a resistance force? I found this:
http://www.lhup.edu/~dsimanek/scenario/rolling.htm

You say resistance force but what is that? There are a lot of Normal reaction force at the points of contact. Is the resultant of all all them which provides a torque opposite to that of the motion called the rolling resistance force? And how do we know that rolling resistance force is very less as compared to kinetic or static friction? I also have a question related to this. Why is moment of Inertia (I) never greater than MR2?

5. Jun 22, 2013

### haruspex

I'm not aware of any standardised meaning of the term "rolling friction". It could mean rolling resistance, or it could refer to the usual sense of tangential friction in the case of an object rolling up or down an incline.
That's a fair description, but perhaps overly complex. If a rolling object is deformable, or rolling on a deformable surface, and there are energy losses associated with the deformation, it will be like it is rolling uphill slightly (or slightly more than the actual slope if not level). The normal force just in front of the point of maximum deformation will be greater than that just behind it, leading to a torque opposing the rolling.

6. Jun 23, 2013