# Energy in rolling motion

When there is purely rolling motion in an inclined plane, it is mentioned that work done by the frictional force is zero!!
Why should it be zero as there is definitely a force as well as displacement in opposite direction of force?
Is this true or for rolling motion there is some other significance?

Plzz help

Hootenanny
Staff Emeritus
Gold Member
When there is purely rolling motion in an inclined plane, it is mentioned that work done by the frictional force is zero!!
Why should it be zero as there is definitely a force as well as displacement in opposite direction of force?
Is this true or for rolling motion there is some other significance?

Plzz help
If the object is purely rolling, i.e. there is no slipping, what can you say about the [relative] velocity of the point of contact on the body and the inclined plane?

arildno
Homework Helper
Gold Member
Dearly Missed
When there is purely rolling motion in an inclined plane, it is mentioned that work done by the frictional force is zero!!
Why should it be zero as there is definitely a force as well as displacement in opposite direction of force?
Is this true or for rolling motion there is some other significance?

Plzz help
Remember that work is directly related to changes in kinetic energy.
Because changes in kinetic energy is related to changes in SPEED, rather than to changes in velocity, it follows that forces that ONLY change the direction of velocitiy (rather than changing its magnitude) do not do any work, in the sense of how work is defined in physics.

what does it mean by relative motion between point of contact and ground?

rcgldr
Homework Helper
what does it mean by relative motion between point of contact and ground?
Should be relative motion at the point of contact with the ground. If the object is slipping, then there is relative motion between the surface of the object and the ground at the point of contact. If the object is purely rolling, then there is zero relative motion between the surface of the object and the ground at the point of contact.

If you're wondering about rolling resistance, it's due to non-elastic (energy lost) deformations at the point of contact.