|WikiProject Physics||(Rated Stub-class, Mid-importance)|
Can we call rolling as pure rotation?
Rolling can be seen as pure rotation. Say, a sphere is rolling on the flat table. At an instant, the sphere can be "seen" as if it is rotating about the point of contact. But as time passes, that "physical point"(or molecule, atom, etc.) changes (due to acceleration of the contact point) into some other. But, if we again see only at "an instant" then we again have rotation about contact point but not about same "physical point" but about same "geometrical point". Hence, the axis is changing all the time but if we see at an instant the sphere can be regarded as rotating about the point of contact.
My doubt is whether it is okay to call this motion as "pure rotation" about the contact point, even if "physical point" does not stay the same? the answer is hidden in the definition of Rotation. According to Wikipedia, rotation is just about an axis, from which I think it can also be about an axis where physical point is varying. But I don't know the internationally accepted definition of Rotation in Physics. Text-books were not of much help.
That last paragraph in the article needs a rewrite to improve the grammar to readability standards.(DF)