Smooth curve

• Jan 30th 2010, 05:39 PM
Euclid's Bridge Habitant
Smooth curve
I keep reading that a smooth curve, say r(t) = (x(t),y(t)), is a curve such that r'(t) is continuous and r'(t) \neq 0.

I understand the "r'(t) is continuous" bit - so there are no "kinks" or anything in the curve (hence, smooth), but I don't understand why we require r'(t) \neq 0. Would anyone be able to explain this part to me please?
• Jan 30th 2010, 07:54 PM
drumist
There's no such restriction. Perhaps you misread or misunderstood?
• Jan 30th 2010, 08:06 PM
Euclid's Bridge Habitant
On page 1070 of Stewart's Calculus (6th Ed, section on line integrals) for instance: "Start with a plane curve given by r(t) = x(t)i + y(t)j and assume that this curve is smooth (ie, r' is continuous and r'(t) \neq 0)"
• Jan 30th 2010, 08:26 PM
drumist
Oh I'm sorry, I see we're talking about a parametric curve. I'm not entirely sure about that, but my intuition says that if r'(t)=0 and we interpret t as time, then the curve "stops" at a particular point. I'm not sure why that is bad though.

I'll let someone else more familiar with parametric curve smoothness answer. (Lipssealed)
• Jan 30th 2010, 08:28 PM
Euclid's Bridge Habitant
Yep, I just can't figure out why that is "bad" :)