# Is this arc length?

1. Mar 11, 2013

### autodidude

Could someone please explain why PQ in the diagram below is rΔθ? Isn't rΔθ arc length?

The best reason I can think of is that it's only an approximation for when the angle is very small, so PQ≈arclength=rΔθ. Not 100% sure though.

http://imageshack.us/scaled/landing/199/feynmanangle.jpg [Broken]

The diagram is from the first volume of the Feynman lectures in 18-3, in the section where he talks about rotation of rigid bodies.

Last edited by a moderator: May 6, 2017
2. Mar 11, 2013

### Staff: Mentor

It is an approximation for small $\Delta \theta$, right. I would expect that it is used in a differential somewhere, where the approximation gets exact.

3. Mar 11, 2013

### autodidude

Thanks mfb

4. Mar 11, 2013