# Geodesic in cylinder

by physics_fun
Tags: cylinder, geodesic
 P: 28 Hi, i'm working on marionþton ch6 question 6.4. "Show that the geodesic on the surface of a straight circular cylinder is a (partial) helix" I used the example of the geodesic on a sphere in the book, but when i calculate the angle phi i get something like phi=b*z+c, where b and c are constants; this is a straight line?! Or does it just mean that the 'speed' of phi doesn't change in time??
 Emeritus Sci Advisor PF Gold P: 4,975 Phi changes linearly with z. Think about the implications of this.
 P: 28 That implies the equation should be linear....and it is! Thanks!
Emeritus
PF Gold
P: 4,975

## Geodesic in cylinder

I still don't think you got what I meant. The equation you came up with shows a linear change in phi with z. Now imagine a cylinder that has a line drawn on its inside surface that changes linearly by 2pi over the total length z. The line drawn on the inside would be part of a helix.

Just making sure you can visualise that.
 P: 28 I think that's just what I meant to say (my English is not always very good...)
 Emeritus Sci Advisor PF Gold P: 4,975 No problem. English is my first language and I struggle to express myself

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