# How to determine moment axis and direction on a rod in 3d

1. May 6, 2013

### amrhu

if i have rod of a certain length. let point (a) on one end of the rod and point (b) on the other end. along the rod axially is the X axis, vertically Y axis and Z is the lateral direction.let force prependicular on x -axis at point (b) and move this force to point (a) which will create a moment. now the question is at which axis will the moment will be? also is the direction of this moment will be clockwise or counter clockwise.

2. May 6, 2013

### SteamKing

Staff Emeritus
In 3D, for a rod, it is not enough to specify the x axis along the length of the rod. A third point, which is not located on the rod itself, must be used as a reference point. For example, your rod is determined by the points a and b, and this segment may be assumed to be the x-axis. By specifying a third point c which is not on the rod, you may construct a perpendicular to the rod, and this direction could be assumed to be the y-axis. With two axes specified, the z axis, which is mutually perpendicular to x and y axes, can be constructed.

3. May 7, 2013

### amrhu

yes you absolutely right i must put a third point in consideration> your assumptions is what i mean now how to determine moment axis and also its direction if i want to move this force to point (a) which assumed to be parallel to y -axis and perpendicular to x -axis at point (b)