You'll see a red outline of the new path. You can see that it worked by using the Node tool F2 and hovering over the line.

(If there were multiple markers, you'll need to do this one-at-a-time for each marker).

With both the line and stroke selected, click Path > Union.

This will convert the stroke into multiple paths-one for the line itself and one for each marker.

Change the stroke width to increase/decrease the size of the arrow.

Choose an arrow for the Start Marker and/or End Marker.Open Fill and Stroke dialog ( Shift+ Ctrl+ F).It also allows you to do things like set a different stroke and fill color for the line (so that it can stand out when it doesn't contrast well with the background). Converting the line to a path helps ensure image fidelity. Light reflected off the surface area of the table top creates an area of reflected light within the spheres shadow. Set the circle's Fill to the ambient color of the sphere. For the first circle, set the Stroke style Width: to 0. To create the sphere, draw 3 circles, all about the same size, as shown above. In my experience, lines with markers can be rendered incorrectly by some SVG applications. Light reflected off the surface area of the table top creates an area of reflected light within the sphere's shadow. You may want it to be a path instead, which I think is what Tomáš Zato was getting at with his comment. When you first create an arrow, it will be a stroke.

Inkscape arrow on sphere