The line it turns about
A door does not turn just anyhow. It turns about one particular line.
As the door opens, its outer edge travels along an arc, but the edge carrying the hinges goes nowhere at all. The vertical line running down the door through the hinges stays exactly where it was.
Axis of rotation
The line that stays still while a body turns is called the axis of rotation of that body.
Figure to be added
How to find the axis
Take anything that turns and ask which part of it does not move. The line along that part is the axis.
| The thing that turns | Its axis of rotation |
|---|---|
| A door | The vertical line through the hinges |
| The head of a tap | The line through the centre of the tap |
| A bicycle wheel | The axle |
| A wooden strip fixed by a screw | The line along the shaft of the screw |
The axis is not a point
Drawn on paper the axis looks like a dot — but it is really a line, running straight into the page. The door's axis runs the whole way from the top hinge down to the bottom one.
From here on, "the distance from the axis" means a distance measured from this line.