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Axis of rotation
Beginner
The line about which a body turns.
Rotation is described about an axis; every point moves in a circle around it with the same angular velocity. The axis can be fixed (door hinge) or moving (rolling wheel).
A spinning top about its tip axis.
Quantities like torque and I depend on the chosen axis.
In more detail
The axis of rotation is the line about which a body turns. Points on the axis stay still, while other points move in circles around it.
Real-world uses
- Wheels rotate about their axles, which determines how they roll.
- Helicopter rotors turn about a central vertical axis.
- A door turns about its hinges, so the hinge line is its axis of rotation and every point on the door moves in a circle around it.
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Frequently asked questions
Must the axis pass through the body?
No. A body can rotate about an axis outside it, such as a swinging door.
Does the choice of axis matter?
Yes. The moment of inertia and torque depend on which axis you use.
Does the axis have to be inside the object?
No. A spinning ring or a rotating plank can turn about an axis that passes outside its material.
Related topics in Rotational Motion
Rigid body · Angular displacement · Angular velocity · Angular acceleration