The present invention discloses a motor comprising: a stator part; and a rotor assembly rotatably disposed with respect to the stator part, wherein the rotor assembly comprises: a ring magnet including an insertion hole with a shaft disposed therein and a plurality of first grooves formed at one side thereof along a first imaginary circle; and a core member including a body portion disposed between the shaft and the insertion hole, and an extension portion covering the plurality of first grooves, wherein the extension portion comprises a plurality of second grooves deviated from a plurality of first imaginary straight lines which pass from a center of the first imaginary circle respectively through the plurality of first grooves.