Let m be the mass of the top,the angular velocity about its own axis Torque of gravity about o: M=F×mg mgrsin 0 Angular momentum about its own axis: L=Jo分 By theorem of angular momentum mg di=M·dt L      Let m be the mass of the top,  the angular velocity about its own axis c r   mg Torque of gravity about o： M r mg      M  mgrsin   Angular momentum about its own axis： L  J r ˆ  By theorem of angular momentum dL  M dt  