r = acosωti + bsinωtj
v = dr/dt = -aωsinωti + bωcosωtj
角动量
L = r×p =r×mv
= m(acosωti + bsinωtj)×(-aωsinωti + bωcosωtj)
= m(abωcos^2ωt + abωsin^2ωt)k
= mabωk常量
质点所受对原点的力矩M为
M = dL/dt = 0