A small particle of mass m is projected at an angle θ with the x-axis with an initial velocity us in the x-y plane as shown in the figure. At a time t

L→=r→×p→ where r→=xi^+yj^=v0cos⁡θti^+v0sin⁡θt−12gt2j^and  p→=mv→=mv0cos⁡θi^+v0sin⁡θ−gtj^So, j→=i^j^ v0cos⁡θtv0sin⁡θt−12gt20mv0cos⁡θmv0sin⁡θ−gt0=−12mgv0t2cos⁡θj^