A particle is projected at time `t=0` from a point `O` with a speed `u` at an angle `theta` to horizontal. Find the torque of a gravitational force on projectile about the origin at time `t. (x,y` plane is vertical plane)

Explanation

`vec(r ) = ( u cos theta)t hat(i) + ( (u sin theta )t - 1/2 "gt"^(2) ) hat(i)`
`vec(F) = - (mg) hat(j) , vec(tau) =vec(r ) xx vec(F)`

` vec(tau) = [(u cos theta ) t hat (i)+((u sin theta) t - 1/2 "gt"^(2))hat(j)] xx mg (-hat(j))`
` vec(tau) = ( u cos theta) t (mg) (hat(i) xx -hat(j))`
`tau = 0 mg ( u cos theta) t (hat(k))`