A particle is projected with a velocity u making an angle θ with the horizontal. The instantaneous power of the gravitational force

At any time,v→=[ucos⁡θ]i^+(usin⁡θ−gt)j^P=F→⋅v→=(−mgj^)⋅[ucos⁡θi^+(usin⁡θ−gt)j^] =mg2t−mgusin⁡θHence, power varies linearly with time.