Question

A particle of mass $m$ is projected from the ground at an angle $\theta$ from vertical. The change in momentum when it is at highest point of its track.

Answer 1

At the highest point the $vertical$ velocity becomes $zero$ for a moment, as we know the gravity being $vertical$ doesn't affect the $horizontal$ velocity so it remains $constant.$

So we get the $change$ in $velocity$ as just $\Delta V=0-uSin\theta =-uSin\theta$

so the change in momentum will be $\Delta P=m\Delta V=-muSin\theta$

In $downward$ direction, i.e. in the direction of force(gravity).