At the highest point in its parabolic trajectory, a shell explodesinto two fragments. Is it possible for bothfragments to fallstraight down after the explosion? Why or why not?

External force is a force exerted on any part ofa system by something outside the system.

If thenet external force on a system is zero, the totalmomentum of the system $\overrightarrow{\mathbf{P}}$is constant or conserved. Each componentof total momentum is separately conserved.

Since no external force is acting on the system in the horizontal direction, so from the conservation of momentum total momentum of the system in the horizontal direction is constant.

For a fragment to fall straight down after the explosion, its horizontal component of momentum shout be zero, but if both the fragments are falling straight down after the explosion the horizontal component of total momentum would not be zero or constant this is a contradiction to the conservation of momentum.

Hence both fragments can’t fall straight down after the explosion.