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A projectile is launched and follows the parabolic trajectory shown in the figure. Midway through its flight it explodes into several fragments due to an internal chemical charge. Air resistance is negligible. Which of the following best describes the subsequent motion of the centre of mass of the fragments? 
AIt immediately falls vertically because the explosion destroys horizontal momentum.
BIt continues along the same parabolic trajectory the unexploded projectile would have followed.
CIt moves along a straight line tangent to the trajectory at the point of explosion.
DIt stops momentarily and then follows a new parabola starting from rest.
Answer & Solution
Correct answer: B. It continues along the same parabolic trajectory the unexploded projectile would have followed.
Explosion forces are internal to the system of fragments, so by Newton's third law they cancel pairwise and contribute nothing to the motion of the centre of mass. The only external force is gravity, which is the same before and after the explosion. Hence $M\mathbf{A}_{CM} = \mathbf{F}_{\text{ext}} = M\mathbf{g}$ throughout, and the COM continues along the original parabola. (A) is wrong: horizontal momentum is conserved by the internal forces. (C) ignores gravity, which is still acting. (D) violates Newton's first law applied to the COM — there is no external impulse at the explosion.
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