# Center of Mass Question

Hi, I was hoping to gather some feedback for one of my homework problems. Any help would be much appreciated. The problem is as follows:

A mortar fires a shell of mass m at speed v_0. The shell explodes at the top of its trajectory as designed. However, rather than creating a shower of colored flares, it breaks into just two pieces, a smaller piece of mass (1/5)m and a larger piece of mass (4/5)m. Both pieces land at exactly the same time. The smaller piece lands perilously close to the mortar (at a distance of zero from the mortar). The larger piece lands a distance d from the mortar. If there had been no explosion, the shell would have landed a distance r from the mortar. Assume that air resistance and the mass of the shell's explosive charge are negligible.

"The two exploded pieces of the shell land at the same time. At the moment of landing, what is the distance x_cm from the mortar to the center of mass of the exploded pieces?"

We're supposed to express it in terms of r. I had no problem w/ the second part of the problem, which wants us to express x_cm in terms of D. That was merely (4/5*m*d)/m, since the other component of the center of mass had a position of 0. I'm stumped on how I can approach the problem expressing it in terms of r, however. The hint given was that "The explosion only exerts internal forces on the particles. The only external force acting on the two-piece system is gravity, so the center of mass will continue along the original trajectory of the shell," which I didn't really find helpful. The position of the first piece would still be 0, wouldn't it? So wouldn't that just leave us with (4/5)*r/m ? This, however, is incorrect.

Edit: After thinking about it some more, would x_cm merely be r?
Edit2: Sweet, it is. Thanks for looking.

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