EXPLOSION please help! I got the right answers for 1 and 2 but I don\'t understa

Question

EXPLOSION please help! I got the right answers for 1 and 2 but I don't understand 3 and 4

An object with total mass mtotal = 6 kg is sitting at rest when it explodes into two pieces. The two pieces, after the explosion, have masses of m and 3m. During the explosion, the pieces are given a total energy of E = 42 J.

1) What is the speed of the smaller piece after the collision? (Answer is 6.48 m/s)

2) What is the speed of the larger piece after the collision? (Answer is 2.16 m/s)

3) If the explosion lasted for a time t = 0.025 s, what was the average force on the larger piece?

4) What is the magnitude of the change in momentum of the smaller piece?

Explanation / Answer

(3) if the explosion lasted for a time t = 0.025s

    the average force on the larger iece is

                      F*t = (3m)(V)

                        V = 3v = 3(6.48) = 19.44 m/s

              F(0.025) = 3(1.5)(19.44)

                         F = 3499.2 N

(4) change in momentum for smaller>

                            (1.5)(6.48) = 9.72 kg m/s

velocity of the center of mass = 0

Again by conservation of momentum
momentum of the system is always 0 the momentum for a system of particles is the total mass of the system
times the velocity of the center of mass, since there is mass velocity of center of mass = 0 m/s

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