The formula for computing the final speed of a perfectly inelastic collision is
m1V1 + m2V2 = (m1 + m2)Vf
in this case the mass are equal so
m1V1 + m1V2 = (m1 + m1)Vf
m1(V1+V2) = 2(m1)Vf
thus
(m1(1000 + 0)) / 2m1 = Vf
Vf = 500 m/s
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