# physics

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A 28000 kg railroad freight car collides with a stationary caboose car. They couple together, and 20 percent of the initial kinetic energy is dissipated as heat, sound, vibrations, and so on. What is the mass of the caboose?

• physics -

initial momentum = 28000 u

final momentum = (m+28000) v

so 28000 u = m v + 28000 v

initial ke = 14000 u^2
final ke = (14000+.5m)v^2

(14000+.5m)v^2 = .8 (14000 u^2)
(14000+.5m)v^2= 11200 u^2
so I have these two equations to work with:
28000 u = m v + 28000 v
and
11200 u^2 = (14000+.5m)v^2
or
.784*10^9 u^2 =(m+28000)^2 v^2
and
11200 u^2 = (14000+.5m)v^2
or
u^2 = 1.28*10^-9 (m+28000)^2 v^2
and
u^2 = 8.93*10^-5 (14000+.5m)v^2
oh, my, v^2 cancels
1.28*10^-9 (m+28000)^2 = 8.93*10^-5 (14000+.5m)
there you go, check arithmetic

• Damon -

When I solved for m I got 1711, but It was wrong. Was I close?

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