Posted by Tully on Sunday, March 16, 2014 at 3:19pm.
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  Damon, Sunday, March 16, 2014 at 3:39pm
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  Tully, Sunday, March 16, 2014 at 4:40pm
When I solved for m I got 1711, but It was wrong. Was I close?