Posted by Hannah on Thursday, January 17, 2008 at 10:07pm.
A 4kilogram mass has a speed of 6 m/s on a horizontal frictionless surface. The mass collides headon and elastically with an identical 4kilogram mass initially at rest. The second 4kilogram mass then collides headon and sticks to a third 4kilogram mass initially at rest.
The final speed of the first 4kilogram mass is...
The final speed of the two 4kilogram masses that stick together is...

Physics  Damon, Thursday, January 17, 2008 at 10:17pm
If they collide elastically the sum of 1/2 m v^2 after has to equal the sum of 1/2 m v^2 s before and the momentums (m1v1+m2v2) must be the same before and after.
This can only happen if the first one stops and the second one proceeds at 6 m/s.
I will leave you to prove that to yourself using those energy and momentum equalities.
Now the second part.
We have no energy conservation now because it is not elastic.
momentum is still conserved though and the two stick so they become one 8 kg mass after
so
4 (6) + 4(0) = 8 (v final) 
Physics  Hannah, Thursday, January 17, 2008 at 10:21pm
so for the first part... the initial m1v1 is (4kg)(6m/s). the final is what i can't get. is the final 12kg? arent we only concerned with the first mass in that question?

Physics  Damon, Thursday, January 17, 2008 at 10:47pm
In the first part of the problem
a 4 kg mass moving at 6 m/s hits another 4 kg mass
that means
4 kg * 6 m/s + 4 kg* 0 m/s = initial momentum
the final momentum is
4 kg * v1 + 4 kg*v2
so
24 + 0 = 4v1 + 4 v2
or
v1 = 6  v2
then energy
initial ke = (1/2) m v^2 = 2*36 = 72 Joules
final ke is also 72 joules
so
72 = 2 v1^2 + 2 v2^2
36 = (6v2)^2 + v2^2
36 = 36  12 v2 + 2 v2^2
v2^2  6 v2 = 0
v2(v26) = 0
so v2 = 0 or v2 = 6
v2 may not equal zero unless mass 1 goes straight through mass 2
so
v2 = 6
(like I tried to tell you :) The first one stops and the second one takes over at the original speed. 
Physics  Damon, Thursday, January 17, 2008 at 10:52pm
In the second part of the problem, the 4 kg mass suddenly doubles in mass, so it has to go half as fast to have the same momentum

Physics  Damon, Thursday, January 17, 2008 at 10:54pm
That is it, ten before 11, I am turning into a pumpkin ! Or at least I am turning into sleep.