# Motion after a collision of objects not involved in it [closed]

I'm confused about collisions where there are parts of the objects not involved in it. How do these parts move after the collision? I'll make an example.

Consider a cart with an incline attached (total mass of the two $m_A$) to it and a cylinder (mass $m$) which is free to move with no friction on the cart. Initially the system is moving at velocity $v_0$. Then the cart collides with a second cart (mass $m_B$), initially steady, and the two remain attached. Find the height on the incline reached by the cylinder.

I was quite sure that in such situation the cylinder just continues its motion with velocity $v_0$ but if I try to solve the problem in this way I get the wrong answer.

The final velocity of the two carts is $v_f=\frac{m_A v_0}{m_A+m_B}$

Then I use conservation of energy for the cylinder (in relative motion on the cart) $\frac{1}{2} m (v_0-v_f)^2=m g h$

$h=\frac{m_B^2 v_0^2}{2(m_A+m_B)(m_A+m_B+m)g}$

What is wrong in thinking that the cylinder will continue its motion at constant velocity? It is not involved in the collision (there is not even friction on the cart).

## closed as off-topic by CuriousOne, John Rennie, ACuriousMind♦, user36790, Qmechanic♦Mar 22 '16 at 12:40

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