Common velocity after truck starts pulling a car

[nikolafmf]

Homework Statement

Truck is connected to a car with a rope of given length. The truck starts accelerating with a given acceleration and at some moment starts pulling the car. What is their common velocity just after the car starts moving?

Homework Equations

I know how to calculate the velocity of the truck just before the car starts moving. Than I use the conservation of momentum or the conservation of energy.

The Attempt at a Solution

I get different results when I use momentum and when I use kinetic energy. Which one is conserved, which is not and why?

 

[Orodruin]

Of course you get different results. In order for the car to instantly get up to the speed of the truck you have what is essentially a completely inelastic collision between the two.

 

[nikolafmf]

Thank you. So conservation of momentum would be the right thing. I have also found that inner forces in the system are relevant for the change of the kinetic energy of a system, but not for the change of momentum. And I do have change of inner forces in this system.