# 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?

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.

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.