 So here we have a YouTube video of a car crash. So we have the car come speeding towards the barrier until it hits it and then the front of the car starts to crumple and you get all these shock waves flowing backwards through the car. So the key thing is the back of the car will not stop until those shock waves reach it. So it's a bit more complicated than that. All the momentum needs to flow somewhere, all the energy needs to be dissipated, but the key point is if you have a car that gets into a crash, the back of the car will not stop moving until some sort of a signal from the front reaches it. Normally it's in the form of shock waves through the metal, but you need some sort of signal to reach it and apply a force causing it to stop. Until that signal reaches the back, it's almost as if it doesn't know the crash has occurred and it'll just keep going forwards. So now let's look at Alice the crash test dummy in her limo speeding towards an adamantium wall or rather in Alice's frame she is stationary while the wall flies towards her. So the wall will hit the front of the car and at this point it will start sending shock waves through the metal. So we know that the car will crumple into the wall of it or in Alice's frame of reference the wall will move forwards crumpling the car until eventually the shock waves reach the back of the car and cause it to come to a halt or in Alice's frame of view once the shock waves reach the back of the car the wall will stop having crumpled say the front part of the car. So let's stick in some concrete numbers. So Alice's limo is seven meters long in her reference frame and the garage has been contracted to a length five over gamma. Now let's suppose Alice is driving so fast that gamma is equal to two. In this case what is Alice's velocity?