This question arises after much debate with some fellow engineering students and we are looking for somewhat of a theoretical answer more than anything.
When you are sitting in a train or a car, what is the actual phenomena that propels the passenger in the direction of motion? We all agree that once the combustion process takes place this drives the piston motion which eventually results in rotational movement through the axle which eventually turns the vehicle's wheels. This pushes the vehicle forward due to the presence of friction which acts against the forward applied force (otherwise their would just be slipping).
Our confusion seems to arise here. How does this motion get translated into the actual human being propelled forward? Obviously we need friction in the cabin of the vehicle or else the human would not move with the movement of the train.
But does this turn into a momentum problem in order to explain the phenonmena on how the human is propelled forward? Some people think it is frictional force that propels the human forward, but from my perspective this does not seem to make sense and can be best explained through the conservation of momentum.
Any Newtonian explanations on this matter would be greatly appreciated. FBD's would be fantastic as well.