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.

  • $\begingroup$ Before I offer an answer, I have a question that will help me understand your reasoning. I'm not quite sure how you would identify the forces from the momentum principle. Can you elaborate on that? $\endgroup$
    – garyp
    Aug 31, 2018 at 18:42
  • $\begingroup$ If you ride a motorbike without a backup, you'll easily check that it is the contact force with the backup. There are funny videos out there, like youtube.com/watch?v=0ozeRnlxvFE For a train or bus, it's the friction of the floor, but it will only work with low accelerations. $\endgroup$
    – FGSUZ
    Aug 31, 2018 at 21:31
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    $\begingroup$ Not clear what you are asking. Why is the propulsion of the vehicle itself and all other contents (eg seats, chassis, spare wheel, engine, luggage, etc) not a problem but the propulsion of the passenger is a problem? What is special about the passenger that requires a separate explanation? $\endgroup$ Sep 5, 2018 at 13:44

2 Answers 2


As you said, the engine of the car makes the wheels rotate, and friction with the ground translates that rotational motion into translational motion of the car.

The force that pushes the passenger forward is simply the normal force with the chair, which results from the fact that you can't go through the chair. If you really want to get into the details, it's because, as you move into the chair, you squeeze its atoms closer together. The resulting electromagnetic repulsion between them is what creates the normal force.

On the other hand, the force that pushes a box sitting on the floor of the car forward is friction with the car floor. So it's not always the normal force.


It is the contact force which is responsible in motion of human being in a vehicle. Contact force has two components, one being friction and other being normal reaction. If you are standing in a bus without holding anything then friction between you and the floor of bus will drive you in the direction of motion of bus. Alternatively you can completely avoid friction and use normal reaction by clinging to a pole tightly with your foot off the ground. Using friction can be explained as follows:

Assume yourself to be the block $2$ and vehicle to be block $1$. $F$ on block $1$ can be considered as the driving force of engine of vehicle. Please note that if there was no friction between then blocks, then $F$ will just drive away block $1$ and block $2$ will stay as it is. It's the frictional force between the blocks that is responsible for driving block $2$.

Obviously for the same force $F$, the velocity of block $1$ would be higher if there was no block $2$. Relating to momentum, for a force $F$ momentum will be same in presence or absence of block $2$.


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    $\begingroup$ "In my opinion..." Maybe this is just your phrasing, but on Physics SE, "in my opinion" make it sound like you are expressing belief or guesswork, not the definitive facts that we like to aim for on Physics SE in answers. $\endgroup$ Aug 31, 2018 at 20:32
  • $\begingroup$ Your choice of,the,word,”component”.in describing the contact force is confusing. The friction or the normal forces are not components but forces on their own. Maybe it would better to speak of two parts of the contact force, or suggest it itself is the sum of two forces. $\endgroup$ Jun 23, 2019 at 12:25
  • $\begingroup$ If you do a vector addition of friction force and normal reaction, you will get contact force. So, friction(F) and normal reaction(N) are two components of the contact force($\vec{C}$). Mathematically, $\vec{C}= F\hat{i} + N\hat{j}$ $\endgroup$
    – Jitendra
    Jun 23, 2019 at 12:58

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