Don't be too sure on what is going on in the bus. For instance, if you have a helium filled balloon tethered by a string floating in the middle of the bus and you brake hard, does the balloon go forward, go backward, or stay in the same place? Most people think it will move forward, because they tend to feel that force on their bodies. Others think it stays in the same place, because they perceive the air to be static. But if you actually do the experiment, you will find the balloon moves toward the rear of the bus when you brake. This is because the ballon is lighter than air, and the heavier air moves forward, while the lighter air moves to the rear. But you ask how can some air be heavier than other air? Well some air is warmer than other air....