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You are most aware of motion in a moving vehicle when it is accelerating. When you are in moving vehicle- which is moving steadily in a straight line then essentially you body is also travelling with the same velocity of the moving vehicle so you won't feel any additional force applied on you. Where as when the moving vehicle is accelerating which means there is a change in velocity of the vehicle, then your body has to overcome inertia to gain the velocity of accelerating vehicle. This causes force on your body, which is felt by you.
Note: Inertia is the resistance of any physical object to any change in its state of motion including changes to its speed and direction or the state of rest
B. If car moves with absolutely constant velocity (nobumps at all) then you would not be aware of motion assuming that you have closed your eyes (i.e. not watching external objects through car windows) and the car windows are closed such that you don't feel the wind blowing. This is because your body has already overcome the inertia when the car accelerated from state of rest to constant velocity. Once the car and your body attains certain velocity which is constantly maintained, then there is no additional force applied on your body. Hence you would not be aware of motion.
Please let me know if you need any clarification. I'm always happy to answer your questions.
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