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Passengers fall forward when bus suddenly stops, why?

When the bus is still in motion. Your body is also in motion. When the bus suddenly stops, your feet will  have less trouble stopping along with the bus because it is in contact with the bus. However, your upper body falls forward because it is still in motion and did not stop with the bus unlike your feet because it is not in closer contact with the bottom of the bus where the force to stop comes from.