They are in contact with the bus--with the air inside. Were they somehow suspended in a vacuum (I don't know how they'd do that), they would indeed hit the back of the bus when it accelerated. But the air is a very important part of the bus and moves along with it; that's the reason you don't feel a breeze on your face when the bus moves forward (assuming the windows are closed).
Learning the sky--one star at a time.
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