Hey Zara, not sure if you are looking for this answer. What you are talking about is the 1st law of motion, i wont give you the definition verbatim cos i am geeky enough. It is simple down to the matter of you moving in a uniform motion with the bus. Uniform meaning same direction, same speed. (hope this doesnt come across in a condescending tone) So when jump up the only force affecting you is gravity and this only affects up and down movements. Once there are no other external factors affecting you or the bus when you jump up you will land in the same place. But, lets say the bus slows down, hits a sticky bit of road or turns just as you jump you will not land in the same place as an external force has stopped from travelling in a uniform motion with the bus.
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Hope this helps from the theory melting your brain.....
or dont tell me, you posted this knowing the answer and you just taking the piss.....