|MadSci Network: Physics|
It's because your head is too heavy! Any object will topple over if the vertical line through the centre of mass of the object does not also pass through its base. (The centre of mass used to be known as the centre of gravity and is the spot which acts as though the total mass of the object were at that point.) In the actions you describe your centre of mass will start out in your chest as you stand against the wall but as you bend forward and lower your head it will move outwards and downwards. You become unstable and topple over easily. Note, if you touch your toes you don't usually fall over - watch someone try it and see how their backside is pushed out as they bend over to maintain stability. However, the wall prevents you doing this in order to obtain the $1 bill.
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