Why does a person lean forward when carrying a heavy backpack?

Answer 1

A person naturally stands in a fashion that places there center of gravity over there feet and just forward of there ankles. For example, It is common to see many pregnant women leaning back because of the extra weight of the fetus in the front of there body. For the same reason back packers lean forward to counter act the extra weight of the pack on there back. How far one leans is determined by how heavy the pack is. :)

Answer 2

Because the fulcrum where the vast majority of the weight (load) is resting is in the lower back area, by leaning forward the overall centre of gravity of the individual (the person + load) is thereby centred, resulting in less strain on the lower back. In addition, if standing up straight the person becomes 'back heavy' making it harder and slower to walk.

Answer 3

because you have to put more force on denser things to make them move