You are trekking on a mountain with a backpack on. Your shoulders exert a force on the backpack, and this force does wor

[answerhappygod]

You are trekking on a mountain with a backpack on. Your
shoulders exert a force on the backpack, and this force does work.
This work is ________________
zero both when walking up and when rwalking down the
mountain.
negative when you walk up the mountain and positive when you
walk down
positive when walk up as well as hen you walk down the
mountain.
negative when you walk up as well as when you walk down the
mountain.
positive when you ride up the mountain and negative when you
walk down the mountain.