Imagine you are asked to push a heavy box across the floor. at first you need to push very hard but once the box has started move you can push less strongly yet still keep it moving. why is it

At first we need to push very hard but once the box has started move we can push less strongly yet still keep it moving because there is a force of static friction which keeps the box at rest but once we overcome static friction it becomes relatively easy for us to move it because now we only have to enconter sliding or rolling friction which is much less than static friction. In fact, static friction has the highest magnitude.

A. to determine if humans can explore mars safely