Zipper Knife

quest#1: why is mechanical work zero when we carry a heavy load across a path?

quest#2: can a negative mechanical advantage value exist?

If the force is going across the path, it is perpendicular to the path and essentially has no effect on the movement.

ex: if we slide a book across the table, gravity is acting across of perpendicular to the path because gravity is acting downward and the motion is vertical. So in this case gravity has 0 work on the book because it does work in the direction of motion

quest2
Using the gravity example, if I were to lift the book off the table, the motion of the book is going against gravity and therefore gravity is doing negative work. This is because the motion is opposite of the force.

how to sew a zipper. creating a knife edge pillow.