Question

Give 2 reasons for a machine not to be 100% efficient

Answer 1

Most simply, because nothing is 100% efficient.

Okay, got that out of the way... Now, practically, why are the simple machines not 100% efficient?

We do a lot of our work with "points" and "spherical cows " to simplify things. This helps us understand the problem/mechanics, but leaves out many of the "dirty" fundamental truths.

These fundamental truths come in many varieties, but in this specific case, The answer isfriction...

Think about it:

a wheel and axle - friction of the rotating member against the non-rotating member (even the wheel against the ground). A pulley is just an extension of a wheel - friction between the rope and the pulley, more between the axle and the block.

An inclined plane - friction between the object on the plane and the plane itself. A screw is a special case of an inclined plane.

A wedge - again, friction (and this is very handy for door stoppers)

A lever - friction against the thing being moved, even with a fulcrum.