Define the term efficiency of a machine. Give two reasons for a machine not to be 100% efficient?

The efficiency of a machine is defined as the ratio of the work done on load by the machine to the work done on the machine by the effort.

$\eta = \dfrac{\text {Work output}}{\text {Work Input}} \\[0.5em]$

It has no unit as it is the ratio of two similar quantities.

A machine is not always 100% efficient due to —

(a) friction

(b) weight of moving parts of machine of a given design.

Due to the above reasons, the velocity ratio of the machine does not change but it's mechanical advantage decreases so its efficiency decreases.