Before revision: If is the power lost to friction, from conservation of energy The mechanical efficiency of a machine (where ) is defined as the ratio of power out to the power in, and is a measure of the frictional energy losses As above, the power is equal to the product of force and velocity, so Therefore, So in non-ideal machines, the mechanical advantage is always less than the velocity ratio by the product with the efficiency η .
After revision: If is the power lost to friction, from conservation of energy The mechanical efficiency of a machine (where ) is defined as the ratio of power out to the power in, and is a measure of the frictional energy losses As above, the power is equal to the product of force and velocity, so Therefore, So in non-ideal machines, the mechanical advantage is always less than the velocity ratio by the product with the efficiency .