Apulley system has an efficiency of 81.8%. if you perform 1100 j of work,
how much useful work does the pulley perform?

The useful work performed by the pulley is, E = 899.8 J

Explanation:

Given data,

The efficiency of a pulley, Ef = 81.8 %

The work input, = 1100 J

The main purpose of the pulley is to change the direction of the force applied.

The formula for useful energy,

                                     

                                     

                                     E = 899.8 J

Hence, the useful work performed by the pulley is, E = 899.8 J

Ascencion means gain in potential energy and descing is the opposite, hence when you lift a book, you apply a positive work."
"You lose energy where as book gains energy. Therefore, positive work on book, negative on you."