Mathematical Example: Efficiency

An electric motor is used to lift a body of weight l0 N at a height 5m. It uses electrical energy of 65 J

a) What is the energy lost by the motor?

b) Find the efficiency of the motor.


Ans: a) Here, energy used = work done

= Force x displacement

= Weight x height

= 10 N x 5 m = 50 J

Then, The energy lost = Energy supplied — energy used

= 65 J — 50 J = 15 J


b) Effective ƞ = (Effective energy / Total input energy) x 100%

= (50 J / 65 J) x 100% = 76.92%


The efficiency of an engine means that how much of the given energy is obtained as effective energy. So, the efficiency means the ratio of effective energy and the total given energy. Usually the efficiency is expressed in percentage.