Question

Calculate the efficiency of the following devices:
A lawnmower which converts 143 J out of every 450 J of electrical input energy into sound and heat. The rest of the energy is converted to kinetic energy of the blades attached to the motor.

Answer 1

The useful work done by the lawnmower is: $450J-143J = 307J$. By definition, the efficiency is the ration of the useful work and total input energy. Thus:

$$\eta = \dfrac{307J}{450J}\times 100\% \approx 68.2\%$$

Answer. 68.2%