The work done in winding the spring gets stored in the wound up spring in the form of elastic potential energy (i.e potential energy due to change in shape). ... During this process, the potential energy stored in it gets converted to kinetic energy. This turns the wheels of the toy car.
Both elastic potential energy and kinetic energy at t initial are converted to internal energy t f.
Explanation:
Apply the principle of Conservation of Energy to the system, where friction is present. Between t i and t f, the elastic potential energy in the spring and the kinetic energy of motion are being transformed into internal energy.
(I can't put t and f together because it got censored so that's why there is a space. Usually they are together.)
