CBSE Class 9 Answered

Wind up toys have a spring inside it which gets wound up with the help of winding keys. So when we wind-up the spring of a toy car by using a winding key, then some work is done by us. As a result, the spring gets coiled up more tightly. 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). When we release the winding key the spring tries to attain its original shape. During this process, the potential energy stored in it gets converted to kinetic energy. This turns the wheels of the toy car.