Materials that combust, such as gas and wood, contain chemical energy. This energy can be transformed into mechanical energy with a change in the chemical itself. Humans use sugar to provide them with energy to allow their body to work.
Cars and jet skis both have combustible engines. The fuel in these engines is used to provide energy for the car and jet skis to operate. This is done with the engine converting chemical energy into heat. Once transformed into heat, the heat is turned into mechanical energy.
All of the given examples can be converted from chemical energy to mechanical energy. Human beings usually need sugar and other types of food in order to become energized. Once they become energized, this can be transformed into mechanical energy that will allow human beings to move.
Cars and Jetskis normally use fuel in order to be powered up. There are different mechanisms within these machines that will transform the chemicals into mechanical energy so that these machines will work properly. It can be a problem when energy is not transformed because they will not reach what the desired action or outcome is.