The spring of a toy car stores 10 j of potential energy. only 8 j of energy changes to kinetic energy as the car moves. what happens to the remaining 2 j of energy?

It will be converted into thermal energy

Explanation:

In an ideal world, energy can be converted from one  form into another without any loss of energy.

However, in the real world this does not really happen, because there are some frictional forces always acting on the system we are considering. Because of this, part of the mechanical energy of the system is always converted into thermal energy (as heat), and it cannot be "retrieved" back, so it becomes a form of energy that cannot be used anymore.

In this example, the initial mechanical energy of the system is the elastic potential energy of the spring, which is

U = 10 J

Later, when the spring is released, this energy is converted into kinetic energy of the moving car. However, as we are told, only part of this energy is actually converted, which is

K = 8 J

However, the law of conservation of energy states that energy cannot be created or destroyed, but only transformed: this means that the other 2 J of energy have not been actually destroyed, but they have only been converted into another form of energy, which is thermal energy, due to the presence of frictional forces.