A figure skater is spinning on ice with their arms extended. When they pull in their arms, their rotational kinetic energy increases. What is the source of this energy?
Solution: E. It is not correct to attribute this increase in kinetic energy to external work because the forces doing work are inside the person. The kinetic energy comes from the person's metabolic processes, a transformation of internal chemical energy into rotational kinetic energy (and some thermal energy as the transformation isn't 100% efficient).
If we look further back, then we can assert that the energy begins as nuclear energy from the sun (which is eventually transmitted and transformed into chemical energy in plants).