Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice test

What happens to the mechanical energy of an object when internal work is done upon it?

1. The mechanical energy increases significantly

2. The mechanical energy remains unchanged

3. The mechanical energy is converted to thermal energy

4. The mechanical energy decreases

The correct answer is: The mechanical energy remains unchanged

When internal work is done on an object, the mechanical energy of that object remains unchanged because the work does not contribute to an increase or decrease in the object's total mechanical energy, which includes both kinetic and potential energy. Instead, internal work redistributes energy within the system but does not alter the overall energy available for mechanical work. For instance, when a person pushes on a compressed spring, while energy is being used to compress it (internal work), the total mechanical energy is conserved in the system. Thus, the potential energy of the spring increases, yet the total mechanical energy does not increase as a result of the mechanical work done — instead, it reflects a transformation rather than a net gain or loss.