Final answer:
The missing 10-J from the slingshot's initially stored potential energy were transformed into sound and thermal energy due to friction and other energy transformations, adhering to the Law of Conservation of Energy.
Step-by-step explanation:
When a rock is fired from a slingshot, 100-J of potential energy was stored in the stretched rubber band. If the calculated kinetic energy of the rock after being fired is only 90-J, the other 10-J were transformed into other forms of energy such as sound and thermal energy. During the release, some energy is inevitably lost due to factors like air resistance, internal friction within the slingshot's band, and sound produced during the release.
In physics, the Law of Conservation of Energy states that energy cannot be created or destroyed, only transformed from one form to another. Therefore, the missing 10-J in this case has not disappeared, but has been converted into other forms of energy, such as the thermal energy from the friction of the rubber band and the sound energy of the snap that is heard when the rock is fired. This explains why the rock's kinetic energy is less than the initial potential energy stored in the rubber band.