This is a ringing that sets in after excessive noise exposure, such as in a rock concert (being too near the speakers) or the explosion of a firecracker. This excessive noise can cause injury to the ear, causing the ringing. The tinnitus will usually clear up on its own, and will last anywhere from minutes to hours.
Mechanical digestion is the actual physical break down of food into smaller pieces. One example of this is mastication (chewing) which occurs in the mouth by the teeth, which act to cut and grind food into smaller pieces. This makes them easier to later digest as it increases the SA of the food molecules.
Mechanical digestion is the physical breaking down of food particles into smaller food particles to increase surface area for chemical digestion. Chemical digestion is the use of enzymes to break down polymers into monomers (through the breaking of chemical bonds)