Why is a fork a wedge?
Because a wedge fulfills its purpose by splitting or separating a solid or intact object.
Wedges, simply put, fulfill its purpose by splitting or separating a solid or intact object.
Like all simple machines, wedges use an initial force or action given by one object or person to result in a force that would make it more effective than doing that same action without the machine. This effectiveness of simple machines is given a value known as "mechanical advantage". Wedges are also designed to be like a triangle or trapezoid to make it easier to cut. The acute angle of wedges makes them even more effective.
The mechanical advantage of wedges is the proportion of blade length and blade width.
Length is the edge of the blade to the base.
Width is how long the base is.
From this equation, a narrow, long wedge would be the most effective wedge.
A popular example of a wedge is an ax or a hatchet. They are used to split wood for fires or to cut down trees. When cutting wood, the hatchet has a force that is directed towards the wood. (In this example, let's say downward). Once the hatchet strikes the log, the downward force translates, or transitions, to a sideways, perpendicular force to the sloped blade. Here is a good diagram from Wikipedia on what's going on.
Note that the sideways force is perpendicular to the wedge and not the initial force on the top.
Forks are no different than hatchets or knives. They simply puncture the food and move the food off to the side to make room for itself. In terms of mechanical advantage, they can have the ability to puncture tough foods like steak given enough force. However, tougher materials like wood or steel would require much more force, a narrow base, or a longer, acute blade or prongs from the fork.