MadSci Network: Physics |
A knife works because it is a combination of two of the six simple machines. The 6 simple machines which make up most machines are; levers, pulleys, wheels-and-axles, screws, ramps (or inclined planes) and wedges. These simple machines provide either a force advantage (making it easier to move a mass) or a speed advantage (moving the mass over a greater distance than the effort force travels). A knife works as a combination of a wedge and a class 2 lever (depending on how you are using it). The blade of the knife is an excellent example of a wedge, which is like a double-sided ramp. Each side of the knife blade carries part of the load (whatever mass you are cutting) along it. The longer the ramp (or wedge) the easier it is to move that load. Since both sides of the blade are sharpened (making them ramps) both sides help to move the mass along the edges of the blade. This helps you to move the sharp edge of a blade through an object fairly easily. The sharper the knife, the more defined the wedge, and the easier it is to cut things with. By resting the point on a cutting board, placing the object to be cut under the blade near the point, and pushing down on the handle, you are also using the knife as a class two lever. A class 2 lever gives the user a force advantage by transferring most of the load of the object being cut to the point of the knife, instead of along the length to the handle (where you apply your effort force. This is how famous TV chefs (like Emil; "BAM!") are able to chop vegetables with those lightning fast moves. They are using the knife in the most efficient way possible (they actually teach you these things in most Chef-Prep courses). By combining this technique with the wedge shape of the blade, you get twice the force advantage! A knife with a serrated edge (like a steak knife) works a little differently. Each serrated portion acts as a wedge in a stronger format (the arch is the strongest shape known to science). This not only makes the knife blade more rigid, but it also increases the surface area of the wedge. This decreases the amount of force needed to cut through the object compared with what you would need with a regular kife.
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