MadSci Network: Physics
Query:

Re: How does a knife work?

Date: Sun May 14 23:06:49 2000
Posted By: Neil Sandham, Secondary School Teacher, Science Teacher and Career and Technology Studies Coordinator, Chestermere Middle School
Area of science: Physics
ID: 957981721.Ph
Message:

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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