Physics of Cutting?

[hlong69]

Gents, ladies, I have a question regarding the physics behind cutting.

I have a good grasp of the concept of forces, and how a material has a breaking force, and if you apply a force (mXa) exceeding it's rating, it will break.

My question is, why does a drawing or pushing motion cut an object, when applying the same force directly down on it does not? For example, I can press a razor sharp knife into my hand and it's won't cut me (unless I am really, really pressing), yet a lite drawing motion will cut, why is this?

I don't think I understand what underlying physics are involved here, and would be interested if someone could explain it to me, and even better, provide me with calculations or the forumla behind it.

I have asked this question on other boards and have never got a satisfactory answer - next stop is Billy Nye

Thanks in advance for any help!

Hugh Long

 

[John L]

I think that the sliding of the micro serrations do the cutting much like the teeth of a saw. ???

 

[rlinger]

because of friction. That is your answer. In further going, the cutting tool need be harder than the work piece being cut. Therefore it can cut because it is less effected by the friction necessary to acheive the cut. A smaller cross section cuts more effeciently than a larger because less force is required to make the frictional cut. If a diamond large enough 2 X 4 pine wood can cut through it if enough of it available and enough energy and time expended to do it. Friction is your answer.

RL

 

[Stacy E. Apelt - Bladesmith]

The reason the knife doesn't cut your hand when pressed flat against your palm is due to the force being applied over the entire contact area.Also,skin is very pliable and forms to the exact shape of the edge.This spreads the force out so the edge won't cut you until you apply too much pressure and overcome the friction of the skin on the edge.This is the same thing as the load pressure for floors.If you put a water bed that has,say 1000 pounds of water,on the floor, the force is spread out over about 40 sq.ft. that is only about .3PSI.If you put all that weight on one sq,in. it would go right through th floor.When you slice with a knife the cutting action is then applied to each spot of the edge simultaneously,thus the cut.All the edge has to do at this point is to overcome friction ,which is done by the extra energy supplied by the moving edge..The smoother and thinner the edge ,the easier it overcomes friction and starts cutting.
RL,you are right about friction,but wrong about wood cutting diamonds.The harder material ALWAYS abrades the softer one.With materials of near same hardness,and ones with weak atomic/molecular bonds,there can be some shearing ,due to friction,that causes wear on the harder substance.With an example of Diamond and Pine the differences are far too vast for the wood to make any wear on the diamond.