[adrian]

adrian  asked the Naked Scientists:

Hi,

My name is Adrian, I'm from Romania, 27 years old.

I'm in 2006 with your podcasts...Thank you for transforming my walks in enthusiastic journeys, thanks to my mp3 player.

Question: Why do you have to move the knife to cut something? (Is it just for increasing the resistance, pressure of the blade on the contact surface to cut?)
 
Regards from Romania,
Adrian


What do you think?

[lyner]

Hi Adrian.
I think this is something like the right answer.
The pressure under a knife is not uniform. Moving it backwards and forwards will shift the high pressure from one place to another and the cutting will be more effective where there is high pressure. Also, particularly when there are serrations,  the local regions of high stress will be even higher (where the teeth 'catch') because there is extra force, laterally. This will 'focus' the force on the tough fibres of the meat / bread etc. If you just push down, the force is shared between all the fibres but, when you are sawing, the blade moves much further and the fibres which are tightest will get  stretched and get more of the share of  force  and get broken first, the slack ones get broken later.

[chris]

That's an excellent answer and it's also an excellent question. I'd never really thought about it before.

Chris

[SquarishTriangle]

That's interesting. The other day, we were trying to saw through a skull (as you do) with more of a downwards-pressing technique than a shearing one. A lot of effort and getting absolutely nowhere. Our lab guy came around with a little advice: "Long strokes, not hard ones!!" ...problem solved!

[lyner]

Well, Dr Frankenstein, I hope the patient recovered.

[lightarrow]

I think there is also a sawing effect of the micro structures which are always present in the blade.

[SquarishTriangle]

Well, Dr Frankenstein, I hope the patient recovered.

Unfortunately not. He or she (couldn't really tell, being just a head and all) was a little too far gone by the time we got to it.

[SquarishTriangle]

...And I do apologise for the morbid jokes. It keeps me (relatively) sane!

[daveshorts]

I think you have to do it with rubbery or fiberous materials. If you have lots of rubbery fibres be they molecules of wood fibres as you push a knife into them you will bend them rather than breaking them until you are pushing against lots of fibres all together which is very difficult to break.

[diagram=336_0]

Even straight knives are serrated on a microscopic scale and serations will catch small bunches of fibres, just form near the surface, one by one and stretch them until the break - a much easier job than trying to cut them all at once.

[diagram=337_0]

[lyner]

I agree. The serrations on a smooth knife are just smaller. I think it's all about selecting each fibre in turn, stressing it so it snaps / cuts, then moving to the next. They self select in just the way you want; this is inverse sods law folks.
There are some weird plastics used in packaging. If you can start a tear, then they are really weak but you can go blue in the face trying to tear a simple crisp packet once you have actually stretched the plastic. A small 'nick' with a knife and it gives way.

[chris]

I agree. And a recent study on some of the most powerful blades ever created, including the sword of Damascus, showed that the steel contains natural carbon nanotubes, which strengthen the steel in lines, creating natural serrations; these allow the blade to become incredibly sharp but also very strong.

Chris