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4.5. Knives


How To Invent (Almost) Anything > 4. Applied Simple Science > 4.5. Knives

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Let us try an example of thinking at the molecular level. If you want to divide something cleanly into smaller parts, then you might use a knife.

So what happens when we cut, when a blade is pressed against another material? If we zoom into the molecular level we can see that the blade is designed to get between the first layer of molecules of the thing being cut and push them apart. Because it is difficult to push lots of molecules apart in one go we tend to slide the knife back and fore, helping the knife to push the molecules apart over a period of time.

But why is the surface so important? Because if we can start a fracture, then it is very easy to propagate it. When we are wiggling or rocking the knife sideways, we are trying to extend the fracture we started at the surface of the thing we are cutting, so that it cracks beyond where the knife blade is sitting.

Fig. 4.8 Cutting action

Think about how different vegetables cut on a wooden or glass cutting board. A carrot will cut well on glass as it fractures. Lettuce needs a wooden board as it is fibrous and the knife needs to slice through all the fibres and hence sink a little way into the board below.

Now that we have recognised the fracturing effect we can think about how we can do this pulling the molecules apart. For example, we can also think about the material if we need to cut. Can we make it softer? Can we make it more fragile? How would using an elastic substance change things? We can also create a pulling force at the edge of the fracture through bending or shearing. By breaking the problem into two parts, starting the fracture and propagating the fracture, we can use a blade to do the starting and bending to do the propagating, as in Fig. 4.9, which is exactly what is done when cutting tiles and glass (where the fracture can also be extended simply by tapping).

Fig. 4.9 Cutting by bending

Let us try a train of thought about cutting with energy. Heat changes the way materials respond to cutting forces. More heat increases the excitation of the molecules, making them push each other away, thus enabling the initial cut, whilst more cold makes fracturing easier. Perhaps, then, if we could heat the surface and cool the interior, it would make the material easier to cut. How could we do this? Maybe by cooling the entire thing, then applying heat through the knife. How could we heat the knife? Friction in the sawing effect happens naturally, but with a different viewpoint, how about eliminating the knife and using only heat, such as from a laser. Lasers heat the molecules so much, they turn them into gas so they are removed completely. If we now think about removing a complete column of molecules, we can look at other methods, such as a water jet or even a simple saw.

How does a pair of scissors work? A few minutes ago it may have been a difficult question, but having just thought about shearing and fractures, it is now easier to understand. Scissors work like a combination of cutting and tearing, with some molecules being moved upwards and some down. Tearing works by a pulling action and scissor cutting by a pushing action, but the direction of forces for both is the same.

Fig. 4.10 Scissor action

Overall, we can think of cutting as ‘fracture management’, where we can think of separating and removing molecules by pushing, pulling, bending and heating different materials. We can also combine some of these, for example if the cutting and separating action is difficult.

What can you think of for inventing which might lead to better ways of separating substances? If you are not sure where to start, begin in the kitchen and work out a better way to crack open an egg!


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