Knives. Kinetic energy. Heat. And the non-fiction tale of how blades can cook food.
1-Minute NomNom
Can the science that created a problem become the solution itself? This might sound too good to be true, a tale of fiction, but it’s not. It is instead a great tale of science friction!
When the weather or an air-conditioned room gets too cold, what is the instinctive thing that many of us will do? We rub our hands together to warm them up. The action generates heat because the two hands are trying to prevent each other from moving. We call this friction. Friction is what converts the moving hands’ kinetic energy (the energy that any moving object has) into thermal energy.
Friction is why food can get warm when we use a blender to chop it up. The friction created when we cut vegetables or meat with a knife is usually too little for us to even notice. But that is not the case with the blender.
The blades of the blender have a lot of kinetic energy as they rotate at high speed. The food, on the other hand, starts out stationary, and hence opposes the movement of the blades. This can create a lot of thermal energy. In some cases, the heat can change or destroy the nutrients in the food, or even cook the food.
This is in fact one of the reasons cold-pressed juice manufacturers give as to why they prefer to use cold pressing instead of blending to make their juices (read all about cold-pressed juices in the 1-Minute NomNom “Hard-pressed to give more“).
Friction does not just generate heat, but it can also cause wear and tear of the surfaces being pushed together. The heat generated by the friction of a knife cutting a piece of vegetable might be negligible, as pointed out earlier, but the effects of friction do build up over time. This is what causes the sharp edge of the knife to become blunt. You can see some cool microscope images of blunted knives at this website by Midwood High School in Brooklyn.
Happily, what caused the problem in the first place can be used to solve the problem. We can use friction to sharpen our knives. When we set our knives’ edges against a whetstone or another knife, the friction of two hard surfaces removes the jagged bits that have damaged the blade. What we get in the end is a sharpened knife, almost as good as new!
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