Farah cuts a tomato with a blunt knife and it squashes
Farah cuts a tomato with a blunt knife and it squashes. Imran hands her a sharp one and it slices under the same press. Imran says she must have pushed harder. Farah says the push was the same and the edge did it. Who is right?
Who do you think is right? Pick one.
The answer: Farah is right
Why
Imran is not imagining the difference: the sharp knife really does cut far better, so 'I must have pushed harder' is a fair guess. What changed is not the force but the AREA it acts on. The same push spread along a wide blunt edge is gentle at every point; squeezed onto a thin sharp edge it becomes an enormous press on that one narrow line, and it is press, force per unit of area, that cuts. Picture standing on soft ground in flat shoes and then on a single narrow heel: the same body weight, but the heel sinks because all of it lands on a tiny patch. Test: push the blunt knife with both hands as hard as you can and it still mostly crushes; more force, wrong shape. Farah is right: a sharp edge concentrates the same force into far more pressure.
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