Peoples Physics Concepts

http://www.ck12.org Chapter 4. Newton’s Laws

Answer:

680 Pa

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1. Why does a sharp knife cut so much better than a dull one?


2. Every day Fakir likes to spend about an hour meditating on his bed of nails. Fakir’s mass is 60 kg, his bed
   contains 2000 nails, and each nail point has a surface area of 4mm^2.
   a. Calculate the total surface area of all the nail points, then convert that area into square meters.
   b. Calculate the pressure exerted on Fakir by the nails. Compare this to your answer from the previous
   question.


3. Estimate the pressure in Pascals that you exert on the ground when you stand in your bare feet flat compared
   to ’tippy toes’. Clearly state your assumptions.

Answers

1. A sharp knife has much less area of contact, so the pressure is greater (pressure is inversely proportional to
   area).


2. a. 0. 008 m^2 b. 75 kPa


3. estimates will vary, but expect about a force five times larger for ’tippy toes’ due to the smaller area. Discuss
   in class.