Science & TechMental model

How many piano tuners are in Chicago? Work it out.

Enrico Fermi estimated unknowns by breaking them into quantities he roughly knew: population, households, pianos per household, tunings per year, working days per tuner. Each guess might be off by a factor of 2, but errors up and down partly cancel. The result is usually right to within an order of magnitude.

Useful: Before you google or believe a number, estimate it yourself in five steps; you'll spot absurd claims instantly.

Source: en.wikipedia.org

Denkwerkzeug MathematikSchaetzen

Difficulty:

Related cards

Check the units before you trust the number.Every meaningful physical equation has the same units on both sides: metres stay metres, seconds …
Fold a sheet of paper 42 times and you reach the Moon.A sheet of paper is 0.1 millimetres thick. Every fold doubles the thickness. After 10 folds it's 10 …
23 people in a room, and two probably share a birthday.With just 23 people, the chance that two share a birthday already exceeds 50 %. With 50 people it's …
Three doors, one car: why switching doubles your odds.A car sits behind one of three doors. You pick one; the host, who knows where the car is, opens …
Why giant movie ants would collapse on the spot.Double a body's size and its surface area quadruples, but its volume and weight increase eightfold. …
How to count to 31 on one hand.In binary, each digit is worth twice the one before it: 1, 2, 4, 8, 16. The number 1011 means 8 + 0 …
Magnitude 7 isn't "a bit stronger" than 6.The magnitude scale is logarithmic. One step up means ten times the ground motion and about 32 …
Evolution wants nothing. It only sorts.A common mistake: species "develop" traits because they need them. In reality, variants arise at …

Keep reading

This card belongs to the topic Science & Tech. Every card is short, cites a source and can be saved – due ones come back by themselves later.

About Doomlearn · Projects · Legal notice · Privacy · Terms