American Journal of Physics®

Back of the Envelope Problems

Edward Purcell

Classical Mechanics
  1. Tides
  2. Baseball thrown from a spacecraft
  3. Tidal friction
  4. Escaping the earth
  5. Balancing a pencil
  6. Escaping from an asteroid
  7. Spinning a water drop
  8. Value of g at surface of a gold nucleus
  9. Sag of a guitar string
  10. Scattering of a hydrogen ion
  11. Length of the day due to cars
  12. Height of a basketball bounce
Thermodynamics and Energy
  1. Electromagnetic energy in your eyeball
  2. Thermal energy in the Earth
  3. Ratio of food to fuel consumption
  4. Squirting water from a heat engine
  5. Energy released by a burning candle
  6. Energy in a light bulb and in oil
  7. Daily energy from the sun
  8. Temperature of the Earth's crust
  9. Entropy and adding creasm to coffee
  10. U. S. electrical energy use
  1. Airplane Collisions
  2. Baseball batting average
  3. Breathing a molecule
  4. Probability of a line from the earth hitting a star
Biological and Medical Physics
  1. Oxygen supply
  2. Energy to feed a person
  3. Transmitting the information in the human genome
  4. Pump power of the heart
Astronomy and Astrophysics
  1. Galaxy collisions
  2. Cosmic Microwave Background
  3. Opacity of intergalactic space
  4. Amount of starlight
  5. Light deflection from a neutron star
  6. Moon's path relative to the sun
  7. Motion of an electron in the solar system
  8. Probability of a line from the earth hitting a star
  9. Star visibility
Condensed Matter Physics
  1. Einstein-de Hass experiment
  2. How many neutrons can a teacup hold?
  3. Drift speed of an electron
  1. Cosmic Microwave Background
  2. Magnetically lifting a snowflake
  3. Reversal of Earth's magnetic field
  4. Radiation from a basketball
  5. Energy transmitted by a power line
  6. Electrical energy in a capacitor
  7. Drift speed of an electron
  8. Energy stored in the earth's magnetic field
  9. Electrons in a raindrop
  10. Raleigh scattering
  11. Radiation from an electron captures by a proton
Kinetic theory
  1. Free path of a molecule in the atmosphere
  2. Diffusion of air
  1. Helium lifting
  2. Drawing a ribbon through oil
  3. Energy in ocean waves
Optics and Acoustics
  1. Light scattering
  2. Hubble Space Telescope
  3. Photons from a light bulb
  4. Vibration amplitude from a loudspeaker
  5. Camera exposure time
  1. A long wire
  2. Amount of paper manufactured
  3. Gasoline in a cylinder
  4. Electrons in a raindrop
  5. Number of molecules in water
  6. Rate of increase of track left by cars
  7. Upper bound on photon rest mass
  8. Placing the contents of the Library of Congress on a postcard
  9. Planck Mass