Compared to the idea of a force....

  • Energy is a more fundamental concept
  • But more abstract:
  • Batteries, stretched rubber bands, books on high shelves all have it

Chicxulub impact

The asteroid or comet that hit the Yucatan probably...

  • had diameter of ~ 10 km
  • was moving ~20 km/s (bullet at speed of sound: ~0.3 km/s)
  • hit so hard that the rock and water were instantaneously heated to a temperature hotter than the surface of the sun,
  • Energy released was > than 10,000 $\times$ US + Soviet nuclear stockpiles ($10^8$ megatons of TNT equivalent).

Energy is not an atom or element

Something has energy if...

  • it has the ability to change itself or its environment,
  • change = damage (sometimes)!
  • or, it can do work, which is force $\times$ distance,
  • it can be turned into heat (well, that's a kind of change in environment too).

Heat is energy that raises the temperature of matter.


We might include a tank of gasoline, a battery, water backed up behind a dam, a stretched out piece of elastic material, or a flying brick.

Energy history of humankind

sunlit field

Plants develop photosynthesis

...They are able use solar energy falling on them to synthesize sugars, which can be burned later when the sun isn't shining.



Animals come along that get their energy by eating the grass, or eating each other. Including our ancestors.



Our ancestors spent a good deal of time eating to get enough energy. but a lot of the energy we ate was used in breaking down the cell walls of raw plant matter. Cooking manages to do a lot of that breaking down for us. We can take in a lot more calories in a shorter period of time. And now they had...


More time for other things


Trade develops


Ships are propelled by wind: air stirred up by heat (thermal energy) from the sun.

Coal powers the industrial revolution

Coalbrookdale by night

Coalbrookdale by night by Phillipp Jakob Loutherbourg

The foundry at Coalbrookdale was one of the first coke-fired furnaces. Coke was made from bituminous coal.

Coal (and oil) are the remains of plants and organic matter, containing a large amount of easy-to-extract energy--just light it!

The industrial revolution came about as people realized they could build machines and huge manufacturing processes based on burning fossil fuels - coal. Only the wealthy could afford to build such infrastructure, and benefitted from it the most, ushering in an era of Capitalism.

Chocolate chips vs TNT

Gram for gram, which substance has more energy (per gram)--Chocolate chip cookies or TNT?

How to measure this energy??


Energy per gram

 Calories (~ Watt $\cdot$ hours)
Bullet (moving at speed of sound)0.01
Battery (auto)0.03
ThermalE in water0.05
Battery (alkaline)0.15
TNT (trinitrotoluene)0.65
Modern explosive (PETN)1
Chocolate chip cookies5
Coal, or Ethanol (alcohol) 6
Butter 7
Natural gas (methane) 13
Hydrogen 26
Asteroid @ 30 km / s 100
Uranium 235 20 $\times 10^6$

From Richard Muller, Physics for Presidents.