After a long day on the job you get into your car, turn on the ignition, put it in gear, press the gas pedal and – move.
- This motion – this velocity – is caused by an acceleration,
- which is caused by a force (static friction with the ground),
- which is caused by an interaction (the momentum transfer from ground to car as the car pushes backwards on the ground).
- But what starts this interaction? How does pushing and pulling suddenly begin?
The car was inactive, parked, but is now suddenly active and moving. As if there was some motion “stored” in the car waiting to be released.
Maybe this ‘“stored motion”’ is a part of a property we already have? But the parked car has no momentum since there’s no speed, and it doesn’t make sense to talk about “having” forces or acceleration. It seems that ‘“stored motion”’ is some kind of a new property.
So, let’s decide to give that property a name: energy.1 Let’s symbolise energy with $E$.
Where is this energy in the car?
- It is of course stored as fuel, the consumable “stuff” the car needs for causing motion.
- Also the human body can create motion. The energy for this is stored within our bodies as the food we eat.2
- ‘Kinetic Energy, Discussion’ (web page), Glenn Elert, The Physics Hypertextbook, www.physics.info/energy-kinetic (accessed May 23th, 2020)
- ‘A Course of Lectures on Natural Philosophy and the Mechanical Arts’ (book), Thomas Young, Johnson, vol. 1, 1807, books.google.dk/books?id=dmM_AAAAcAAJ&pg=PA78, page 78
- ‘Online Etymology Dictionary’ (dictionary), Douglas Harper, www.etymonline.com
- ‘Different Meanings of the Term Energeia in the Philosophy of Aristotle’ (article), Chung-Hwan Chen, Philosophy and Phenomenological Research, International Phenomenological Society, vol. 17, issue 1, 1956, www.jstor.org/stable/2104687, DOI 10.2307/2104687, page 56
- ‘General Chemistry: Principles, Patterns, and Applications’ (book), Bruce Averill & Patricia Eldredge, Saylor Foundation, 2011, open.umn.edu/opentextbooks/textbooks/general-chemistry-principles-patterns-and-applications, ISBN 9781453322307, chapter 5.4