# Why energy rate did not replace power = Force times velocity?

After reading the history of horse power (and power), the physical definitions for them and after testing the theory in rally races, I'm curious what were the reasons for selecting this word (power) as a "badge" for $$\mathbf F\cdot \mathbf v$$ later called energy rate.

According to https://hsm.stackexchange.com/a/3255/19097 the power might be "ability to act or do." I ask because energy rate is more transparent than "power" if we take into consideration their meanings.

https://en.m.wikipedia.org/wiki/Horsepower

Why wasn't joule/second enough? Why the need for a new unit (watt)? The energy concept is younger than power. Speed is also a rate: m/s or km/h or rpm and so on. Speed does not have an analog in terms of measuring unit.

Example: The car travels at a speed of 10 Names (Name Surname of the guy who never defined this so far).

Found this interesting question that relates to power Etymology of "power" (math.). And this When was the concept of “Power” defined?. So according to https://hsm.stackexchange.com/a/13309/19097, Watt was not aware of energy and he thought that power is force multiplied by velocity.

Also if energy is younger 1840 than power 1770, why didn't joule/second replace the watt? https://hsm.stackexchange.com/a/2554/19097

• Question should not look like revision histories, see meta.stackexchange.com/questions/323116/… Commented Oct 5, 2023 at 14:05
• @Mauricio Thank you for your help. It feels like you need a colledge to know all the rules for asking a question. Commented Oct 5, 2023 at 16:54

## 2 Answers

The equality $$dE/dt=vF$$ can be considered a nontrivial insight, and not simply a definition of the term on the right (ot left). An analogous question would be: in light of the equality $$E=mc^2$$, why do we need the concept of energy since we already know mass and speed of light?

Or maybe I misunderstand the question. There seem to be several ones. One seems to be: why did people call $$dE/dt$$ the power and not just energy rate. There is probably an interesting history to this, but similarly you could ask why people called $$dp/dt$$ the force and not simply the momentum rate.

• I meant why the power word stick even after Joules introduced the energy concept that better describes what power really is (a rate not a force from the common language perception). I guess the human brain prefers this word...power... But you gave a good point. Never thought of it that way. Until now this is the accepted answer... Commented Nov 8, 2023 at 13:54

The number of named units is arbitrary and depends on history, tradition, context, and convenience. One could equally well ask why the SI system has any named derived units, since every measurable quantity can be quantified in terms of the seven base units: metre, second, kilogram, ampere, kelvin, mole, and candela. Who needs the volt, ohm, newton,pascal, hertz, …? On the other hand, why is there no named SI unit for momentum? (There is a suggestion to name such a momentum unit after Emmy Noether.)

The watt and joule were simultaneously proposed by electrical engineer Carl Wilhelm Siemens in 1882, to replace long-existing but arbitrary units: the horsepower and the calorie. I think Siemens recognized that you can't eliminate a common existing unit without replacing it with succinct alternative. The fact that both horsepower and calories are still in common use shows how strong tradition and convenience are.

• Thanks for the feedback. There is one big problem with power: It is missleading. Why use power in thermal engines field when torque curve is the clearer and more accurate attribute for how an engine behaves (because the rotational speed changes)? Energy rate betrayes power for what it realy is. Watt and HP are not. marius-ciclistu.ro/pdf/… Commented Oct 8, 2023 at 5:04
• Also "power transmission" is used in industry. Energy rate CAN'T be transmitted. Energy or Force can. Commented Oct 8, 2023 at 5:07