# Another way to tell engineers from physicists (not a joke) **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [November 13, 2013, 8:11pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852 "2013-11-13T20:11:47Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [November 13, 2013, 8:11pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/1 "2013-11-13T20:11:47Z")
I was reading an article about automotive fuel economy and an engineer was making the point that to conserve gas, minimize acceleration because acceleration uses a lot of fuel. It occurred to me that a physicist would say to minimize braking, because braking converts kinetic energy into heat. They are both correct, and probably the more you brake the more you have to accelerate. But one focuses on when you’re using the fuel, and the other focuses on when you’re throwing away energy. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [November 13, 2013, 8:54pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/2 "2013-11-13T20:54:01Z")
I don’t see that either discpline would focus exclusively on one fuel economy approach or another. FWIW, I once read an SAE paper that analyzed fuel economy, and found that good fuel economy for urban driving correlates strongly with “microtrip” time. A “microtrip” was defined as travelling from one stopping point to another, e.g. the light turns green, you advance to the next traffic light, you have just completed one “microtrip.” Your entire journey from start to finsh would be composed of a series of microtrips. Long microtrip times are had by gentle accelerations, modest speeds, and (consequently) the least possible braking requirement at the end of each microtrip. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [November 13, 2013, 8:55pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/3 "2013-11-13T20:55:53Z")
I don’t see that either discpline would focus exclusively on one fuel economy approach or another. Why wouldn’t both of them look at all the relevant factors? FWIW, I once read an SAE paper that analyzed fuel economy, and found that good fuel economy for urban driving correlates strongly with “microtrip” time. A “microtrip” was defined as travelling from one stopping point to another, e.g. the light turns green, you advance to the next traffic light, you have just completed one “microtrip.” Your entire journey from start to finsh would be composed of a series of microtrips. Long microtrip times are had by gentle accelerations, modest speeds, and (consequently) the least possible braking requirement at the end of each microtrip. ---
**Author:** ![Finagle](https://avatars.discourse-cdn.com/v4/letter/f/b19c9b/32.png) [@Finagle](https://boards.straightdope.com/u/Finagle)\ **Post date:** [November 13, 2013, 9:25pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/4 "2013-11-13T21:25:52Z")
> [@CookingWithGas](#): > > I was reading an article about automotive fuel economy and an engineer was making the point that to conserve gas, minimize acceleration because acceleration uses a lot of fuel. > > It occurred to me that a physicist would say to minimize braking, because braking converts kinetic energy into heat. Well, a physicist would be modeling the car as a frictionless sphere rolling along in a vacuum which doesn’t correlate well to my driving experience. ---
**Author:** ![runningdude](https://avatars.discourse-cdn.com/v4/letter/r/e9c0ed/32.png) [@runningdude](https://boards.straightdope.com/u/runningdude)\ **Post date:** [November 13, 2013, 9:26pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/5 "2013-11-13T21:26:32Z")
> [@CookingWithGas](#): > > I was reading an article about automotive fuel economy and an engineer was making the point that to conserve gas, minimize acceleration because acceleration uses a lot of fuel. > > It occurred to me that a physicist would say to minimize braking, because braking converts kinetic energy into heat. > > They are both correct, and probably the more you brake the more you have to accelerate. But one focuses on when you’re using the fuel, and the other focuses on when you’re throwing away energy. Braking is a form of acceleration ( specifically negative acceleration/deceleration). In your example, both the physicist and the engineer are advocating for the same thing - avoid acceleration! ---
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [November 13, 2013, 9:30pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/6 "2013-11-13T21:30:00Z")
> [@Machine\_Elf](#): > > I don’t see that either discpline would focus exclusively on one fuel economy approach or another. Why wouldn’t both of them look at all the relevant factors? > > FWIW, I once read an SAE paper that analyzed fuel economy, and found that good fuel economy for urban driving correlates strongly with “microtrip” time. A “microtrip” was defined as travelling from one stopping point to another, e.g. the light turns green, you advance to the next traffic light, you have just completed one “microtrip.” Your entire journey from start to finsh would be composed of a series of microtrips. Long microtrip times are had by gentle accelerations, modest speeds, and (consequently) the least possible braking requirement at the end of each microtrip. You can say that again! 😛 ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [November 14, 2013, 4:21am UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/7 "2013-11-14T04:21:56Z")
> [@runningdude](#): > > Braking is a form of acceleration ( specifically negative acceleration/deceleration). In your example, both the physicist and the engineer are advocating for the same thing - avoid acceleration! Yes, but only the physicist is likely to use the word “acceleration” in that manner. An engineer, being more practical, would use “deceleration,” as that’s the term most people use, and thus would accomplish the advice job more easily. ---
**Author:** ![runningdude](https://avatars.discourse-cdn.com/v4/letter/r/e9c0ed/32.png) [@runningdude](https://boards.straightdope.com/u/runningdude)\ **Post date:** [November 15, 2013, 6:09pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/8 "2013-11-15T18:09:51Z")
> [@BigT](#): > > Yes, but only the physicist is likely to use the word “acceleration” in that manner. An engineer, being more practical, would use “deceleration,” as that’s the term most people use, and thus would accomplish the advice job more easily. I am an engineer, lol… :dubious: ---
**Author:** ![Procrustus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/procrustus/32/2994_2.png) [@Procrustus](https://boards.straightdope.com/u/Procrustus)\ **Post date:** [November 15, 2013, 6:11pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/9 "2013-11-15T18:11:51Z")
An economist would simply _assume_ a car that got better mileage. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [November 15, 2013, 6:52pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/10 "2013-11-15T18:52:02Z")
The physicist might also need to consider acceleration part of the equation in terms of the efficiency of the engine at turning chemical energy into kinetic energy. The brakes are clearly 100% efficient in their conversion, but different acceleration methods might lead to different fuel consumption for the same increase in kinetic energy. ---
**Author:** ![Marion\_Wormer](https://avatars.discourse-cdn.com/v4/letter/m/d07c76/32.png) [@Marion\_Wormer](https://boards.straightdope.com/u/Marion_Wormer)\ **Post date:** [November 16, 2013, 6:07am UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/11 "2013-11-16T06:07:41Z")
Perhaps they would gather their data, form their hypothesis, then turn the whole problem over to operations research who would analyze population distributions, male driving technique vs female technique, tall vs short, hilly vs flat, city vs highway then do a linear program to come up with a solution that works most of the time. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [November 16, 2013, 1:21pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/12 "2013-11-16T13:21:28Z")
I’d hope one of them would ask why anybody expects gentle forward acceleration was better. In the case where the top speed is constrained (say, to be 55 mph because we’re on a highway), then the acceleration we use to achieve it should be using the engine at a speed and torque where its fuel use is most efficient, not smallest. For typical engines I think that’s at a major fraction of full power. In other words, the engine is definitely going to have to come up with the mechanical energy that the car stores in its motion at top speed, and it should do so at the most power efficient point on its curve, which IIRC is often at around 3/4 of full power. ---
**Author:** ![Bosda\_Di\_Chi\_of\_Tricor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bosda_di_chi_of_tricor/32/16845_2.png) [@Bosda\_Di\_Chi\_of\_Tricor](https://boards.straightdope.com/u/Bosda_Di_Chi_of_Tricor)\ **Post date:** [November 16, 2013, 1:41pm UTC](https://boards.straightdope.com/t/another-way-to-tell-engineers-from-physicists-not-a-joke/673852/13 "2013-11-16T13:41:07Z")
> [@](#): > > Another way to tell engineers from physicists Just look for that tattoo behind their left ear–engineers have a bar code, scientists are in Binary.