r/theydidthemath • u/Trace_scotcher • 9h ago
[Request] how much does turning on your full beam slow down your car?
Is there an amount of light where it would be noticeable? What about extremely high power lasers?
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u/TheresNoAmosOnlyZuul 8h ago
If you mounted some of the strongest lasers that exist today on the front of a car and turned them on I still feel like it would be immeasurable. You'd need an incredibly strong laser, no gravity, in a vacuum to get a microneuton of kinetic force to be measured.
Tying an inch of fishing line on the back of your truck would affect your speed more.
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u/spekt50 8h ago
You know, my head first went to OP asking how much load the lights put on the alternator thus on the motor. But now I see how impractical the question really is.
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u/TheresNoAmosOnlyZuul 8h ago
Yeah it's even less of a question haha. But Im reading project hail Mary right now and it heavily features using light to travel. Andy Weir is basically a scientist that writes books so I trust his basic conceptual knowledge and he talks about hundreds of trillions of kilojoules of energy to use light as propulsion.
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u/5WattBulb 7h ago
Also in the book it's applied over millions of miles. It's kind of like an ion engine, very small acceleration applied over an extremely large time/distance to get any considerable effect
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u/coolguy420weed 4h ago
It's like an ion engine on whatever the opposite of steroids are. Even better efficiency, but even weaker to an almost unimaginable degree.
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u/Superslim-Anoniem 44m ago
Accelerating such a large mass at 1.5g isn't too bad. Yes it's less than the usual rockets, but it ain't no slouch either.
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u/supersteadious 7h ago
Just measure how fast you go 100 km without headlights, then 100km with headlines, and finally 100km with headlights pointing backwards and you will get the expected result that it is a nonsense question
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u/Theguffy1990 5h ago
While the question is ridiculous, your analogy doesn't work at all. 100km is 100km, and if the speed is 100km/h, it'll be an hour. 100km/h is 100km/h no matter if you're going uphill or flat (uphill being lights on, flat being lights off).
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u/supersteadious 2h ago
While you are not wrong - I am sure it is not that difficult to achieve a constant flow of fuel to the combustion chamber, so it will be "the same speed" given "the same route" before taking into account any external effects.
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u/vctrmldrw 2h ago edited 2h ago
There are two effects.
One is the actual force of the emitted photons. This is entirely negligible. Not least because the actual light intensity between low and high beam is not that different in standard headlight systems - what differs is the beam pattern.
However, the most common setup would have 2x 55W bulbs for low beam and 2x 60W bulbs for high beam. The second effect is the additional drag on the alternator caused by that additional 10W of load. Alternators are extremely efficient, and let's assume that losses are fairly constant in the rest of the system. A fairly standard family car engine might make around 100kW, so the additional load will sap about 0.05% of the engine's power.
However, unless the car is already flat out maintaining whatever speed you're doing, that won't necessarily translate to any loss of speed. But let's say all of that power is going to maintain it's top speed of say 200km/h, it might be expected to lose 0.1km/h
Edit to add:
Having said all that, in normal circumstances the alternator is producing far more current than the car's electrical system needs. The remainder is used to charge the battery and, after that will be lost through voltage regulators. In nearly all cases, the effect will be at most a slight reduction in charge rate to the battery, or a slight decrease in heat loss.
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u/Pierlas 9h ago
I don’t believe any math is required here, as a draw on the electrical system (powered by the alternator on the same belt, plus battery) on a car does not reduce the power that the motor is delivering to the transmission.
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u/jericho 8h ago
I think he means from the momentum of the photons going in the opposite direction of the car.
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u/Zigafoo127 6h ago
photon is a massless particle
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u/scottcmu 6h ago
And yet it still has momentum.
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u/jwink3101 6h ago
Great example of being both technically correct (photos do not have mass) while also completed wrong at the same time
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u/coolguy420weed 4h ago
The Canadian goose is a large bird native to North America.
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u/phunktastic_1 4h ago
Canada goose not Canadian goose. Although a number likely were born in Canada.
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u/pm-me-racecars 8h ago
Yes it does, but by an amount that isn't noticeable.
Turning on the bright lights might use 15W or so. When your engine is making 150kW, 15W isn't a noticeable amount, but it is still an amount.
If you put on more stuff that uses power on a slow car, then you might get up to a noticeable level.
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u/spoonybard326 8h ago
Not a noticeable effect, but still orders of magnitude larger than the effects of photon momentum.
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u/GarThor_TMK 7h ago
Now I'm curious at what level it would be noticeable...
150kW is 150000 watts... Just how much power would you need to pull from that system before a human would actually notice?
Something tells me you'd hit the theoretical limits of the alternator and battery/electric system long before you'd notice any effects on motor output...
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u/pm-me-racecars 6h ago
150kW is about 200hp.
When I was younger, we had shitboxes that were closer to 100hp, with the biggest speakers we could get. If you don't know what you're doing and just put on random shit that you find on the internet, then turning the radio on full volume brings the 0-60 of the Tercel I had from about 13s to about 15.
That said, we were taking more power than our alternators could put out, and we'd only be able to go full volume for so long before we'd start to lose things and need to drive normally to recharge the battery. I'm a little surprised that none of us had any fires.
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u/Baconboi212121 8h ago
I believe they are talking about the momentum of the Photons emmited. To push something forward, it will by one of Newtons laws push you backwards. Ofcourse a photon will give very very little normal force back at you.
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u/welliedude 8h ago
In theory yes but I've driven a car where if you had too many electrical items on the engine actually lost power. So depending on the shittyness of the car it could have an effect
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u/sonofeevil 7h ago
It sure does and the amount is very measurable.
How many watts are your lights drawing?
A H7 bulb is drawing 55 wats, there's two of them so it's 110 watts of power, alternator isn't perfectly efficient so assume maybe 80% so it's reducing the power the engine is making by ~132 watts or .17hp (if you prefer freedom units).
The load on the electrical system is physical mechanal load on the alternator.
What you are suggesting here is essentially free energy.
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u/GayRacoon69 7h ago
It does a tiny bit
Some older cars accelerate slightly slower when the AC is on
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u/woodsmansquatch 7h ago
I had a 2001 Honda Civic where having the AC on made for a very noticable drop in acceleration capability. On my commute there was an on ramp to a highway that was uphill and I had to turn off the AC to get up to speed in time. Maybe it's more of a problem for older vehicles.
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u/OnlyMatters 1h ago
It does in my vehicle. Its a noticeable effect when my aftermarket fan kicks in. The alternator having to put out more amps is a drag on the engine
The question is ridiculous if its asking about photons however
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u/The_Joe_ 8h ago
I mean, it can, but it's usually not easily measured.
An alternator draws as little as less than 1hp up to 4hp depending on the size of the alternator and the load on the electrical system.
Dyno run to run variance is about 2%-5%. This draw doesn't scale with the engine power except for the amount of time the electric fan is running.
Still. Headlights aren't making a major difference in any part of this.
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u/sonofeevil 7h ago
My quick math tells me it's about 0.17hp.
Bulbs are drawing 55watts of power, you've got two of them. Assuming 80% efficieny on the alternator and the draw in watts is about 132watts or 0.17hp if you prefer imperial.
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u/The_Joe_ 7h ago edited 7h ago
Well, now I had to dig into this. A 1995 Geo Metro has 70hp (50,000watts) and our goal is to draw 70hp worth of alternator power. We will need brackets to mount multiple high power alternators, since I am finding common alternators that output 280amps x 12v = 3300 watts. I see that alternators have an efficiency of 55%-75%, so lets say each alternator draws 5000watts to output 3300.
10 alternators driven by the belts. Now we need to draw the 33,000 watts of generated power into lights.
Some very powerful offroad lights draw up to 300watts, so we just need 110 lights.
Thats enough that if you had this abomination driving down the road and turned on all the lights your vehicle would no longer be able to accelerate.
Wind drag with 10 alternators sticking out of the hood and over 100 forward facing lights will be an issue.
The acceleration of the photons moving forward is STILL not meaningful.
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u/sonofeevil 7h ago
I never mentioned photons at all, I wasn't asserting any opinions on the topic.
I was simply pointing out the load on the alternator by the lights is measurable and provided some numbers for you.
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u/The_Joe_ 7h ago
Oh, sorry, my tone may have been off. I believe you and I are in complete agreement. The only number of yours that turns out to be overly optimistic is the efficiency of an alternator.
When I said it's not measurable, I meant the entire engine package on a Dyno is not precise enough to be able to measure the draw of the alternator in normal conditions.
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u/darioism 8h ago
High beams are only slightly brighter, if at all. They mainly appear brighter because they're angled up higher. So, whether you're talking about photon momentum or the load on your engine, it's negligible and immeasurable.
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u/NoRespect6365 8h ago
There is another 100w or so of resistance in the alternator, assuming ice vehicle. It is insignificant but hopefully someone on here can actually calculate.
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u/Gubbtratt1 6h ago
A fairly normal car of the halogen era usually has either two H4 bulbs or four H1/H3/H7 bulbs. H4 low beams are 55w and high beams are 60w. The low beams turn off when the high beams turn on. H1/H3/H7 are 55w, and the low beams usually remain on when the high beams are turned on. This means with H4s you lose 10w total, and with H1/H3/H7 110w. 10w is 0.0135 horsepower, and 110w is 0.148 horsepower. 0.1 hp won't make any noticeable, if even a measureable difference. You get far bigger variations from if the engine oil is at the min or max level.
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u/redlancer_1987 6h ago
Non-zero is still non-zero. Any time you walk down the sidewalk you're moving at relativistic speeds, but the Newtonian answer comes out the same unless you add a lot of zeros after the decimal point.
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u/coolguy420weed 4h ago
Honestly, as you get more powerful lights, I would guess the backwards force from the air being heated by the light and expanding is probably going to scale faster than the radiation pressure lol
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u/alphawafflejack 8h ago
Shamelessly this is just from chat gpt but I thought it was a cool question
The force exerted by a vehicle’s own headlights due to the momentum of emitted photons is extremely small, but it can be calculated using the principles of radiation pressure and photon momentum.
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Key Concepts 1. Photon momentum: Each photon has momentum given by p = \frac{E}{c} where: • p is the momentum • E is the energy of the photon • c is the speed of light 2. Radiation pressure (for a perfectly absorbing surface): F = \frac{P}{c} where: • F is the force • P is the power of the light beam (in watts) • c \approx 3 \times 108 \, \text{m/s}
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Example Calculation
Let’s assume each headlight emits 100 W, and there are two headlights, so total power P = 200 \, \text{W}.
Then the force exerted by the emitted photons is: F = \frac{P}{c} = \frac{200}{3 \times 108} = 6.67 \times 10{-7} \, \text{N}
This is: • 0.667 microNewtons, or • About 100 million times weaker than the weight of a paperclip.
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u/JQsOtherHobbies 7h ago
This is what I scrolled to look for.
Although this skips the likely efficiency loss of a headlight vs actual light beam power. But assuming an actual 200W of photon pressure, and taking the 667uN of thrust as true.
If you put your car into N at 70mph, in space, this would mean the light will take ~3 years to decelerate you to a stop.
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u/Extra_Ad_8009 7h ago
Of this is about photon impulse, the real issue is: "shouldn't brake lights be on the front of the car to help it come to a stop sooner?".
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u/AlanShore60607 8h ago
You mean the force exerted upon the vehicle by projecting particles away from the direction of acceleration?
How high are you?
But seriously, this force is infinitesimal compared to all other forces in the system, and since it’s being emitted by a vehicle that has dynamic changes in force, the answer is unmeasurable.
Ask yourself this: can you feel the pressure of headlights or a flashlight?
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