[wanderingmind]

100 percent can relate on the relentless curiosity of a child. I was dumbfounded yesterday by this question -> if cars go at high speed, and breaks stop it suddenly, how come the friction doesn't generate enough heat to start a spark?

[My_Name]

They generate enough energy to, I mean have you seen racing brake discs?

It's just that the energy is spread out over a larger area (brake discs/pads as opposed to a tiny spark) that is also constantly being cooled by air movement, so the energy in any one point is not enough to ignite any of the materials involved. It's why they used to put asbestos in the brake pads, but they now use ceramics and other heat-resistant materials.

Also, engineering.

[Terr_]

I'd approach it by looking at the definition of a "spark", and the things we can change to avoid tripping that definition.

For a visual spark, it has to be something thrown off which is the right size, material, and a temperature to glow visibly to the human eye under normal light.

For a spark that starts a fire, it has to be something thrown off which has a certain temperature by the time it lands or travels to some other surface.

In both cases, hot very fine particles will cool quickly just by hitting the ambient air. If the underlying surface (e.g. brake disk) gets glowy-hot, we wouldn't call that a spark--it's too big, and fixed.

[LIMEVINCE]

They actually might spark if the materials of the brakes weren't purposefully chosen to avoid a mini firework show when you suddenly brake.

Oof, the guy who went into the definition of sparks actually makes me realize this question branches off into so many different rabbitholes.

[samuell]

I think it is simply designed that way.

In my experience sparks are generally generated when quite small metal surfaces get in contact at high speeds, meaning they can generate a lot of friction in a very small point, and thus not have enough braking effect despite the local friction in one small point (think rail wheels squeeking in curves).

Brakes OTOH are designed to spread out the friction on a somewhat larger area, which allows more material to take up the friction and the heat, and spread it around.

I know brakes can get extremely hot too though, and after a really hard braking you might have to worry about deformed brakes sometimes.

Race cars tend to have really large breaks (and brake pads) for this reason.

[anamax]

Car brakes can get hot enough to start a fire (wood, paper, maybe even gasoline - see race cars) but a spark is a hot piece of material that is large enough so it can keep its heat for a while.

Brakes generate very fine dust that cools off almost immediately.

[ColdStream]

The concentration of energy is vital. Sort of like how if you fire a gun it will only push the person back a few centimeters but that energy condensed into a very small spot and fly through a car.

[rawgabbit]

I would answer that heat is a form of energy. A horse pulling a carriage a 1000 yards requires more energy than pulling 30 yards.

When a car slams on the brakes the distance is too short to generate enough heat to deform. I would also suspect the treads would partially melt and deform before it catches on fire.

[KunYuan]

My kid is like this too. They love asking questions, and one answer usually leads straight to another. Sometimes it feels endless.

[rramadass]

Here are a couple of ways you can nurture it - https://news.ycombinator.com/item?id=49081436

[ErroneousBosh]

Well, they do - you can get brakes red hot.

https://www.youtube.com/watch?v=CUnEeiZK7bY

And, if you let the pads wear down too far so they're metal on metal they absolutely will spark.

How cool are those rally cars though?

[Razengan]

It's ironic that AI today may give curious kids answers quicker if not better than asking humans or searching the web did.

[dd8601fn]

Why not. May as well get those last edge cases of child rearing that haven’t already been outsourced.

Only kidding. Kinda.

[testbjjl]

May being the operative word, for now anyway for certain.

[testbjjl]

Brake pads?