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Turning speed into heat

How Does Rubbing Stop a Car?

Brakes do not remove a car's energy; they convert it into heat in the discs.

A steel disc on thewheelPads squeeze both facesFriction fights thespinMovement becomes heat
Brakes do not remove speed. They turn it into heat.

  1. Step 1 of 5

    A moving car holds energy that must go somewhere.

    A rolling wheel Energy leaves asheat, not nothing

    Slowing down does not make that energy vanish, because energy never vanishes. Braking is really the job of moving it somewhere harmless.

  2. Step 2 of 5

    Pads squeeze a disc bolted to the wheel.

    The wheel from theside Two pads gripping thedisc

    A steel disc turns with the wheel, and a clamp holds a pad on each side of it, ready to pinch the disc from both faces at once.

  3. Step 3 of 5

    Rubbing converts motion into heat.

    1 Pads press thespinning disc2 Rubbing resiststhe turn3 The disc getshot

    The pads scrape against the turning disc, and every bit of that scraping leaves energy behind as warmth in the metal instead of speed in the car.

  4. Step 4 of 5

    Your foot's push is multiplied by fluid.

    Your foot on the pedala modest pushBrake fluid in a narrowpipepressure passedalongWide pistons at thewheela much biggersqueeze

    A modest push on the pedal squeezes fluid down a narrow pipe to much wider pistons at each wheel, which turns a small force into a large one.

  5. Step 5 of 5

    Too much heat and brakes stop working.

    Cool discs Full gripOverheated discs Fade — much less grip

    Discs shed heat into the air, but on a long descent they can take more than they lose, and overheated pads grip far less well.

    Stopping a family car from motorway speed dumps about 700,000 joules into the discs, enough to heat two litres of water from cold to boiling.

The short version

Brakes stop a car by squeezing pads onto a spinning disc, converting the car's movement into heat that the discs shed into the air.

Try it yourself

Rub your palms together hard for ten seconds. That warmth is exactly what brake discs do with a car's motion, on a far larger scale.

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