Stopping Distances and Braking Energy
Stopping distance splits into thinking distance, set by reaction time and speed, and braking distance, set by the work the brakes must do to empty the kinetic store. Braking distance grows with the square of the speed.
What a learner can do afterwards
- Calculates thinking distance from a reaction time and a speed
- Uses the kinetic store and the braking force to work out a braking distance
- Explains why doubling the speed roughly quadruples the braking distance, and lists what makes each part longer
1 · Read
Stopping distance splits in two for you. Thinking distance is the ground covered while you notice danger and react, before your foot touches the brake. Braking distance is the ground covered while the brakes drain the kinetic store to zero.
Thinking distance is speed times reaction time. At 20 m/s with a 0.6 s reaction, you travel 20 times 0.6 = 12 m before braking even starts. Tiredness, distraction, and drink stretch the reaction, and higher speed covers more ground in the same time.
Braking distance comes from work: brake force times distance = kinetic store removed. Since the store grows with speed squared, doubling speed from 15 m/s to 30 m/s roughly quadruples the braking distance. Worn brakes, worn tyres, wet roads, and heavy loads all stretch it further.
Answer stopping questions in three beats: thinking distance, braking distance, then the total. Say which part each factor touches: phones and drink hit thinking, rain and bald tyres hit braking.
Reaction times speed for thinking, speed squared for braking, and the total is both added.
More to read
How to Brake and Stop a Car in the Shortest Distance
Slamming the brakes is not the fastest way to stop. If your car has ABS (anti-lock brakes), press the pedal down firmly and keep pressing, even if it pulses under your foot. That pulsing means the system is stopping your wheels from locking up, so your tyres keep gripping the road instead of sliding. A sliding, locked tyre actually grips worse than a rolling one, so it takes longer to stop. Smooth, firm, steady pressure lets the brakes do the most work in the shortest distance. Worn tyres, wet or icy roads, and worn-out brake pads all make the brakes less able to soak up the car's kinetic energy quickly, so the braking distance grows.
3 Ways to Brake Properly on a Motorcycle
Before you even touch the brakes, your body needs time to notice danger and react. A typical reaction time is about 0.62 seconds. During that time your bike or car is still moving at full speed, covering what's called the thinking distance. The faster you're going, the further you travel in that same 0.62 seconds, so higher speed always means a longer thinking distance. Once you do start braking, a higher starting speed also means it takes longer to come to a complete stop, because there's more kinetic energy for the brakes to remove.
Kinetic Energy and the Work-Energy Theorem
Moving objects carry kinetic energy, energy of motion. When a force acts on something over a distance, it does work, and that work changes the kinetic energy. A car's engine does work to speed the car up, filling its kinetic energy store. When you brake, the brakes push back against the car's motion, doing negative work that drains that kinetic energy store back down to zero. The braking distance is exactly how far the car travels while the brakes empty that store: a bigger kinetic energy store (from a higher speed) needs more distance for the same braking force to empty it.
Key Terms: Work and Kinetic Energy
Here are a few words worth knowing. Kinetic energy: the energy of motion, found from one-half the object's mass times its speed squared. Work: what happens when a force acts on something and pushes it through a distance. Net work: the total work from every force acting on an object combined. Work-energy theorem: the total (net) work done on something equals the change in its kinetic energy. This is exactly the idea behind braking: the brakes do negative net work on the car, and that work equals the kinetic energy the car loses as it slows to a stop.
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2 · Watch
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Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.