Why speed squares your stopping

Science · Forces & Motion · ages 15-16

Name ______________________ Date ____________

A car travels at 20 m/s with a reaction time of 0.6 s. What is the thinking distance in m?
Answer: ______________
What is thinking distance?
- The gap to the car ahead
- Ground covered while reacting, before braking
- Ground covered while the brakes work
Stopping distance is thinking distance plus braking distance.
Circle one: True False
Speed doubles from 15 m/s to 30 m/s. What happens to braking distance?
- Roughly quadruples
- Doubles
- Stays fixed
Which pair lengthens thinking distance?
- Drink and higher speed
- Wet road and worn tyres
- New brakes and low speed
A car's kinetic store is 90000 J and its brakes push with 6000 N. What is the braking distance in m?
Answer: ______________
Why does 30 m/s need far more than double the braking distance of 15 m/s?
- Thinking adds up
- Store grows with speed squared
- Brakes fade with speed
Bald tyres in rain: which stopping part grows, and why?
- Thinking, reactions slow
- Both equally
- Braking, weaker force drains the store slower

Answer key

For grown-ups. Fold this page away before handing over the rest.

12 · Speed times reaction time: 20 times 0.6 = 12 m.
Ground covered while reacting, before braking · Thinking distance is covered while you react, before braking starts.
True · The stop splits into reacting ground plus braking ground.
Roughly quadruples · The kinetic store grows with speed squared, so the brakes need about 4x the distance.
Drink and higher speed · Drink stretches the reaction and speed covers more ground in the same time.
15 · Force times distance = store removed: 90000 divided by 6000 = 15 m.
Store grows with speed squared · Doubling speed quadruples the store the brakes must remove.
Braking, weaker force drains the store slower · Less grip means a weaker braking force, which needs more distance to drain the same store.