GCSE · Physics · Edexcel · Spec 1PH0

Factors affecting stopping distance

A hazard appears. Before your foot even reaches the brake, the car carries on at full speed. So what makes a stop longer, and which factor does double damage?

Stopping distance

Slower driver, or weaker stop?

Every stop comes in two parts. First the thinking distance: the car keeps going while the driver reacts. Then the braking distance: the brakes slow the car down. For each factor, choose the part it makes longer, or both.

  • A Longer thinking distance
  • B Longer braking distance
  1. The driver is tired
  2. The driver has taken drugs
  3. The driver has drunk alcohol
  4. The driver is distracted
  5. The driver is using a phone
  6. Worn tyres
  7. A wet road
  8. An icy road
  9. A fully loaded van (greater mass)
  10. A vehicle in poor condition
  11. Driving faster

Thinking distance

What is the car doing while you react?

A driver is travelling along at a steady speed when a hazard appears ahead. Over about one second, the driver notices it, decides what to do, and moves their foot onto the brake pedal.

During that second, which is closest to what you think the car is doing?
How sure are you?

Adding up a stop

Problem

A car is travelling at 30 mph. With a reaction time of about 1 second, its thinking distance is about 13 m and its braking distance is about 14 m. (a) Find the stopping distance. (b) Now suppose, as an example, the driver is using a phone and their reaction time doubles. Find the new stopping distance.

Braking distance

?

Reason it through

Why do worn tyres, a wet road or an icy road make the braking distance longer?

Link 1 of 4

First link · your turn

Worn tyres, water and ice all change one thing where the tyre meets the road. What is it?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Predict, then check

The reaction time and the road stay exactly the same. Only the speed changes.

A car's speed doubles. What happens to the two parts of its stopping distance?

Same speed, same road: car vs heavy lorry

CarvsHeavy lorry

The only difference is mass. Watch how that one difference runs down every row.

Focus

Kinetic energy at the same speed

Car

Less

Heavy lorry

More

The insight

Kinetic energy = ½ × mass × speed², so a greater mass means more energy to get rid of when stopping.

Energy transferred to the brakes when it stops

Car

Less

Heavy lorry

More

Braking distance with the same braking force

Car

Shorter

Heavy lorry

Longer

Risk of the brakes overheating and failing

Car

Lower

Heavy lorry

Higher

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why does dropping from 30 mph to 20 mph outside a school make such a difference?

What you need to know

  • Stopping distance = thinking distance + braking distance.
  • Reaction time is the time between noticing a hazard and acting on it: notice, decide, move your foot onto the brake. The brakes aren't on yet, so the vehicle doesn't slow.
  • Thinking distance is the distance travelled at constant speed during the reaction time: thinking distance = speed × reaction time.
  • Tiredness, drugs, alcohol and distractions (including using a phone) make the reaction time longer, so they increase the thinking distance.
  • Braking distance is the distance travelled after the brakes are applied. Less grip (worn tyres, wet or icy roads) gives a smaller braking force; deceleration = force ÷ mass; a smaller deceleration gives a longer braking distance.
  • Speed affects both parts. Doubling it doubles the thinking distance and increases the braking distance by a factor of about four.
  • Braking transfers energy from the vehicle's kinetic store to the thermal stores of the brakes. Hot brakes are less effective and can fail.

The big picture

A vehicle's stopping distance is the thinking distance plus the braking distance. Thinking distance is how far the vehicle travels, at constant speed, while the driver reacts; braking distance is how far it travels while the brakes slow it to a stop. Tiredness, drugs, alcohol, distractions and phone use lengthen the reaction time, so they increase the thinking distance. Worn tyres, wet or icy roads, greater mass and the poor condition of a vehicle increase the braking distance. Speed increases both: doubling it doubles the thinking distance and makes the braking distance about four times as large.

Key points

1Driver factors → thinking distance. Tyre, road and vehicle factors → braking distance. Speed → both.
2A wet road can up to double the braking distance; an icy road typically makes it more than 10 times as large.
3At 30 mph, with a reaction time of about 1 s, the thinking distance (about 13 m) is roughly as long as the braking distance (about 14 m).
4Stopping distance at 30 mph is more than twice that at 20 mph, which is the reason for 20 mph limits near schools.
5Kinetic energy = ½ × mass × speed². Doubling the speed makes it four times as large, and more mass means more energy too.
6Brakes are more likely to overheat after a lot of braking from high speed or frequent braking down a steep road, and a heavy lorry's brakes more than a car's.

Worked example

Problem

On a dry road, a car's thinking distance is 12 m and its braking distance is 12 m (example figures). It starts to rain, and a wet road can up to double the braking distance. What is the largest stopping distance you should expect on the wet road?

⚠ Watch out

Saying that doubling the speed doubles the stopping distance. Only the thinking distance doubles. The braking distance goes up about four times, so the whole stop more than doubles.

🧠

Memory hook

Brain, then brakes. Anything that slows the brain stretches the thinking distance. Anything that cuts the grip or adds mass stretches the braking distance. Speed stretches both.

✓

Check yourself

A driver has drunk alcohol and is driving a loaded van. Which part of the stopping distance does each increase? (Alcohol: longer reaction time, so thinking distance. More mass: smaller deceleration, so braking distance.)

Flashcards

(14)
What two distances add up to the stopping distance?
Thinking distance + braking distance.
Define thinking distance.
The distance the vehicle travels, at constant speed, during the driver's reaction time.
Define braking distance.
The distance the vehicle travels after the brakes are applied, until it stops.
What happens during a driver's reaction time?
They notice the hazard, decide how to react, then move their foot onto the brake. The brakes aren't on yet, so the vehicle doesn't slow.
Equation for thinking distance
Thinking distance = speed × reaction time (m = m/s × s).
Four things that make a driver's reaction time longer
Tiredness, drugs, alcohol and distractions (such as using a phone).
Why does using a phone increase the thinking distance so much?
The driver is concentrating on the phone, not on what's in front of them, so their reaction time is much higher.
How does less grip lead to a longer braking distance?
Less grip → smaller braking force → smaller deceleration → longer braking distance.
Deceleration equation, and what greater mass does to it
Deceleration = force ÷ mass. For the same force, a greater mass gives a smaller deceleration.
Wet road vs icy road: effect on braking distance
Wet: up to double. Icy: typically more than 10 times as large.
Speed doubles. What happens to thinking distance and braking distance?
Thinking distance doubles; braking distance increases by a factor of about four.
Where does a vehicle's kinetic energy go when it brakes?
Work is done on the brakes, transferring the energy to the thermal stores of the brakes (and of the tyres and road), which heat up.
Why can brakes fail after heavy use?
They overheat, and hot brakes are less effective. It is most likely after a lot of braking from high speed or frequent braking down a steep road.
Why are a heavy lorry's brakes more likely to overheat than a car's?
Its greater mass gives it more kinetic energy, so more energy is transferred to its brakes when it stops.

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