Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Linked equations: s = vt, W = Fs, Eₖ = ½mv²
Notes
Stopping distance = thinking distance + braking distance. For a given braking force, the greater the speed, the greater the stopping distance.
Thinking distance
How far the vehicle travels during the driver’s reaction time (typically 0.2 s to 0.9 s). Affected by speed and anything that slows reactions: tiredness, drugs, alcohol and distractions such as mobile phones. Reaction time can be measured with a ruler-drop test.
Braking distance
How far the vehicle travels once the brakes are applied. Affected by speed, adverse road and weather conditions (wet or icy roads) and the condition of the brakes and tyres.
When the brakes are applied, friction between the brake and wheel does work, reducing the kinetic energy of the vehicle and increasing the temperature of the brakes. Stopping quickly from high speed needs a large braking force – the large deceleration can make the brakes overheat or the vehicle skid.
Questions
Q1 (F) What is thinking distance?
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The distance a vehicle travels during the driver’s reaction time, before the brakes are applied.
Q2 (F) Give three factors that increase a driver’s reaction time.
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Any three of: tiredness, drugs, alcohol, distractions (e.g. using a phone).
Q3 (F) Give three factors that increase braking distance.
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Any three of: higher speed; wet or icy roads; worn tyres; worn brakes; a heavier vehicle.
Q4 (F) A car’s thinking distance is 12 m and its braking distance is 24 m. Calculate its stopping distance.
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12 + 24 = 36 m
Q5 (F/H) A car travels at 20 m/s. The driver’s reaction time is 0.7 s. Calculate the thinking distance.
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F: s = v t
I: s = 20 × 0.7
A: 14 m
Q6 (F/H) Explain why the brakes get hot when a car stops.
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Friction between the brakes and the wheels does work, transferring energy from the car’s kinetic energy store to the thermal energy store of the brakes, so their temperature increases.
Q7 (F/H) Describe how to measure reaction time using a ruler.
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A partner holds a ruler vertically with the zero between your thumb and finger. They drop it without warning and you catch it. Record the distance it fell – a longer distance means a longer reaction time (convert using a table or s = ½gt²). Repeat and calculate a mean.
Q8 (H) Explain why doubling a car’s speed roughly quadruples its braking distance.
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Kinetic energy = ½mv², so doubling v makes the KE four times bigger. The brakes must do four times as much work (W = Fs). With the same braking force, the braking distance must be four times as long.
