Stopping distance

Question 1

What is thinking distance?

Answer 1

the distance the car travels while the driver reacts to the danger and applies the brakes (reaction time)

Question 2

What is stopping distance?

Answer 2

the total distance over which a vehicle comes to rest

stopping distance = thinking distance + braking distance

Question 3

What is braking distance?

Answer 3

the distance the car travels after you apply the brakes/while it is slowing down

Question 4

What happens to the thinking and braking distances when you double the speed of a car?

Answer 4

the thinking distance doubles, but the braking distance increases by a factor of 4

Question 5

Thinking distance and braking distance are factors that affect the stopping distance.

(a) When does thinking distance (reaction time) increase?
(b) When does braking distance increase?

Answer 5

(a) when:
- the driver is tired
- the driver is distracted
- the driver has taken alcohol or drugs
- there is low visibility (due to fog, mist, rain or low light levels)
- the car is going faster

(b) when:
- the amount of friction between the tyre and the road decreases
- the road is icy or wet
- the brakes or tyres are worn
- the mass of the car is bigger
- the car is going faster

Question 6

What is braking distance proportional to?

Answer 6

(speed)²

Question 7

What happens when a force is applied to the brakes?

Answer 7

the kinetic energy is transferred to thermal energy and the vehicle comes to rest over a certain distance

Question 8

What is the equation for the work needed to bring a vehicle to rest?

Answer 8

F × d = ½ × m × v²

Question 9

A car is moving at a constant speed. Explain how the stopping distance changes when:

(a) the speed of the car increases
(b) the car has more passengers
(c) the driver has been drinking alcohol

Answer 9

(a) the stopping distance increases because the thinking distance and braking distance increase (distance = speed × time)
(b) the mass of the car will be greater so the braking distance will increase, which means the stopping distance will increase as the momentum will be greater (momentum = mass × velocity)
(c) the stopping distance will increase because the thinking distance will increase due to the driver’s reaction time decreasing

Question 10

Explain how the thinking and braking distance of a car changes when the speed of the car increases from 20 mph to 80 mph.

Answer 10

Thinking distance increases (by a factor of 4), braking distance increases (by a factor of 16)

Question 11

Calculate the braking distance of a car of mass 1250 kg travelling at 12 m/s if the average force applied to the brakes is 1800 N.

Answer 11

F × d = ½ × m × v², d = 1250 kg × (12 m/s)² ÷ (2 × 1800 N) = 50 m