8.4 - on the road

what is thinking distance (s₁)?

• the distance travelled by a vehicle in the time it takes the driver to react

• s₁ = vt₀ (thinking distance = speed of vehicle x reaction time)

what is the distance travelled by a vehicle in the time it takes the driver to react?

thinking distance (s₁)

what is the symbol for thinking distance?

s₁

what is s₁ ?

• thinking distance

• v t₀

what is the equation for thinking distance?

s₁ = v t₀

• s₁ = thinking distance

• v = speed of vehicle

• t₀ = reaction time of driver

what is t₀ ?

the reaction time from the driver

s₁ = v t₀

equation for thinking distance

what affects thinking distance?

• distractions

• drugs

• alcohol

what do distractions, drugs, and alcohol affect?

the thinking distance

why are drivers in the UK banned from using mobile phone or being under the influence whilst driving?

because it impacts a driver’s thinking time, which impacts the thinking distance, therefore the stopping distance, meaning the driver is less likely to stop the car in time in case of an emergency, and therefore more likely to get into / cause an accident

what is the braking distance?

• the distance travelled by a car in the time it takes to stop safely, from when the brakes are first applied

• s₂ = u² / 2 a

when is braking distance measured from?

the moment the brakes are first applied

what is the symbol for braking distance?

s₂

what is s₂ ?

• braking distance

• u² / 2 a

s₁ = ?

s₂ = ?

s₁ = thinking distance = v t₀

s₂ = braking distance = u² / 2 a

s₂ = u² / 2 a

equation for braking distance

what affects braking distance?

• the condition of the road, e.g., icy / greasy roads

• the brakes’ condition

• the tires’ condition (tread depth, tire pressure)

how does the condition of the road affect braking distance?

if the road’s surface doesn’t have enough friction (e.g., if its icy), then the tires require a larger braking force and time from the car to get enough grip to reach the limiting frictional force to successively stop the car

how does the brakes’ condition affect braking distance?

if the brakes are poorly maintained or broken, because they aren’t working at the efficiency of newer brakes, therefore increasing braking distance

unable to provide the correct braking force to the tires, meaning the limiting frictional force is harder to reach

how does the tires’ condition affect braking distance?

if the tire doesn’t have enough grip, it cannot generate enough friction with the road to slow and stop the car

why is it dangerous to drive on icy roads, or roads with little friction?

because it takes the tires more time and force to generate enough grip with the road to slow and stop the car, meaning braking distance and therefore stopping distance is increased

what is the equation for braking distance?

s₂ = u² / 2 a

• u = initial speed (before brakes are applied)

• a = constant deceleration to zero speed

what is acceleration (a) in the stopping distance?

the constant deceleration of the car from car’s speed when the brakes are first hit (u) to zero

is a (acceleration) the acceleration of the car in stopping problems?

no, it’s the constant deceleration of the car’s speed from when the brakes are first applied to zero

is the car’s deceleration constant in stopping problems?

yes

why is the car’s deceleration constant in stopping problems?

to make modelling easier

what is stopping distance?

• thinking distance + braking distance

• u t₀ + u² / 2 a

u t₀ + u² / 2 a

stopping distance

what affects stopping distance?

• magnitude of thinking distance (and therefore everything that affects thinking distance)

• magnitude of stopping distance (and therefore everything that affects stopping distance)

what is the shortest stopping distances on dry road when going 30 mph (according to the highway code)?

75 ft / 22.6 m

what is the shortest stopping distances on dry road when going 50 mph (according to the highway code)?

175 ft / 52.5 m

what is the shortest stopping distance on dry road when going 70 mph (according to the highway code)?

315 ft / 94.5 m

why is the short stopping distance on dry road 75 ft (22.5 m) when going 30 mph (according to the highway code)?

here

why is the short stopping distance on dry road 175 ft (52.5 m) when going 50 mph (according to the highway code)?

here

why is the short stopping distance on dry road 315 ft (94.5 m) when going 70 mph (according to the highway code)?

here

what is a front-wheel drive vehicle?

engine turns the front wheels via the transmission system

what is a rear-wheel drive vehicle?

engine turns the rear wheels via the transmission system

how does the engine turn the wheels?

via the transmission system

what does the transmission system do?

turns the car’s wheels (front or rear, depending on the vehicle’s drive) using the power from the engine

what is wheel spin?

the car ‘slipping’

what prevents wheel spin?

the friction between the tyres and the road, so the driving wheels therefore roll along the road

why do wheels roll along the road as opposed to sliding?

because of the friction between the tyres and the road

when we describe a car skidding, what is actually skidding?

the wheels

when does a car skid?

• when a driver tries to accelerate too fast

• if the upper limit of friction (the limiting frictional force) between the tyres and the road is reached

why do the wheels of a car skid when a driver tries to accelerate too fast?

because there’s an upper limit to the amount of friction between the tyres and the road

what increases the friction force between the tyres and the road?

increased braking force

what happens when the brakes are applied in a moving vehicle?

the wheels are slowed down by the brakes due to increased frictional force between the tyres and the road, therefore slowing the vehicle down to a complete stop

when does a car skid when braking?

if the upper limit of friction (the limiting frictional force) between the tyres and the road is reached

what is the upper limit of friction?

• the maximum frictional force between the tyres and the road

• the limiting frictional force

how else can the upper limit of friction be referred to as?

the limiting frictional force

how else can the limiting frictional force be referred to as?

the upper limit of friction

what happens when the limiting frictional force is reached?

• the brakes lock and the vehicle slides uncontrollably forward

• the wheels spin

when a vehicle’s brakes lock and the vehicle slides uncontrollably forward, what has happened?

the vehicle’s wheels have reached their upper limit of friction with the road, meaning the brake’s have locked and the wheels skid

what is ABS?

anti-lock brake system

what can prevent skidding from a brake lock?

an anti-lock brake system (ABS)

how does an anti-lock brake system (ABS) work?

• a speed sensor on each wheel is linked to a central electronic control unit that controls hydraulic values in the brake system

• when the brakes are applied, if a wheel starts to lock, the control unit senses that the wheel is rotating much slower than the other wheels

• the valves are activated so the brake pressure on the wheels is reduced to stop it locking

what does it mean for a brake to lock?

the brakes lock up (stop applying a braking force to the wheels) and the wheel skids, with the vehicle sliding uncontrollably forward

how is an anti-lock brake system (ABS) composed?

speed sensor on each wheel linked to a central electronic control unit that controls hydraulic values in the brake system

in an ABS, what is on each wheel?

a speed sensor linked to a central electronic control unit

in an ABS, where are the speed sensors?

on each wheel

in an ABS, are there speed sensors on all the wheels?

yes

in an ABS, why are speed sensors on all four wheels are opposed to just the two driving wheels?

• because any of the four wheels can lock

• the speed of the wheels are compared to each other because if a wheel is moving considerably slower, it means it is locking

in an ABS, what are the speed sensors connected to?

the driving wheels

in an ABS, what is the central electronic control unit connected to?

the speed sensors on each driving wheel

in an ABS, what does the central electronic control unit do?

control hydraulic valves in the brake system

in an ABS, what controls the hydraulic valves in the brake system?

the central electronic control unit

what does an anti-lock brake system (ABS) do when a wheel starts to lock?

the control unit senses that the wheel is rotating much slower than the other wheels, so it activates the valves so the brake pressure on the wheel is reduced to stop it locking

how does decreased brake pressure on the wheel from ABS stop the wheel from locking?

with decreased brake pressure, there is less frictional force between the tyres and the road (as frictional force is proportional to braking force), therefore the frictional force decreases and the wheel will not lock

in terms of forces, how exactly does a car move forward?

by pushing backwards on the road

does a car move forward by pushing forward or backwards on the road?

backwards

where is the frictional force on a car when moving?

opposite the direction of motion

what practical can you do to test for limiting friction?

• pull a block on a rough surface with increasing force (using a newton-meter) until the block slides

• the limiting frictional force between the underside of the block and the surface it is on is equal to the pull force on the block just before sliding over

here

when measuring the limiting friction in a box practical, what is the frictional force between?

the underside of the box and the surface

when measuring the limiting friction in a box practical, why is the surface rough?

so there is a frictional force opposing the box’s movement between the box and the surface

when measuring the limiting friction in a box practical, what is the limiting frictional force equal to?

equal to the pull force on the block just before sliding occurs

when measuring the limiting friction in a box practical, what is the pull force on the block just before sliding occurs equal to?

the limiting frictional force

when measuring the limiting friction in a box practical, what does limiting friction vary with?

box weight

how should you brake on an icy / greasy road?

apply the brakes with less force than on a dry road

why should you brake by applying the brakes with less force when on an icy / greasy road?

otherwise skidding is more likely to occur

which has a larger braking distance - a car braking on a dry road or a car braking on an icy / greasy road?

car braking on an icy / greasy road

why does braking distance increase on an icy / greasy road as opposed to a dry one?

the brakes need to be applied with less force when on an icy / greasy road than on a dry road otherwise they will skid, therefore braking distance is larger as it takes longer for the vehicle to come to a full stop

how should a driver position their car in fast moving traffic?

by leaving a large enough gap between them and the gap in front

why should a driver keep a large gap between them and the car in front when in fast-moving traffic?

to ensure they can have enough braking distance to slow down safely if the vehicle in front slows down

what makes an tire unsafe?

• tread depth < 1.6 mm

• tread covers < ¾ of the tire’s width

• pressure is too small / too great

what are the legal requirements for a tire’s tread in the UK?

• absolute minimum depth of 1.6 mm

• tread must cover ¾ of the tire’s width all the way around

what is the tread of a tire?

grooves on the tire’s surface to grip roads

why must tire tread’s be of a certain depth?

otherwise any grease / oil / water on the road considerably reduces the friction between the road and tire

what happens if the pressure on a tire is too small / too great?

• if the pressure is too small / too great, the tire will wear down unevenly

• if the pressure is too large, braking force is reduced as there is less tire area in contact with the road

what happens if a tire is unbalanced?

it will wear down unevenly

why does increased tire pressure decrease braking force?

it means the area of the tire that’s in contact with the road is reduced, therefore braking force between the tire and the road is decreased

why is skidding more likely on an icy / greasy road?

the limiting frictional force between the road and the tires is reduced from its dry value

which has a larger limiting friction force between the road and the tires - a car on a dry road or a car on an icy / greasy road?

a car on a dry road

which has a smaller limiting friction force between the road and the tires - a car on a dry road or a car on an icy / greasy road?

a car on an icy / greasy road

why does a car on a dry road have a larger limiting frictional force between the road and tires than a car on an icy / greasy road?

there is more friction along a dry road because the surface is rougher, meaning the tires can get a better grip than they can on ice / grease