Forces and Motion: 5.1
Introduction  defining and explaining the stopping
distance
and speed of a road vehicle  diagram table of
values
Doc Brown's Physics exam study revision notes
INDEX of physics notes:
reaction times, stopping distances of road vehicles, Newton's 2nd Law,
KE calculations
5.1
Introduction to stopping distances
and the speed of
road vehicles
What is the formula for stopping
distance?
When driving a car you obviously need to be
alert to any sudden change in your situation, particularly if you have to do an
emergency brake to come to a halt.
In this situation you want to stop the
car (or any other road vehicle) in the shortest time as possible to
effect an appropriate emergency stop!
This means applying the maximum force
on the brake pedal.
The longer it takes for you to react
and longer it takes to come to a halt, the greater the risk of crashing
into an object in your path. Everybody's 'thinking' reaction time to a situation
requiring a quick physical response is different, though typically in the range
0.2 to 0.8 seconds. In biology you may have studied
the nervous system including the reflex arc
(GCSE level biology notes).
The distance it takes to stop a road vehicle
in an emergency situation is given by the following formula:
STOPPING DISTANCE = THINKING DISTANCE
+ BRAKING DISTANCE
The thinking distance is how far
you travel during your reaction time, which is the time interval from you
perceiving a hazard and actually starting to take action e.g. apply the brakes.
The braking distance is the actual
distance you travel from when you first apply the brakes until you come to a
halt.
The stopping distance is the total
time required from initial visual stimulus to actually stopping moving.
The chart above gives typical or average values
for thinking distances, braking distances and stopping distance and are
quoted from the UK Highway Code
guidance booklet.
You can see the thinking distance is
quite an appreciable portion of the total stopping distance, particularly at
lower speeds, BUT, look how dramatically the total stopping distance
increases with increase in speed.
These values should be doubled for
wet roads and multiplied by 10 for ice coated roads. Snow would be
somewhere in between, but where?, so just take great care in driving in
any of these adverse driving conditions.
Later in this page I've used this
data to do some graphs and calculations relating stopping distance to
speed and the kinetic energy of a car.
INDEX of physics notes on
reaction times, stopping distances of road vehicles, Newton's 2nd Law,
braking friction force, KE calculations
Keywords, phrases and learning objectives for
the physics of road vehicles 
stopping distance
Know that stopping distance = thinking distance + braking
distance
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INDEX of physics notes on
reaction times, stopping distances of road vehicles, Newton's 2nd Law,
braking friction force, KE calculations
