AQA GCSE Stopping Distances(Physics)
Stopping Distances
The stopping distance is the sum of the thinking distance and braking distance
Below an equation has been written to show this and the two other rearrangements.
Thinking distance
Thinking distance is the distance travelled by a car in the time it takes the driver to react.
The reaction time is the time period that occurs when a person sees an event and then react to that event.
If a driver sees a car in front braking, and then applies his foot to the brake pedal 0.3 seconds later, then the reaction time is 0.3 seconds.
Braking distance
Braking distance is the distance travelled by the vehicle whilst a braking force is being applied to stop the vehicle.
Factors that affect stopping distance
As speed increases, stopping distance will also increase, assuming that braking force is constant.
Tiredness, drugs, alcohol, distractions all increase the reaction time or thinking distance and therefore increase stopping distance.
Adverse road conditions such as wet or icy roads will increase the braking distance, which increase the stopping distance.
Poor condition of vehicle brakes or tyres will increase braking distance, which increases stopping distance.
Estimating stopping distances
You do not need to learn the table below, but you need to be able to estimate stopping distance for different speeds. So, the table below is to guide you.
Graphs and Stopping Distance.
You will need to be able to interpret graphs that link speed to stopping distance, lets show you an example!
Normally the area under a velocity time graph will give the distance travelled. However, the area under a speed-time graph will also give the distance travelled.
Thinking distance
During the thinking distance the car is moving at constant speed. During this time period, the car is still moving forward whilst the driver reacts.
In the graph below, the thinking distance section has a rectangle shape. To calculate the thinking distance we will work out the area 8m/s x 3s = 24m.
Braking distance
During the braking distance, the brake pedal is applied, so a braking force is decelerating the vehicle and the speed is decreasing to zero.
In the graph below, the braking distance has a triangle shape. To calculate the braking distance we will use area of a triangle formula
1/2 x 6s x 8m/s = 24m
Stopping distance
Stopping distance = thinking distance + braking distance
Thinking distance = 24m
Braking distance = 24m
Stopping distance = 24m + 24m = 48m.
Practice Question
1.State the definition of the following:
a)Stopping distance
b)Thinking distance
c)Braking distance
d)Reaction time
2. A car at a particular speed has a stopping distance of 50m and a thinking distance of 15m. Calculate the braking distance.
3. Estimate the thinking and braking distanec of a vehicle at 80mph.
4. There are two cars; Car A and Car B. The brakes on Car A will function normally, but the brakes on Car B are worn out and need replacing.
Draw two speed time graphs on the axes below to represent car A and car B. Assume that in both cases the thinking distance is the same for both.
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