Stopping Distances (Edexcel GCSE Physics: Combined Science)

Revision Note

Test Yourself
Ashika

Author

Ashika

Expertise

Physics Project Lead

Thinking & Braking Distances

  • The stopping distance of a car is defined as:

The total distance travelled during the time it takes for a car to stop in response to some emergency

  • It can be written as an equation involving two distances:

Stopping distance = Thinking distance + Braking distance

  • Where:
    • Thinking distance = the distance travelled in the time it takes the driver to react (reaction time) in metres (m)
    • Braking distance = the distance travelled under the braking force in metres (m)
    • Stopping distance = the sum of the thinking distance and braking distance, in metres (m)

  • For a given braking force, the greater the speed of the vehicle, the greater the stopping distance

Worked example

At a speed of 20m/s, a particular vehicle had a stopping distance of 40 metres. The car travelled 14 metres whilst the driver was reacting to the incident in front of him. What was the braking distance?

A   54 m

B  34 m

C   26 m

D   6 m

ANSWER: C

Step 1: Identify the different variables

    • The stopping distance is 40 metres
    • The car travelled 14 metres while the driver was reacting
      • This is the thinking distance

Step 2: Rearrange the formula for stopping distance

Stopping distance = Thinking distance + Braking distance

Braking distance = Stopping distance – Thinking distance

Step 3: Calculate the correct braking distance

    • Braking distance = 40 – 14 = 26 metres

Dangers on the Road

  • A large deceleration can be dangerous for a number of different reasons
  • For any object to decelerate, a force is required and for heavy objects, large decelerations will require large forces
  • This is true according to Newton’s Second Law:

F = ma

  • Where:
    • F = force in newtons (N)
    • m = mass in kilograms (kg)
    • a = acceleration in metres per second squares (m/s2)

  • This equation can be rearranged with the help of a formula triangle:

Fma Formula Triangle, downloadable IGCSE & GCSE Physics revision notes

Force, mass, acceleration formula triangle

Overheating of Brakes

  • Vehicles use brakes to decelerate
  • These reduce the speed of the vehicle by creating a friction force between the brake and the wheel
  • The kinetic energy of the vehicle is converted to thermal energy of the brakes
  • If brakes get too hot then they can fail, meaning that they will not work effectively the next time they are used

Braking and Friction, downloadable IGCSE & GCSE Physics revision notes

Work done by breaking transfers kinetic into thermal energy

Loss of Control and Injury

  • When a vehicle undergoes a deceleration then the driver and passengers also experience a deceleration
  • This can cause injuries such as whiplash is a neck injury caused when a persons head moves suddenly, relative to their body
  • It is also more difficult to control a vehicle that is decelerating
  • Losing control can cause a collision

Worked example

A passenger travels in a car at a moderate speed. The vehicle is involved in a collision, which brings the car (and the passenger) to a halt in 0.1 seconds. Estimate:

a) The acceleration of the car (and the passenger).

b) The force on the passenger.

Part (a)

Step 1: Estimate the required quantities and list the known quantities

A moderate speed for a car is about 50 mph or 20 m/s

    • Initial velocity ~ 20 m/s
    • Final velocity = 0 m/s
    • Time, t = 0.1 s

Step 2: Calculate the change in velocity of the car (and the passenger)

change in velocity = Δv = final velocity − initial velocity

Δv = 0 − 20

Δv = −20 m/s

Step 3: Calculate the acceleration of the car (and the passenger) using the equation:

Step 4: Calculate the deceleration

a = −20 ÷ 0.1

a ~ −200 m/s2

Part (b)

Step 1: Estimate the required quantities and list the known quantities

An adult person has a mass of about 70 kg

    • Mass of the passenger, m ~ 70 kg
    • Acceleration, a = −200 m/s2

Step 2: State Newton's second law 

    • This question involves quantities of force, mass and acceleration, so the equation for Newton's second law is:

F = ma

Step 3: Calculate an estimate for the decelerating force

F = 70 × −200

F ~ −14 000 N

Exam Tip

Remember that resultant force is a vector quantity. Examiners may ask you to comment on why its value is negative - this happens when the resultant force acts in the opposite direction to the object's motion. In the worked example above, the resultant force opposes the passenger's motion, slowing them down (decelerating them) to a halt, this is why it has a minus symbol.

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

Join the 80,663 Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Ashika

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.